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Articles from 2008 In April


Motion Control Supplement: Killer Apps

April 30, 2008
Supplement Spotlight:
 
Non-Linear Control Advances Servo Performance
Over the last few years, YET (Yaskawa Eshed Technology Inc.) developed a new non-linear adaptive control technology that improves a servo system’s dynamic precision. Full Story
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NEW MOTOR MOUNTS FROM BISHOP-WISECARVER

Bishop-Wisecarver Corporation now offers a complete new line of motor mounts to complement its wide range of linear actuators. Each motor mount is supplied as a kit, complete with shaft coupling and mounting hardware. Motor mounts are available to fit any manufacturer's motor or gearbox. Click Here.
Motion Control Supplement Features:
Innovative Tools And Techniques to Implement Flexible Automation
A high percentage of Automation Engineering Inc.'s customers' manufacturing applications require flexible automation to accommodate rapidly changing products with shorter market life cycles. Full Story
State-of-the-Art Applications State-of-the-art automation and motion control applications, including Ethernet, robotics and motor-drive controllers, are driving higher levels of machine control performance. Full Story
Modular Machines Automation motion technologies shape new machinery designs. Full Story
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Robotic Jellyfish Swim and Fly at Hannover Fair - VIDEO The biggest draws at Festo’s Hannover Fair exhibits have been biologically inspired robotic creatures that show off cutting-edge automation technologies. Watch Now

Solving Sensor Interfacing Issues in Portable Multi-Sensor SystemsPart of Design News' new Sensor Solutions website! As the number of sensors increase and specifications tighten, the design challenges of the sensor interfacing portion of the system increase. Guest expert Andreas Heldwein from ZMD AG will explain the problems and a solution that can simplify interfacing multiple sensors. Watch Now
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A Study in Innovation

Customer and supplier engineering teams collaborated to assure innovative features were incorporated into this snow-blower subassembly's final design. Powder metallurgy provided the manufacturing features and capability needed to marry reliability and safety in a demanding application environment. Click Here.
Sponsored Technology Content
Partnering with Iowa State's Institute for Physical Research and Technology In partnership with Groschopp Inc.
Groschopp Inc. engineers and manufactures customized, high-reliability fractional horsepower electric motors and gear motors for some of the world's most demanding Original Equipment Manufacturers. Find out why the company has turned to IPRT Company Assistance to help it maintain its reputation for products of the highest quality. Read More

Design Tips For Engineers: Taking Aluminum to the Next Level In partnership with Hydro Aluminum
Find out how aluminum extrusions can be designed to minimize the need for further processing, making assembly and final production easier, more cost-effective and efficient. Read More

Sensorless Field Oriented (FOC) Control for AC Induction Motors In partnership with Microchip Technology Inc.
The requirement of low-cost, low-maintenance, robust electrical motors have resulted in the emergence of the AC Induction Motor (ACIM) as the industry leader. Learn how to apply sensorless field-oriented control of induction motors using a dsPIC® Digital Signal Controller (DSC) from Microchip Technology. Read More
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Non-Linear Control Advances Servo Performance
Innovative Tools And Techniques to Implement Flexible Automation
State-of-the-Art Applications
Modular Machines
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Copyright 2008, Reed Business Information, a division of Reed Elsevier Inc.

New Steels Cut Weight, Boost Safety Performance

New Steels Cut Weight, Boost Safety Performance

It was apparent at the recent Great Designs in Steel seminar that steel plans on stealing a page or two from the plastics’ playbook in the key automotive battleground. Steel has several advantages to start with. The manufacturing infrastructure to make steel parts exists, and in fact represents a significant capital investment. Steel also has a strong recycling track record (to say nothing of performance). It seems intuitive that the high gas prices will kick start already existing efforts to reduce weight of cars. But not so fast. New grades of steel reduce weight, and also play into the trend to boost safety performance, particularly for the sides and rear of vehicles. For example an ultra high strength steel (boron-alloyed 22 MnB5) cuts 2 kg for a side crash panel in BMW’s new X6 Sports Activity Coupe. The seven-passenger Acura MDX body structure contains 56 percent high-strength steels, including several new advanced grades. It may surprise some, but some of these new grades are significantly more formable than your father’s steel, allowing creation of complex shapes previously only possible with plastic.

Supplement: Sensing Consumer Needs

April 29, 2008
Supplement Spotlight:
 
Small Sensors Make a Big Difference in Consumer Products
New features and functionality result when consumer products include sensors. Full Story
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EXERGEN'S EXTREME IRt/c

Exergen's Industrial Division introduces the new Extreme IRt/c, which can withstand extreme ambient temperature changes and hold true accuracy, with thermal shock. The rugged design allows for drift free output of a K type thermocouple, under very adverse ambient extremes. Click Here.
Automotive News:
Sensors Address System-Level Design Issues
The ability to sense common or unusual parameters is a great start for any control system. However, those sensors that take into account system-related aspects distinguish themselves from the competition. Full Story
Pulse-Width-Modulation Digital-to-Analog Converter vs. Stand-Alone DAC Is the almost-cost-free Pulse-Width-Modulation Digital-to-Analog Converter good enough for an application or does it need a higher-performance, stand-alone DAC? Full Story
The System Aspects of Sensing Author Rob Lineback discusses the increasing focus on the system aspects of sensors, including interfacing, communication (including wireless), packaging and ease of use. Full Story
Total Recall Using In-Sight ID technologies that have already proven themselves in other industries are now opening up previously untapped potential in hospitals for both safety and cost efficiencies. Full Story
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Solving Sensor Interfacing Issues in Portable Multi-Sensor SystemsPart of Design News' new Sensor Solutions website! As the number of sensors increase and specifications tighten, the design challenges of the sensor interfacing portion of the system increase. Guest expert Andreas Heldwein from ZMD AG will explain the problems and a solution that can simplify interfacing multiple sensors. Watch Now

Robotic Jellyfish Swim and Fly at Hannover Fair - VIDEO The biggest draws at Festo’s Hannover Fair exhibits have been biologically inspired robotic creatures that show off cutting-edge automation technologies. Watch Now
ADVERTISEMENT
Sponsored Technology Content
Partnering with Iowa State's Institute for Physical Research and Technology In partnership with Groschopp Inc.
Groschopp Inc. engineers and manufactures customized, high-reliability fractional horsepower electric motors and gear motors for some of the world's most demanding Original Equipment Manufacturers. Find out why the company has turned to IPRT Company Assistance to help it maintain its reputation for products of the highest quality. Read More

Design Tips For Engineers: Taking Aluminum to the Next Level In partnership with Hydro Aluminum
Find out how aluminum extrusions can be designed to minimize the need for further processing, making assembly and final production easier, more cost-effective and efficient. Read More

Sensorless Field Oriented (FOC) Control for AC Induction Motors In partnership with Microchip Technology Inc.
The requirement of low-cost, low-maintenance, robust electrical motors have resulted in the emergence of the AC Induction Motor (ACIM) as the industry leader. Learn how to apply sensorless field-oriented control of induction motors using a dsPIC® Digital Signal Controller (DSC) from Microchip Technology. Read More
ADVERTISEMENT

Small Sensors Make a Big Difference in Consumer Products
Sensors Address System-Level Design Issues
Pulse-Width-Modulation Digital-to-Analog Converter vs. Stand-Alone DAC
The System Aspects of Sensing
Total Recall Using In-Sight

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STAFF: Editor-in-Chief: John Dodge | Executive Editor: Liz Taurasi | Web Editor: Regina Lynch
PRIVACY MANAGER: Reed Business Information 2000 Clearwater Drive Oak Brook, IL 60523 | Fax: 630-288-8394
Copyright 2008, Reed Business Information, a division of Reed Elsevier Inc.

The Mythical Battery In The Basement

The Mythical Battery In The Basement

At a recent social gathering, a lawyer told me the “truth” about electric vehicle (EV) batteries.

            “A friend of mine knows someone at Google, and he said that General Motors could build great electric cars right now if it wanted to,” he said. “The battery technology is ready. The problem is GM is in bed with the oil companies.”

            Ah, yes, the old auto-industry-in-bed-with-the-oil-companies conspiracy theory. Twenty years ago, we kept hearing about the 200-mile-per-gallon carburetor. Now it’s the killer battery. 

            The amazing thing about this bit of technological folklore is that it lives on, even among engineers. Over the past 10 years, I’ve received countless e-mails from readers who are convinced there’s a battery in a basement (usually at GM), wrapped in oily rags, hidden on a shelf somewhere. The battery is a veritable powerhouse, capable of propelling a truck for 400 miles on a 15-minute recharge. But the evil scientists at GM are rubbing their hands together and twitching with delight while they take payoffs from the oil companies for hiding it. It’s reminiscent of the final scene in the movie, “Raiders of the Lost Ark,” in which the government hides the Ark of the Covenant in a non-descript wooden crate in an unnamed warehouse somewhere.

            It is, of course, a great yarn. And it lives on because so many people at cocktail parties believe it and nod their heads knowingly. GM, after all, must be in bed with the oil companies, as well as with J. Edgar Hoover and Darth Vader.

            In the stories, it’s funny how the blame almost always falls at the doorstep of GM. It rarely, if ever, gets attributed to Honda, Toyota, or Nissan — all of which built and abandoned electric cars in the late 1990s. It’s also interesting to note that Google has emerged as a savior in this area, probably because it serves as an embodiment of the future, while GM is seen as a relic of an oil-thirsty past.

            I know that many of our readers will be consumed by anger when they read this, but there is no such battery in a basement. Not even at GM. The truth is, a lot of very bright electrochemists have been working on the EV battery for a long time, and they still haven’t come close to the 400-mile, 15-minute recharge battery.

            Recently, we published a story on the status of the EV battery effort. If you’re a conspiracy theorist, you probably didn’t like it. We interviewed experts in electrochemistry at Argonne National Laboratory, Cal-Berkeley and elsewhere. Their collective conclusion: Building a plug-in hybrid battery (not even a pure EV battery) is difficult enough.

            Elton Cairns, a professor emeritus of chemical engineering at Berkeley and a former battery researcher in NASA’s Gemini program, put it best. “If you ask, ‘Technically, can we do it by 2010?’ The answer is yes,’” he said. “But is the battery affordable by consumers? The answer is no.”

            And that’s for a 40-mile (ITALICS) plug-in hybrid battery.        

Virtually everyone in our group of experts agreed that with enough hard work, an affordable 40-mile lithium-ion battery pack is within sight. None know of a 400- or 500-mile battery with a 15-minute recharge time. Most said the path to such technology is long, torturous and unpredictable.

            But the truth is complicated. Boring, too.

Unfortunately, it’s a lot easier to cite mythical conspiracies than it is to build that magic battery.   

Styrofoam, a Lifesaver and a Scourge

Styrofoam, a Lifesaver and a Scourge

Styrofoam has been with us for 67 years and it’ll probably be here well after humans have been driven from the planet.

Styrofoam, developed by Dow Chemical in 1941, is a scourge. I realized that not after my employer rightfully banned Styrofoam coffee cups a couple of years ago, but after I picked trash this weekend at 60 acres of conservation property near my home. The tick-infested area is bordered  by a big bend in the Merrimack River and it catches all manner of trash coming down stream, almost all of it plastic - bottles, bottle caps, recycling bins from towns upstream, tampons, needles, condoms, pens, wrappers, combs, balls, toys, and many types of plastic twist-off seals. There was very little metal, save a transmission or two. 

But the bits of Styrofoam mostly from fast-food packaging is what made the clean-up seem hopeless. You could work on a three-square feet for 30 minutes and not get it all. This stuff is like an low-grade infection in the environment. Now I have even a greater appreciation for sustainable and recycleable plastic.

Styrofoam is used in a variety of applications: coffee cups, packaging, boat floatation, insulation and boogie boards. Indeed as boat floatation, it has saved lives. It definitely has great characteristics such as buoyancy, but my vote goes to Starbucks which uses cardboard coffee cups instead of Styrofoam like Dunkin’ Donuts. There’s quite a bit of talk about recycling Styrofoam, but it’s difficult. Californians Against Waste has a very detailed web site on the harm polystyrene (Styrofoam is a derivative) poses to the environment and human health. The Styrene compound, they claim, is found in fat, blood  and even breastmilk! Polystyrene, the polymerization of styrene, was discovered by a German pharmacist in 1839.

If Styrofoam saves sailors and keeps people warm, that’s great. We should use it. But fast food packaging is needless and harmful. Somes uses of Styrofoam should be restricted if not banned outright.

The System Aspects of Sensing

The short list of system aspects includes interfacing, communication (including wireless), packaging and ease of use. As the author of IC Insights' annual “Optoelectronics, Sensor/Actuator and Discrete (O-S-D) Report,” where he annually addresses market forecasts as well as technology trends, Rob Lineback has a unique view on what is happening in the sensor areas, especially where microelectromechanical systems (MEMS) and thin-film semiconductor technology is involved. He discussed the increasing focus on the system aspects of sensors with Design News.

Is there another system aspect that should be included in the list?

I would include the cost issue. Designers are being asked to be aware of what the cost might be, especially if something is going into manufacturing in any kind of volume. Also, system level test of some sort (is important), where the product can be easily debugged during design and even tested in manufacturing. The ability to reprogram or to reconfigure a design platform for various iterations or versions of a product for various price points in the market place, as well as geographical markets, is another one.

Have you observed any major transitions in MEMS sensing?

For a long time, the emphasis was mainly talking about all the fantastic things that could be done with MEMS-based sensors and thin-film-based sensors and a lot of the attention was placed on the amazing manufacturing capabilities to make these devices. Now, the focus is more and more from a component level — essentially, a part that is used to produce a piece of equipment. I think that is a real big change in the way a lot of these technologies are being handled right now.

Is one sensor type more likely to have more system-level integration orapplication-specific packaging?

I think the accelerometers are definitely at the forefront when it comes to having multi-sensing capability packed into the package. Also, multi-die or chip solutions, much more application-specific functions, are built into these accelerometers for handheld devices for human interface control, where you shake it or you tap it, like an MP3 player or a cell phone and then also freefall detection, in case you drop it, it will actually shut down the hard drive or do different things to protect itself. I think we are seeing the accelerometer really being at the forefront when it comes to being tailored and having application-specific functions, low-cost packaging and other things in order to hit the price target in a small size that is required for the applications that are going after these types of devices.

Is there such a thing as a plug-and-play sensor today?

The integration level in the sensor products has reached a point where the need for actually interfacing directly to the sensor has become less and less of a factor. What you are really interfacing to is the electronics side, since the microcontroller and the A-to-D and even the wireless transmitters are actually inside these packages now. It is really becoming a subsystem, more than it is a sensor. The sensor is embedded inside the package. That, in some ways, has made it easier to do what could be thought of as plug-and-play — but the plug-and-play is really just plugging and playing with another integrated circuit that is connected to the serial bus and no longer worrying about the specific sensor part that is inside the package.

Author Information
Rob Lineback is a senior market research analyst at IC Insights Inc. He has 28 years of experience as an industry analyst and editor covering semiconductor business trends, technology and global suppliers. He can be reached at rob@icinsights.com.

Small Sensors Make a Big Difference in Consumer Products

Forecast to have an increasing impact on solid-state sensor applications (according to IC Insights' O-S-D Report), consumer electronics add a new twist to sensor manufacturers' and system designers' plans. While consumer electronics products have many different requirements and constraints from other markets, the added use of sensors will benefit not only the products that incorporate them but other applications, as well. Market segments that can benefit include those that can take advantage of the unique aspects of the consumer electronics market such as reduced time-to-market and reduced time-to-volume, as well as the physical attributes of consumer electronics' sensing products, including the requirements for small size, low power consumption and more. Motion is a growing sensor opportunity but many other sensor technologies are finding their way into consumer electronics products.

Motion Makes Sense

In the exploding consumer market for solid-state sensors, motion sensing has changed the way users interact with products and also enabled entirely new products. Adding a single-, dual- or three-axis accelerometer provides the ability to sense motion. From simply detecting a free-fall to lock a hard disk drive and prevent damage, to sensing complex motion in games from golf to bowling, small microelectromechanical systems (MEMS)-based accelerometers have had a big impact on consumer products.

With the first sound effects system where guitarists could control Wah and Phaser/Flanger effects using hand and body motions, Source Audio's Hot Hand redefined the way guitarists personalize their performance.

More recently, the company's Soundblox guitar effects pedals liberated guitarists from a fixed position and provided them more dynamic sound capabilities. Analog Devices' ADXL3xx family of multi-axis accelerometers provides the motion sensing for Hot Hand but these newer consumer applications have not happened overnight. ADI's initial accelerometers were developed for sensing in automotive air bags.

“Thanks to nearly two decades of manufacturing experience that has helped ADI dramatically reduce the size, cost and power consumption of MEMS devices, motion sensing technology today is giving designers options they never had before,” says Harvey Weinberg, applications engineer, Micromachined Products Div., Analog Devices Inc.

ADI's ADXL3xx family of multi axis, low-g iMEMS® motion sensors were developed for consumer product applications. The ADXL330, a monolithic three-axis accelerometer with signal conditioned voltage outputs, senses acceleration with a minimum full-scale range of ±3g. The three-axis accelerometer can measure the static acceleration of gravity in tilt-sensing applications, as well as dynamic acceleration resulting from motion, shock or vibration.

“Design cycles for consumer products can be months or years depending on the product and competitive environment. In contrast to the auto industry, however, they go to market right away,” says Weinberg. “Product life cycles are measured in months and not years.” With the new consumer accelerometers, other markets can take advantage of the new performance capability sooner than if automotive was driving the development.

More than guitar players are excited about the new functionality from motion sensing. Apple popularized the display screen that knows its place (orientation) in the iPhone. The highly integrated phone uses STMicroelectronics' LIS302ALB three-axis linear accelerometer. Operating from a 3.3V supply, the ±2g accelerometer provides an analog output. Similar to other MEMS accelerometers, the LIS302ALB has potential applications in appliances and robotics.

A Touch of Class

In addition to providing a flexible, reliable and cost-effective user interface, touch-sensitive controls are sealed from spills and dust and have no moving parts to wear out. With these controls, engineers can design touch-based rotary wheel and linear slider interfaces with the ability to hide or illuminate buttons and “morph” touchpad patterns in cell phones, portable media players, remote controls and more. However, home appliance and industrial control equipment can also take advantage of the technology.

Products like Freescale Semiconductor’s MPR083, a capacitive eight-position rotary touch sensor controller and MPR084, a capacitive eight-pad touch sensor controller simplify the design process.

“These parts were developed specifically for consumer applications but based on one developed initially for automotive,” says Bryce Osoinach, systems and applications engineer, Freescale Semiconductor. Freescale’s initial products were automotive-qualified electric-field products. Recently, the company announced its first microcontroller (MCU)-based state machine products for the consumer market.

“Automotive applications such as occupant detection that utilize Freescale’s automotive qualified electric-field products can take several years to go into production. However, consumer sensor applications can be as short as three to six months,” says Darrell Simms, senior product marketing manager, Freescale Semiconductor.

An Open-Shut Case for Sensors

A folding type cell phone case enables a larger display but something has to alert the phone when the case is opened or closed. Instead of a mechanical switch that would be subject to wear out, many cell phone makers are opting for a Hall Effect sensor to indicate an open or closed position. For this application, ROHM's BU52000 series provides a compact and thin-profile package with a mounting area of only 1.1 x 1.1 x 0.5 mm. Besides improved reliability, the non-contact sensing technique also provides increased design flexibility. The tiny Hall Effect device can open possibilities in other markets.

How Hot is It?

Automotive still provides a driving force for new sensing technology that subsequently gets modified for consumer and other applications. For example, expanding on its automotive-proven family of intelligent infrared, non-contact thermometers, Melexis recently introduced the MLX90614xAC with a narrow Field of View (FOV). A silicon chip with a thin, micromachined membrane sensitive to the infrared radiation of a distant object provides the sensing element. In the same package, a custom signal conditioning chip amplifies and digitizes the signal and calculates the object's temperature. With a Field of View of only 35 degrees without the use of costly infrared optics, the infrared thermometer may enable new applications in microwave ovens, true non-contact fever temperature measurement and industrial temperature control.

Pulse-Width-Modulation Digital-to-Analog Converter vs. Stand-Alone DAC

In today’s highly competitive electronics environment, designers are constantly looking for ways to reduce overall system costs. One of the most commonly asked questions analog specialists at digital microcontroller (MCU) companies hear from customers is, is the almost cost-free Pulse-Width-Modulation (PWM) Digital-to-Analog Converter (DAC) good enough for my application or do I need a higher-performance, stand-alone DAC, instead?”

The generation of an analog voltage using a digital Pulse-Width-Modulated signal is known as a PWM DAC. As most designers’ PCB boards have a microcontroller with a built-in PWM-output feature onboard, a simple digital-to-analog data conversion can be easily realized by adding a few passive components at the MCU’s PWM-output pin, as an alternative to using a stand-alone DAC.

However, in the MCU application environment, system designers can have DAC functionality nearly free of charge. PWM DACs are widely used in very low-cost applications, where accuracy is not a primary concern. Stand-alone DACs, however, are used for applications requiring higher accuracy.

Although the PWM DAC can be realized with the simple addition of a few passive components, implementing a PWM DAC for system applications is not a simple task. There are many limitations associated with this. Understanding the complexities of using the PWM DAC and its effects can save significant development time and effort.

This article presents a technique for converting a PWM pulse to an analog voltage using a simple RC low-pass filter. It also reviews the PWM DAC’s limitations and its key design constraints with regard to resolution, frequency, ripple, settling time and current consumption, which are very important design parameters that are largely affected by the resistor (R) and capacitor (C) values, as well as the PWM duty cycle and frequency.

Stand-alone DACs

Figure 1 (in gallery below) shows an example of a stand-alone DAC. Its analog output voltage is given by:

Where Dn is the digital code. For example, with a 12-bit DAC, the user can get Vout = 2.5V with Vref= 5V and Dn = 1,000-0000-0000. Typical stand-alone DACs provide good linearity and a short settling time, which is the time required to update each output voltage.

How PWM DACs Work

Figure 2, below, shows a basic configuration of the PWM DAC. The MCU outputs a PWM signal to an RC low pass-filter. The PWM pulse train’s digital value becomes an analog voltage, when it passes through the RC filter. At a given period of time, the analog output is proportional to the PWM pulse’s high durations.

A PWM signal is defined as a digital signal with a fixed frequency, but a varying duty cycle. Figure 3 illustrates a PWM signal. The PWM period (T) is the time interval required to complete one full PWM cycle. The duty cycle is the ratio of the high duration (t) to the total period (T).

The PWM signal and RC-filter circuit parameters affect the analog output’s resolution, amplitude, settling time and ripple. The PWM DAC’s limitations are clearly demonstrated by analyzing the interaction of the PWM parameters and the RC filter. A better understanding of the relationship between these parameters enables designers to optimize the PWM to best suit their application’s requirements, while minimizing design time.

PWM DAC Bit Resolution

The PWM counter length (L) and the smallest duty-cycle change in the PWM counter (C) determine the PWM DAC’s bit resolution. The following equation expresses the maximum bit resolution of the PWM DAC:

Where:
R = Resolution in Bits
L = Length of Counter in Bits
C = Smallest Duty-Cycle Change

For example, if the system generates an analog output voltage from a PWM DAC with a counter of 4,096 (L) and a minimum count step of one (C), the PWM DAC’s bit resolution is 12 bits.

When the PWM resolution is determined, it is possible to calculate the Least Significant Bit (LSB) size. The LSB size is dependant upon the PWM resolution and the PWM’s output-high level voltage (VOH) and can be calculated using the following equation:

For example, a 12-bit PWM DAC with a VOH of 5V has an LSB size of 1.2 mV.

RC-Filter Design Considerations for PWM DACs

One key design consideration when determining the PWM’s resolution is output-voltage ripple. Ripple occurs due to overshoot and undershoot as the PWM charges and discharges the RC circuit. One way to approximate the charge characteristics is to modify the equations to charge and discharge an RC-filter circuit. As the effects of this are cumulative, the following equations can be used as approximations:

VLH is the voltage increase for a specific PWM period and VHL is the voltage decrease for a specific PWM period. The values of VLH and VHL are dependant upon not only the RC-filter values, but also upon the PWM frequency and duty cycle. The PWM frequency and duty cycle determine the time available for the PWM to charge (tcharge) and discharge (tdischarge) the capacitor. Vripple is the difference between VLH and VHL for the same PWM period.

Figure 4 illustrates the magnitude of the voltage ripple across the output capacitor versus time. The vertical axis displays the magnitude of the ripple voltage, while the horizontal axis provides the corresponding time. The plot shows how the ripple voltage settles at approximately 125 mV in a time interval of approximately 40 ms for R = 10KO and 250 mV at 20 ms for R = 1 KO. In the previous example, an LSB size of less than 1.2 mV for a 12-bit system was needed. This ripple is greater than 100 LSB for a 12-bit DAC with a 5V reference, meaning the resulting PWM DAC solution has an effective resolution of less than 6 bits due to the ripples.

The ripple can be reduced by increasing the capacitor and resistor values or by increasing the PWM frequency.

As shown in Figure 4, the ripple decreases as the resistance value increases. However, nothing comes without a price — settling time doubles as the ripple decreases by 50 percent.

For applications that require faster settling time (increased bandwidth) and higher resolution, a second RC filter can be added. Obvious trade-offs include the cost of additional components and the increased board space occupied. Figure 5 shows a model for a two-pole RC low-pass filter. Figure 6 shows the analog-output voltage of this model.

There are a couple of things to keep in mind when designing the filter. First, make sure the RC pole is set at a much greater frequency than that of the signal being generated. Secondly, if you are designing a two-pole filter, make sure that R2 > R1.

The 3dB corner frequency of the RC filter is given by:

There are a couple of additional things to consider. Increasing the PWM frequency will also decrease the ripple, but the trade-off is increased settling time. Figure 7 shows this case for 10 MHz and 5 kHz.

The worst-case ripple occurs at a 50 percent duty cycle. The ripple will decrease as the duty cycle moves closer to 0 or 100 percent. Figure 8 shows the peak-to-peak magnitude of ripples on the PWM DAC output. The ripple decreases almost two times as the duty cycle changes from 50 to 85 percent.

VOH = 5V, C = 10 µF, R = 1 kO, PWM Frequency = 1 kHz, Duty Cycle = 50 percent (Solid Curve) and 85 percent (Dotted Curve).

PWM DACs and Power Consumption

Many electronic products today are portable or handheld devices. These devices are battery-powered and many have strict constraints with regard to power consumption. Therefore, it is a good idea to minimize the PWM DAC’s power consumption. The current consumed in the PWM solution is simple to approximate, using the following equation:

Figure 9 shows the current and voltage plots. As shown in the plot, the PWM DAC with a lower resistor draws a significant amount of current (in the range of a few mA). This high level of current consumption is unacceptable for many battery-powered applications. Current can be decreased by increasing the resistor value.

In Figure 10, the resistor value has been increased by a factor of 10, which has likewise decreased the current consumption by a factor of 10.

As the resistor limits the current available to charge/discharge the capacitor, decreasing the amount of current available (increasing resistance) to the circuit will increase the settling time.

Another factor to consider is the filter’s pole. As the resistor value increases, the 3 dB frequency decreases by the same magnitude. This can be compensated for by reducing the capacitor value by the same magnitude, which offsets the increased settling time and maintains the original pole of the filter. Figure 11 demonstrates this. Note that, as the capacitor value reduces, the circuit becomes more susceptible to loading. This is another important design consideration.

Conclusions

Although the PWM DAC is simple and low-cost, using it to generate a stable analog voltage output is not a simple, straightforward task. Various design constraints must be considered and understood before realizing the circuit. In some cases, the design steps become very time-consuming and tedious work for busy system designers.

The limitations associated with PWM DACs are low bit resolution, slow settling time, poor power efficiency and time-consuming design steps, when compared to stand-alone DACs.

PWM DACs can be used for low-cost, low resolution (less than 10-bits) and very low frequency applications (less than 1 kHz) where power consumption is not a critical parameter. On the other hand, a stand-alone DAC should be considered when high accuracy, short settling time, low power consumption and short design cycles are needed.

Author Information
Youbok Lee, technical staff engineer with Microchip Technology’s Analog & Interface Products Div., has more than 20 years of industry experience in RF, embeded circuits and remote-sensing applications.
John Austin is the senior products marketing manager with MicrochipTechnology’s Analog & Interface Products Div.

Sensors Address System-Level Design Issues

The ability to sense common or unusual parameters is a great start for any control system. However, those sensors that take into account system-related aspects distinguish themselves from the competition. While usually more expensive than units that just provide the sensing function, the cost is more than made up at the system level, in reduced time-to-market and other advantages. Sensors with design considerations for and specific focus on system-level details for interfacing, communication, packaging and ease-of-use make life easier for system designers and end users.

Interfacing Sensors

While many sensors include some or all of the identified key system factors, one aspect frequently stands out as a primary design consideration. In most cases, system-level integration, like many of the other design considerations, is not revolutionary but more of an incremental improvement to the company's previous generation of sensors.

System-level integration typifies a growing interface trend from several sensor manufacturers. In addition to its low-profile and surface-mount design that is compatible with standard printed circuit board (PCB) assembly methods, Kavlico's P6050 allows system designers the ability to select a standard analog voltage output or a digital serial peripheral interface (SPI) output.

Using a 5V dc supply, the amplified analog output is 0.5 to 4.5V dc, linearly proportional to pressure. With its integrated 10-bit analog to digital converter and digital output, the digital output version allows system designers to eliminate additional components. This option saves space, component cost and reduces overall design complexity.

Communicating Data

Communicating sensed information to the system through either a wired or wireless network is another system aspect sensor manufacturers have addressed with another level of integration. In some instances, the communication function is separate from the sensor but targets specific sensor types. For example, MicroStrain's G-Link® Micro Datalogging Transceiver system, combined with a MEMS accelerometer, provides a high-speed, wireless, triaxial accelerometer node, designed to operate as part of an integrated wireless sensor network system.

Triggering the sampling and logging of data from 70m is simplified with the bi-directional RF communications link. Additionally, the link can request the transmission of real-time data to a host PC for acquisition and analysis. The system can handle simultaneous real-time streaming from up to 16 nodes in the 2.4 GHz range.

Application-Specific Packaging

Packaging that addresses unique requirements of specific applications has been one of the first system-level capabilities implemented by sensor suppliers. In many cases, packaging determines whether the sensor will work in an application.

For example, Banner Engineering's WORLD-BEAM Q20 photoelectric sensors, in a IP67-sealed housing with industry-standard 25.4 mm mounting, delivers a variety of sensing modes. With a water-tight, small rectangular housing, the Q20 is rated up to 1,200 psi for wash-down environments and provides reliable sensing in a space-saving package.

Other application-specific packaging addresses system requirements with industry standard form factors and standard mechanical interfaces. Designed specifically for use in heavy-duty vehicle engine exhaust gas recirculation (EGR) systems, Honeywell's one-piece R300 Series temperature sensor has a M14 x 1.5 mounting thread and an integral connector. With a continuous working temperature range of -40 to 275C (-40 to 527F) and the ability to handle excursions up to 300C (572F), the unit also qualifies for sensing temperatures in other heavy-duty vehicle engine applications including oil, coolant, fuel and air inlet.

Similarly, Kavlico's P528 family of ceramic capacitive refrigeration pressure sensors also have a threaded body and integral connector but these units sense pressure in refrigeration compressors, rooftop chillers and refrigerant recovery systems. Handling pressures in the 0-15 to 0-500 psia ranges, the sensor has brass housing with a ¾-inch external hex and pressure ports that include 1/8-27 NPT, ¼-18 NPT and ¼ SAE female threads with Shrader deflators. The integral electrical connector is a Packard Metri-Pak 150.

Ease of Use

Perhaps the ultimate system level design feature is any aspect that makes the sensor easier to use. In spite of increased functionality that could have resulted in a vision sensor that was more complex and difficult to use, simplicity was a primary design aspect of Pepperl+Fuchs' VOS300 Series vision sensors.

Although the VOS300 combines a camera, illumination, digital outputs, process data and five evaluation methods into a single housing, the sensor is easily configured without programming knowledge and operated without the need for detailed image processing experience. Targeting inspection applications beyond the capability of traditional photoelectric sensors, the vision sensor provides a single discrete output indicating general “pass/fail” status. Using an Ethernet connection, additional data can be read from the sensor to determine the pass or fail condition of each feature check.

Addressing specific system requirements gets easier with improvements on existing products. This is the case with Cognex Checker 232. Compared to the previous Checker 202, the 232 has the same features, plus functions to inspect smaller features and a wider field of view. With a much longer working distance that allows mounting further from the inspection area, the sensor simplifies detection and inspection measurements without requiring a complete vision system.

With its PK pressure sensors, Turck demonstrates easily programmed sensors for pneumatic and robotic applications. Detecting the switch point in air and inert gas, in pressure ranges from -29.5 to 0 inches of Mercury (-1 to 0 bar) and 0 to 145 psi, the sensor uses simple push-button programming. The menu can be inverted for additional programming flexibility.

As sensor designers add more attributes to sensors that address system-related issues, system designers can focus on even more advanced systems.

Auto Industry Working Hard to Make an Electric Vehicle Battery

To an engineer, it looks obvious.

Gasoline packs 80 times more energy per kilogram than a lithium-ion electric vehicle battery. It holds 250 times more energy than a common lead-acid battery. So, it’s a no-brainer. Batteries can’t possibly deliver the energy needed to power the future of the auto industry, right?

Wrong. With vehicle exhaust being blamed for global warming and with concerns over foreign oil availability growing, the auto industry has re-ratcheted up its efforts to develop an electric car and the battery still sits smack-dab in the middle of Alternative Energy Highway.

“The battery is central,” says Mark Verbrugge, director of the Materials and Processes Lab. at GM Research Labs. “We know and understand all of the technologies that are needed, other than the battery.”

Indeed, battery technology is still, to some degree, a mystery. But automakers don’t want to wait. General Motors has promised a 2010 delivery date for the Chevy Volt, a “plug-in hybrid” vehicle that uses lithium-ion batteries. Meanwhile, Toyota and Ford are working on plug-ins while Chrysler has placed a handful of Dodge Sprinter plug-ins in a test fleet. All will draw power from batteries.

Questions remain, however: Can today’s battery technology sustain an EV market? Do the batteries pack enough energy? Is their cost low enough? Is the durability there? Are they safe?

The answers are complex and varied. Most automotive engineers and electrochemists agree on one point, however: A big, full-featured, battery-powered car isn’t feasible yet. Energy densities are still too low; range is too short; recharge time, too long. Because no one as yet can build an electric vehicle with a 300-mile range and 15-min recharge time, batteries aren’t about to replace the internal combustion engine.

“We’d like to have a direct replacement for what we have today,” says David Swan, president and engineer for DHS Engineering Inc., a consultant to the EV industry. “But creating an electric vehicle that matches our current vehicles — performance for performance, price for price — is extraordinarily difficult.”

Still, there’s a market there, albeit a niche market. Such companies as Global Electric Motorcars (GEM), Zap! Electric Cars and Zenn Motor Co. are producing tiny, battery-powered neighborhood vehicles. Daimler is testing a diminutive EV in London.

Moreover, plug-in hybrids are on the rise. Plug-ins, which use internal combustion engines to extend range, make it easier to build an EV battery because they eliminate concerns over specific energy.

Even so, makers of plug-in batteries say the task is not a slam dunk. “This is a big, big challenge,” says Mohammed Alamgir, director of research for Compact Power, Inc., a battery maker for the GM Volt project. “People in this industry are accustomed to teeny-weeny cell phone batteries. Now we’re looking at a battery that has to be forklifted. It’s a huge jump in scale.”

The Energy Density Battle

The drive to make an electric vehicle battery is hardly new. Legend has it that Thomas Edison and Henry Ford collaborated on the challenge a century ago. Given five years, they said, they could lick the battery problem. But while they developed a product, their battery’s energy density was just a fraction of that of a gallon of gas, and the EV gradually disappeared.

During the 1980s, the auto industry again made a collective effort to beat the battery problem. Again, it failed, as EVs from Chrysler, Ford, GM, Honda, Nissan and Toyota were shelved in the late 1990s.

The issues facing EV batteries of a decade ago were the same as those of today: Energy density, recharge time, cost, durability and safety were the big challenges.

Energy density was prime among those, mainly because it directly translates to vehicle range: the higher the energy density, the greater the range between recharges. In a full-size sedan, for example, a specific energy of 100 W-hr/kg translates to approximately 100 miles of range. To boost range, automakers need to pack more batteries on board, which can dramatically increase mass.

Mass-related issues were the reason that battery power long failed to capture the fancy of automotive engineers. Many looked at the numbers and scratched their heads. Today’s best batteries, for example, offer a specific energy of approximately 150 W-hr/kg. In contrast, the accepted specific energy of gasoline is about 12,722 W-hr/kg. Engineers often argue about how much of gasoline’s energy is usable, but even if only 4,000 W-hr/kg is usable, gasoline still packs 25 times more energy than a lithium-ion battery. That, in turn, means that the mass of a good EV battery is 25 times that of gasoline.

Worse, batteries recharge slowly. Using a 110V outlet, an EV battery typically hits full recharge in more than six hours.

“You have this great inequity of the density of the energy (source),” says Larry Oswald, chief executive officer of Global Electric Motorcars, a Chrysler company. “A battery is like a heavy fuel tank with a very small neck in it.” During the 1990s, battery makers skirted the energy density deficiencies by stretching the truth. They talked about ranges of 400 miles and recharge times of 15 min. Neither, however, came to pass.

“We hurt ourselves badly by exaggerating where we were, where we were going, and how long it would take to get there,” says Swan, who owns three electric vehicles. “The battery makers would internally calculate the range based on a car that used very little energy. They made all kinds of great assumptions and, lo and behold, on paper they were getting 400-mile ranges and 15-minute recharge times.”

Dealing With Cost Issues

That’s why the plug-in hybrid has emerged as such an important alternative. With the plug-in, range becomes a non-issue. The United States Advanced Battery Consortium (USABC), an organization formed by American automakers, has set goals for plug-ins with 10- and 40-mile ranges. With the shorter range requirements, it’s not necessary for battery makers to achieve specific energy levels approaching 300-400 W-hr/kg. Rather, the USABC has set a goal for the 10-mile plug-in to reach 56 W-hr/kg and for the 40-mile vehicle to achieve 96 W-hr/kg.

By backing up the battery with an internal combustion engine and a generator — as the plug-in hybrid does — auto executives say they could dramatically improve the driving range of EVs. GM execs, for example, say the Chevy Volt could have a range of 400 miles. “If you lived 30 miles from work and charged your vehicle every night when you came home or during the day at work, you could get 150 miles per gallon,” GM Vice Chairman Robert Lutz told Auto Show attendees in 2007.

Still, there’s an unresolved cost issue. To keep costs reasonable, the USABC has set goals of $293/kW-hr for a 40-mile plug-in and $500/kW-hr for a 10-mile vehicle. Here, too, shorter range has its advantages. Because short-range battery packs can be smaller, battery makers no longer need to shoot for the exceptionally difficult figure of $100/kW-hr, which was the long term goal of a decade ago.

Nevertheless, all acknowledge it won’t be easy. Experts asked by Design News to estimate the going rate for today’s lithium-ion battery said it ranges between $500 and $1,000/kW-hr. Lithium-ion cells alone, they say, typically cost $300/kW-hr. But EV batteries costs must necessarily include packaging, protective circuitry, cooling systems and dealer mark-ups, along with the cell itself.

“It’s not a simple matter,” says Elton Cairns, a professor emeritus of chemical engineering at the University of California-Berkeley, as well as a former developer of fuel cells for General Motors cars and a designer of batteries for the Gemini spacecraft. “When you put electronic circuitry and packaging in, you’re probably right around a $1,000/kW-hr.”

Most experts agree, however, the brunt of the remaining work is engineering, not invention. “The issue now is one of scaling,” says David Cole, director of the Center for Automotive Research. “From our perspective, it appears some of the critical inventions have been made. What remains is some good engineering development.”

Safety Solutions

Completing that engineering before the publicly announced start dates, however, is another matter. General Motors, in particular, has stuck to its original proclamations of a 2010 introduction date for the Chevy Volt. Given vehicle development times, however, battery makers must have their products ready now, or very soon, to meet that schedule.

The good news is that makers of lithium-ion batteries say they’ve licked the safety issue that has grabbed headlines in the past. Thermal runaway, which has reportedly plagued lithium-ion in laptops and cell phones, has been eliminated through a change in chemistries. Instead of using cobalt oxide in the positive electrode, EV battery makers are employing alternatives. A123 Systems, for example, employs a nano-phosphate material in its cathode while LG Chem and Compact Power Inc. use a manganese-spinel chemistry. Such chemistries are said to prevent overheating of the battery during recharge, which can reduce life and possibly even cause fires.

Battery makers are also dealing with heat issues by adding cooling systems to next-generation battery packs. Such battery packs typically use liquid coolant that flows in channels between the cells, thus drawing off heat. They’re also employing battery management electronics that help keep voltages in line as the batteries cycle.

“Safety is a huge concern,” says Donald Hillebrand, director of the Center for Transportation Research at Argonne National Lab. “But there are chemistries out there that will solve the problems.”

Still, the 2010 schedule presents a monumental challenge to solving those problems. Automotive engineers worry that there won’t be sufficient time to study and test battery packs in everyday conditions.

“The big risks we have to overcome if we expect to see widespread implementation are quality, reliability, and durability,” says Verbrugge of General Motors. “We’d like to get at least three to four years (of testing) on these batteries.”

Battery makers, some of whom have already delivered battery packs to tier-one suppliers, say they performed accelerated life tests on the batteries with exposure to various ranges of temperature. Executives at A123, however, say their designs are not “locked down,” meaning that changes could still be made.

For automakers, the durability issue is inextricably linked to cost. “The auto industry is very concerned about the cost numbers because, ultimately, they not only have to buy the battery, they have to warranty it,” says Hillebrand of Argonne. “If the warranty is 120,000 miles or 10 years, they don’t want to have to start swapping out batteries at that point. That’s one of the reasons they’re so nervous about the cost numbers.”

Hard Work Ahead

With such struggles still looming on the horizon, few experts are looking past the plug-in hybrid. Most say automakers have their hands full now. ?They’re not going to start talking about big, full-featured battery-powered cars just yet.

“When you listen to the big automakers talk about their plans for plug-ins, EVs and hybrids, they all say the same thing,” Hillebrand says. “They say they are committed to production, they really intend to do it, but then they pause and add, ‘... if the battery technology is available.’ Anybody who is seriously involved in this is still staring at that battery issue.”

Moreover, experts say battery makers and the auto industry need to work together to keep battery production in the U.S. “Right now, we are concerned about using imported petroleum,” Hillebrand says. “We haven’t accomplished anything if we trade our dependence on imported oil for a dependence on foreign-made batteries.”

Experts also agree on another point: Commonly repeated stories of a magic battery, suppressed by big oil companies and hidden in a basement in Detroit, are folklore. Battery improvements will be eked out in tiny increments over time, largely through the sweat and hard work of electrochemists and automotive engineers. There is no other way.

“It would be wonderful if that magic battery in the basement existed,” Swan says. “But it doesn’t. We just have to keep methodically making improvements.”