In order to understand why resolution is critical, it is important to review how the speed controller works. The speed controller is typically a P-I controller, whose input is the difference between the speed set point and actual speed (encoder) values. The output of the speed controller generates the torque set point, which determines how much force the motor shaft will exert on its load. Therefore, the torque command to the motor is directly proportional to the difference between actual speed and speed set point. In order to smoothly control the motor's load, you never want the input to the speed controller to instantly have a large value.
Current servo drives have speed controllers that are updated in the 100-200µsec range. For the example shown below, we will assume a 125µsec speed controller on a motor running at 30rpm.
Motor RPM = 30
Deg/sec = 180
Deg/125µsec = 0.023
Resolver/Sin/cos
Feedback resolution: 10,000/1 million
Pulses/deg: 27.78/2.777.78
Pulses/125µsec: 0.625/62.5
The calculations show that, at this low speed, the resolver feedback is so low that consecutive scans of the speed controller can actually occur without the resolver registering a difference in angular position. Since actual speed is defined as ∆ Distance/∆ Time, this registers with the drive as zero speed over the previous 125µsec. This causes the speed controller to immediately generate a large output to try and reduce the perceived difference at the input. During the next scan of the controller, there is an incremental change, and the controller then reduces its output, because the perceived difference at the input is gone or greatly reduced. It should be obvious how this behavior could cause erratic movements at this low speed.
To avoid this undesired consequence, the engineer tuning the drive is forced to keep the gain of the speed controller very low. A low gain slows the response time of the controller so that, when two scans occur on the same encoder increment, the controller delays its response time long enough to see the new pulse during its next scan. This stops the erratic movements but in some cases causes a new problem.
Suppose the motor needs to stop the load quickly. An example might be a machine where the inch button causes the machine to move at a low speed, but the operator needs the machine to stop the instant the inch button is released. Dynamically stopping a heavy load requires a fast injection of negative torque, and this requires a speed controller with a fast reaction time. If the feedback device is limiting the controller's gain, this abrupt stop may not be possible. Changing the encoder to a sin/cos can allow for the speed controller gain to be increased by as much as 300 percent.
Resolvers are less expensive and more durable than optical encoders, so there will always be a need for them with servomotors. However, when specifying the servomotor for an application, make sure all operating scenarios are considered before deciding on the feedback device. A good rule of thumb to use is to select a feedback device that can deliver 5-10 pulses at the lowest rpm required for the application for the scan time of the speed controller.
Marcus, thank you for an informative article. I was just at a seminar given by a semiconductor vendor on a new microcontroller targeted at the motor control market. These incorporate motor control timers as well as fast A/D converters. All of these are built in to the SoC, so that the measurement and correction strategies you discuss can be implemented.
As long as a servo drive permits access to and real time changes in its gains, then it's not strictly necessary to have a modern servo drive in order to take advantage of the concept. The system controller could change the gains and feed them to the drive. It's not as elegant as a self-contained drive, but it does permit use of this neat concept.
Wish I could afford a driver like this one. I accidentally hit the end of travel on one of my servos on my CNC mill. The momentary stall ended up letting all the "magic smoke" out of one of my servo drivers. A driver that would properly compensate for drive error like the above one would be most handy.
Good points about adaptive gains, tuning and resonances. I've often seen that the effect of mechanical resonances and changes over time are not taken into consideration when tuning a servo. The problem is that nine times out of ten you can get away with it, so the tenth time seems like a "mystery" when it occurs.
Don't forget friction! Friction is a nasty thing that makes accurate position control difficult. Slip stick itself is one of the reasons for growling at low speeds. However, sometimes a dither (vibration) signal is added to the torque reference in order to keep the mechanical system in a small movement all the time. This avoids the slip stick phenomena but unfortunately has audible noise as byproduct.
By the way, there is a slight error in the explanation of the Bode plot (apparently this is a transfer function from motor air-gap torque to motor speed). The plot line below about 20 Hz describes the motor and load moving together as one piece. Around 29 Hz only the motor moves but not the load (this frequency is also known as anti-resonance frequency). Between 29 Hz and 53 Hz the load is more and more accelerating when the motor is decelerating and vice versa thus finally reaching the resonance at 53 Hz. Above 53 Hz the plot shows more and more only the inertia of the motor.
You can check this with the rubber band and ball. If you move your hand very slowly up and down, the ball will follow. When you increse the frequency you will find there is a frequency where your hand moves but the ball does not move. This is the anti-resonance frequency. Increasing the frequency still you will finally get into resonance frequency where the ball is moving very much even when your hand movement is small. Increasing the frequency yet higher (if you can) you will find that the ball is again more or less standing still although your hand moves a lot.
I have been thinking more about this rubber string analogy and there was an error in my earlier post too.
The system consists of two masses, the ball and your hand, and a spring between them, the rubber band. Your muscles provide the force to your hand that is moving it. In a servo drive the force is the air-pap torque, the hand corresponds to the inertia of the servomotor's rotor and the ball corresponds to the driven load. The rubber band corresponds to the shafts and the coupling between the motor and the load.
Now when you slowly use your muscles to move your hand up and down the ball will follow. When you increase the frequency of the movement you will find that it becomes more and more "stiff", that is, more difficult to move your hand although the ball is moving up and down. This is the anti-frequency resonance where it is difficult to get the hand tomove. By increasing the frequency further it becomes again easier to move your hand and the amplitudes of the hand and the ball oscillations increases a lot when the frequency is approaching the resonance frequency. Above the resonance frequency the ball movement decreases and your muscles are moving mainly your hand.
The anti-resonance frequency makes it difficult to control the motor movement near and above it and thus limits the dynamics that is possible to be achieved. Note that the anti-resonance frequency is defined only by the torsional stiffnesses of the coupling and the shaft and the inertia of the load. Thus the servo control system cannot improve the control response beyond that. As a rule of thumb the shortest possible response time of the speed control is the inverse of the anti-resonance frequency.
Reduction gear is often used with servo drives. The reduction gear decreases the ioad inertia seen on the motor shaft. Thus in practice the anti-resonance frequency is important only for servo drives that do not have reduction gear.
In a world that's going green, industrial operations have a problem: Their processes involve materials that are potentially toxic, flammable, corrosive, or reactive. If improperly managed, this can precipitate dangerous health and environmental consequences.
An analysis of what’s needed to implement Design for Disassembly and Design for Recycling results in eight strategies engineers can use to design an intentional end-of-life stage into their products.
Government regulations, coupled with growing consumer sensitivity about data and identity theft, require that data storage organizations demonstrate proper protection and due diligence in protecting sensitive information stored inside datacenter enclosures.
When a crane doesn't have a monitoring system, crane owners schedule service every six months and simply scrap the parts they replace, even if a part has had little use and doesn't need replacing. This can cost thousands.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 3
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
0 
