USB Interface Simplifies Sensor ConnectionsUSB Interface Simplifies Sensor Connections

Systems designers and integrators are familiar with thetypical sensor platform architecture that includes an amplifier/signalconditioner with an active analog filter (requiring an external, stable andregulated power supply), a multi-meter (display) and data storage. In automatedsystems use, this sensor architecture also requires instrumentation to convertoutput into digital numbers as well as specific programming to display,manipulate and use the data collected by the sensor. It is this device connection that oftenintroduces the most complexity into the system design for engineers dependingon the application.
USB Interface Option
Using a USB connection-the same one we're all familiar withon our PC peripherals-all configuration is done in the background of the device.Industrial level USB connections bring this familiarity to complicated sensorsystems so they can all be connected in one way, eliminating potential formisconnection along the sensor chain in an industrial system.

Any computer or other USB-enabled readout device loaded withinterface software will allow full sensor system interface. An additional benefitof using a USB sensor chain is that it utilizes the computer's power supply.Depending on the sensor's power requirements, most systems will not need anadditional power supply.

The USB interface also removes the need for an amplifier(signal conditioner), analog filter, power supply, and multi-meter.

Other advantages of USB, besides the cost, include the highresolution digital output, an integrated SINC digital filter to achieve smoothlow pass filter, the ability to increase the sampling rate for high speed applications,and capability to store calibration values inside the onboard, non-volatilememory (which is required to apply the real-time calculation to account fornon-linearity). As a result, the TEDSchip typically used to store these and other configurations is no longer needed.

High quality USB modules also have long-term stability and alow temperature coefficient compared to analog amplifiers due to the eliminationof the analogue circuits. These modules are also not as sensitive as analogamplifiers to noise because they use digital processing. They also have a higherCommon Mode Rejection Ratio (CMRR) and Power Supply Rejection Ratio (PSRR)compared to analog amplifiers. CMRR and PSRR directly cause the signal-to-noiseratio (SNR), which defines the level of output noise.

In the best conditions, most analog amplifiers have morethan 1 millivolt internal noise, while the internal noise of high quality USBmodules is approximately 1 microvolt.

A high speed, bi-directional USB link gives system designersthe ability to communicate with sensors via PC-based software andmonitor/control the functionality of features such as tare, peak, valley andselectable averaging, without having to manually change hardware settings. Thistype of system verifies that the sensor is functioning properly, so it removesany risk of not installing the sensors correctly.

Another benefit of USB technology is that, in addition toits ability to handle any brand, type or model of sensor, it can also handlemultiple sensors. One of the biggest headaches for sensor design engineers isthat every different sensor typically requires a different controller. Usingthe USB approach, most systems can use one platform. This is true for all standardsensors, however, some unique sensors may require special USB connectors.

Application Considerations
A USB connectionallows any sensor with analog output to interface cost effectively with digitaldevices. For example, a 100 gram load cell can be interfaced with USB modulesto resolve at 1 milligram resolution-and potentially even 100 microgramresolution (0.1 milligram)-for less than $3,000. A similar conventional sensorsystem would require a large package of precision instrumentation costing up to$20,000.

Toillustrate the cost savings benefits of USB sensors, consider this industrialapplication example: In the process of automating hydraulic or pneumaticpresses for feedback control, traditionally one load cell is used for heavyduty presses with a few million pounds of load capacity. This typically requiresa large, expensive custom sensor. Using USB technology, multiple standard,off-the-shelf load cells with in-line external USB interfaces could beinterfaced using a hub. This approach not only simplifies installation, butminimizes total cost. It also allows the operator to check the uniform loadingof a large platform while monitoring the output of each sensor individually. Twoadded benefits are easier troubleshooting plus the ability for the system toremain operational if a sensor is removed for repair or replacement.

The USB solution works for most sensors, including oldersystems still in the field. These sensors can be updated with a USB interface andstill use the same sensor configuration. Using a hub, up to five levels ofsensors can cascade to a chip with a single USB port.

Beyond all of the cost saving benefits, the bottom line forany sensor application is validation, i.e., measuring an important area of aprocess or system. This can become a major issue when sensors are recalibrated.For an ISO 9000 regulated company, any shift in the sensor control system that was not identified wouldmean that all parts would be subject to recall because they were not madeexactly to spec.