USB Interface Simplifies Sensor Connection

Systems designers and integrators are familiar with the typical sensor platform architecture that includes an amplifier/signal conditioner with an active analog filter (requiring an external, stable and regulated power supply), a multi-meter (display) and data storage. In automated systems use, this sensor architecture also requires instrumentation to convert output 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 often introduces the most complexity into the system design for engineers depending on the application.

USB Interface Option

Using a USB connection - the same one we're all familiar with on our PC peripherals - all configuration is done in the background of the device. Industrial level USB connections bring this familiarity to complicated sensor systems so they can all be connected in one way, eliminating potential for misconnection along the sensor chain in an industrial system.

Any computer or other USB-enabled readout device loaded with interface software will allow full sensor system interface. An additional benefit of 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 an additional 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 high-resolution digital output, an integrated SINC digital filter to achieve smooth low pass filter, the ability to increase the sampling rate for high-speed applications, and capability to store calibration values inside the onboard, non-volatile memory (which is required to apply the real-time calculation to account for non-linearity). As a result, the TEDS chip typically used to store these and other configurations is no longer needed.

High-quality USB modules also have long-term stability and a low-temperature coefficient compared to analog amplifiers due to the elimination of the analogue circuits. These modules are also not as sensitive as analog amplifiers to noise because they use digital processing. They also have a higher common mode rejection ratio (CMRR) and power supply rejection ratio (PSRR) compared to analog amplifiers. CMRR and PSRR directly cause the signal-to-noise ratio (SNR), which defines the level of output noise.

In the best conditions, most analog amplifiers have more than 1 millivolt internal noise, while the internal noise of high-quality USB modules is approximately 1 microvolt.

A high-speed, bi-directional USB link gives system designers the ability to communicate with sensors via PC-based software and monitor/control the functionality of features such as peak, valley and selectable averaging, without having to manually change hardware settings. This type of system verifies that the sensor is functioning properly, so it removes any risk of not installing the sensors correctly.

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

Application Considerations

A USB connection allows any sensor with analog output to interface cost effectively with digital devices. For example, a 100 gm load cell can be interfaced with USB modules to resolve at 1 mg resolution - and potentially even 100 microgram resolution (0.1 mg) - for less than $3,000. A similar conventional sensor system would require a large package of precision instrumentation costing up to $20,000.

To illustrate the cost savings benefits of USB sensors, consider this industrial application example: In the process of automating hydraulic or pneumatic presses for feedback control, traditionally one load cell is used for heavy-duty presses with a few million pounds of load capacity. This typically requires a large, expensive custom sensor. Using USB technology, multiple standard, off-the-shelf load cells with in-line external USB interfaces could be interfaced using a hub. This approach not only simplifies installation, but minimizes total cost. It also allows the operator to check the uniform loading of a large platform while monitoring the output of each sensor individually. Two added benefits are easier troubleshooting plus the ability for the system to remain operational if a sensor is removed for repair or replacement.

The USB solution works for most sensors, including older systems, which can be updated with a USB interface and use the same sensor configuration. Using a hub, up to five levels of sensors can cascade to a chip with a single USB port.

Beyond all of the cost-saving benefits, the bottom line for any sensor application is validation, i.e., measuring an important area of a process or system. This can become a major issue when sensors are recalibrated. For an ISO9000 regulated company, any shift in the sensor control system would mean that all parts would be subject to recall because they were not made exactly to spec.

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