Programmable resistors: the flexible approach to sensor simulation in test

Author : Paul Bovingdon (Simulation Product Mgr) & Bob Stasonis (Product Specialist) | Pickering Interfaces

01 May 2019

Figure 1. PXI programable resistor with resistor chain diagram

Sensors are ubiquitous in our world today. In all electronics applications, a thorough test strategy is important, but in certain cases, such as avionics, engine management units, semiconductor manufacture & satellites, it is essential.

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However, sensors add a layer of complexity to a test strategy, as they must be exercised across their complete operational range when testing the controller circuit board that makes decisions based on a sensor’s response. Replacing actual sensors with simulation can reduce costs, by creating smaller test systems.

Figure 2. PXI 18 channel RTD simulator module

For example, if actual temperature sensors are used, a heat source will be required to make the sensor react. Similarly, a strain gauge will require a mechanical fixture that flexes a plate on which a strain gauge is mounted. Not only will the test system be much larger, chances are it will be slower and less repeatable than testing based on sensor simulation. Also, it may not be practical to incorporate actual sensors into a test system, so external hardware that is designed to replace these sensors is used. As Paul Bovingdon, Simulation Product Manager & Bob Stasonis, Technical Product Specialist at modular signal switching & simulation experts, Pickering Interfaces explain, such systems are commonly based on programmable resistors.

What is a programmable resistor?
As the name suggests, it is a resistor that is changed in value by a test program. The programmable resistor can be a device that plugs into a modular test format, such as PXI. It can also be a standalone device that is controlled via USB or Ethernet. For most programmable resistors, the values are changed using relays to switch into or out of a chain of discrete resistors with increasing resistance values based on a binary series. There are many choices of programmable resistors in terms of range, power, accuracy and resolution.

Figure 3. Single channel precision resistor diagram

Programmable resistor types and parameters
Figure 1 shows a PXI-based programmable resistor module (Pickering Interfaces’ model 40-295) based on relay chains. If we take a single channel, we see that it is composed of multiple resistors, with individual relays connected across each resistor. When all the relays are closed, all the resistors are shorted out and the series resistance is effectively zero ohms. In actuality, it can be a few hundred milliohms, as each relay, as well as the PCB itself, will exhibit a very low resistance. As one or more relays are opened, the value of the resistor chain will equal the series resistance value of the resistors associated with these open relays. Resistor chain modules are commonly available in 8-bit, 12- bit, 16-bit and 24-bit, where the number of bits equals the number of resistors in each chain. The larger the bit level, the greater the resolution or range that can be achieved, but with fewer channels per module.

Need more accuracy?
A precision programmable resistor module is designed to meet highly accurate test applications (Figure 2 shows Pickering Interfaces’ model 40-262). The design is similar to the resistor chain mentioned above, as it does have a resistor chain for the coarse adjustments (see Figure 3), with the addition of a digital potentiometer for fine adjustments. Using software that knows the value and accuracy of the resistor chain, the module can make fine adjustment settings to the channel being programmed. This configuration provides resistor settings orders of magnitude better in terms of resolution and accuracy than standard resistor chains. The resolution on a precision resistor module can be as low as 2 milliohms versus 250 milliohms on standard resistor chains, and at a typical accuracy of 0.03% versus ±0.3% on standard resistor chains. A precision resistor costs more than a standard resistor chain and may have fewer channels because of the added circuitry, but if the application demands this kind of accuracy, there are choices.

Figure 4. Series switching diagram

Fault injection
Some versions of programmable resistors include two additional relays per channel to inject faults into the test program. These relays can simulate a sensor short or an open. Potentiometer simulation is created by connecting two resistor chains together, as shown in Figure 4. The test code should raise the value of one resistor chain and lower the other resistor chain’s value, just as a potentiometer works.

Sensor type
Finally, there are programmable resistor modules configured for a particular sensor type. The two most popular simulate an RTD (resistive temperature detector) or the resistor bridge circuit in a strain gauge. These modules can have additional software drivers that allow you to set temperatures or program the value required to balance the bridge, and the setting of the simulator can be varied above and below this value to simulate extension and compression of a strain gauge (Figure 5).

Figure 5. Strain gauge simulator diagram

If high-accuracy settings are required, look for resistor modules with a calibration port. In the schematic seen in Figure 6, the calibration port is a multiplexer that switches the port to each channel, one at a time. Connecting a precision DMM to the port allows the user to verify the value of every setting required for their test program.

Depending on the manufacturer and model, there is another feature that may be of benefit. Some programmable resistor modules feature uncommitted relays that can be used for several different reconfigurations of the resistor module. These include:

•  Linking two resistor chains together – in series or parallel – to either extend the range of a resistor channel or improve its resolution.
•  Adding additional bits to extend the range.
•  Add a fixed resistor to be switched in to provide an offset resistance.
•  Add additional fault conditions.

Figure 6. RTD simulator with MUX and calibration port diagram

In a modular test platform, this approach is like having two modules in one chassis slot, which can free up a slot for additional instruments or switching.

There are many things to consider when selecting programmable resistor modules. Pickering Interfaces offers industry’s broadest range of programmable resistors, available in both PXI and PCI formats – from standard resistor chains and precision resistors to specialised modules – so designers should be able to find a sensor simulation tool suite for their specific application. The company is also happy to consider full-custom designs.

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