Before we delve into RTD simulation, let's review the links to previous articles in our six-part series on sensor simulation in ECU validation.
- Part 1: Understanding Sensor Simulation in ECU Validation presents an overview of the importance and role of sensors in real-time control systems and an introduction to Electronic Control Units (ECUs) and how they rely on sensor data.
- Part 2: Thermocouple Simulation in ECU Validation examines the Seebeck Effect, the importance of simulation accuracy, and cold junction compensation.
Part 3: RTD Simulation in ECU Validation
While thermocouples convert a difference in voltage potential to a temperature, Resistance Temperature Detectors (RTDs) are sensors that essentially perform the same function but exhibit a resistance proportional to temperature. RTDs are often used in applications that demand a high degree of accuracy, though this comes with a price premium compared to thermocouples.
Figure 1: RTD Measurement Circuit
The basic principle of operation of an RTD is that the resistance of a metal changes as its temperature changes. Many popular RTDs are made up of platinum wire with a nominal resistance (at 0 degrees C) of either 100, 500 or 1000 ohms, and the wire length (and its resistance) changes as the temperature changes. The sensor is nominally connected to a data acquisition system in the control unit, which sources a current to measure the resistance.
From an application standpoint, RTDs are more accurate, stable, and linear than thermocouples within a temperature range of up to 600 °C. They are better suited to lower temperature ranges, where higher accuracy, stability, and repeatability are required. In contrast, thermocouples are more cost-effective, less accurate, less stable, and can drift over time.
However, thermocouples have a faster temperature response, are more rugged, and can withstand harsher conditions, such as vibration and temperatures up to and over 2000 °C, depending on the thermocouple type.
There are obviously many parallels between thermocouples and RTD simulations, but now the sensor simulator must accurately simulate resistances rather than voltages. While the thermocouple simulator must be flexible enough to support Type J, K, and T, among others, RTD simulators must have the range required to simulate RTD types such as PT100, PT500, and PT1000. Most resistance simulators, such as Pickering’s PXI RTD Simulator series (model 40-263), achieve the range/resolution required using programmable resistor ladders.
Figure 2: A simplified representation of an RTD simulator channel.
The representation of an RTD simulator channel above comprises a series of resistors that can be switched in or out of the series by relays to create the desired resistance value, representing a specific temperature. Isolation and shorting relays allow a channel to simulate an open or shorted sensor condition. In this diagram, ‘N’ is a base resistance value determining the resolution, while ‘x’ determines the range.
Generally speaking, there will be a trade-off between range and resolution, i.e., tighter resolution will result in a limited range. This is evident when looking at the options table for the model 40-263 series, which have a different range for each sensor type, with resolutions that vary by up to one order of magnitude.
It’s important to note that absolute accuracy is affected by the contact resistance of the relays that are switched in to bypass each resistor. Typically, this will be around 100 megaohms; however, for increased accuracy, some designs will add parallel relays to reduce the overall contact resistance. Pickering achieves the high channel densities shown in the table (up to 12 RTD channels per PXI chassis slot) by using small form factor electromechanical or reed relays in its designs.
Figure 3 (right): Note that while three different types of RTDs are represented in the table, the simulated temperature range is consistent across the board.
This article is part 3 of a 6-part series on sensor simulation.
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