Before we conclude our series on sensor simulation in ECU validation with an exploration of current loop simulation, let's quickly revisit our series' key points and links.
- 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 explores proportional resistance to temperature in simulation and suitability for low-temperature ranges where higher accuracy, stability, and repeatability are paramount.
- Part 4: Strain Gauge Simulation in ECU Validation discusses the importance of simulating strain gauges accurately across various applications and how they work.
- Part 5: LVDT, RVDT & Resolver Simulation in ECU Validation dives into the specific makeup and variables involved in simulating linear and rotational movement.
Part 6: 4-20 mA Current Loop Simulation in ECU Validation
While the sensor types discussed in this series are typically dedicated to measuring a specific function (i.e., thermocouples are used to measure temperature), 4 – 20 mA current loop transmitters and transducers cover various measurement types. They are typically used in industrial settings, such as factory automation, where accuracy and resolution are not critical, but immunity to electrical noise is an important factor. 4 – 20 mA transmitters may be used to measure a diverse range of physical properties, including velocity, vibration, pressure, temperature, and fluid level and flow. The fundamental principle is fairly basic: the transmitter is connected to a power source–typically in the range of 10 to 30 volts–and outputs a DC current between 4 – 20 mA that is proportional to the unit of measurement being made. For example, a pressure sensor might provide a linear output between 4 – 20 mA for a pressure range of 0-15 PSIg, as shown in the image below.
Figure 1: Pressure transducer with 24V power supply and 0-15 psig
The use of current to deliver the measurement instead of voltage has the advantage that the value does not degrade over long distances, so the current signal remains constant through all components in the loop. Also, using 4 mA as a "Live Zero" enhances the signal-to-noise-ratio at low levels and makes a loop failure more apparent as a current loop with an open termination has zero current flow, which is outside the valid 4 to 20 mA signal range.
While 4 – 20 mA is a very popular sensor type used in automation, these transmitters may operate in other current modes. Additionally, it is quite common to have transmitters that provide a conditioned voltage output proportional to the measurement being made. Therefore, incorporating as much flexibility as possible into a simulator is desirable.
Pickering’s 41/43-765 series is an example of a current loop/voltage simulator with several built-in features that expand the application space in which it can be used. With up to 16 simulation channels in a single-slot PXI or PXIe module, high channel count systems can be simulated in a small footprint. The outputs can work in 4-20 mA, 0-24 mA and +/-24 mA current modes, the latter enabling the simulation of both sourcing or sinking sensors, with 16-bit resolution providing output control better than 1 uA. There are also optional low-power voltage modes of 0-5 V, +/-12 V, or +/-5 V. The outputs are always actively sourcing or sinking, so no voltage source is required in a current loop, simplifying the test system. Since these sensors often operate in environments that span large distances, the module can be programmed for various slew rates to simulate different wire lengths. As with Pickering’s other simulator modules, onboard relays are included to simulate short and open fault conditions.
Summary
Efficient design validation of safety-critical control systems requires the ability to simulate real-world environments to extreme operating conditions efficiently. To ensure that the control system behaves safely and predictably, all potential fault conditions—such as wire breaks or short circuits—must be simulated. Simulating the sensor outputs that feed a control system is much more efficient and safer than building models and changing the environmental settings.
Pickering has developed a broad range of sensor simulation products that help design and validation engineers optimize their control algorithms. These products can be used in systems with many control options, such as PXI, PXI Express, Ethernet/LXI, and USB, with Windows, Linux, and Real-time application options.
Learn more about Pickering's 4-20 mA Current Loop Simulation Module
This article is part 6 of a 6-part series on sensor simulation.
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