Simulating Millivolt Sensor Signals
This configuration serves as a valuable tool for simulating millivolt (mV) sensor signals, particularly in industrial control systems. Modern sensors often incorporate some form of ‘intelligence’ directly at the measurement point, where the sensor interacts with the object it measures. During this interaction, the sensor signal undergoes conditioning and digitization. Subsequently, the digitized signal is transmitted to a microcontroller. Which then relays a digital representation of the sensor value to the remote control system. Despite this trend, numerous ‘legacy’ control systems still exist in the field where the intelligence is located away from the sensor head. These older systems rely on field wiring to convey the measured signal back to the central control system.
Simulating Sensor Signals for Commissioning
During the commissioning of plants equipped with sensors, it is essential to simulate the sensor signal to ensure its accurate transmission. This is particularly crucial in setups where the sensor signals pass through various junction boxes before reaching the control system. Simulating the sensor signal is vital not only to confirm that it reaches the correct terminals on the control system but also to validate the control system’s response to the signal.
Design Overview and Bench Testing
The design presented here serves as a reliable tool for ‘bench testing’ control systems before their installation. It’s important to note that this design is suitable for simple simulation purposes and is not precise enough for calibration tasks. The system is powered either by a ‘plugtop’ PSU during bench testing or a battery. Three current sources (diodes) receive power. Among these, I1 generates a 1.00 mA current signal, creating a 100-mV signal when switched across the 100-Ω potentiometer. Similarly, I2 generates a 0.25-mA signal, resulting in 25 mV across the potentiometer. I3, with 3.0 mA, illuminates the LED to indicate power. The chosen current source is switched using S2 in conjunction with the 10-turn potentiometer. Switch S1 ingeniously swaps the output signal’s polarity. Utilizing the Type MTA206PA DPDT switch from Knitter provides a center-off position, shorting out the output signals (S1 pins 2 and 5), ensuring a zero output signal.
Considerations and Circuit Operation
The current sources, although relatively costly, have a 10% tolerance, making them unsuitable for calibration purposes. To adjust for higher output, a bleed resistor (R1, R2) can be added, as depicted in the diagram. These current sources are manufactured by Vishay/Siliconix and are available through Farnell. The circuit consumes approximately 4.25 mA.