Build a PID Controller out of Stuff Already on Your Workbench

Nearly all modern process actuators (valves, agitators, heaters, motors, etc.) and process meters are electronic, which makes them easily integrated into Proportional-Integral-Derivative (PID) based control systems. PID control is at the heart of many processes, and it’s important to get it right. When designing a process system, it can be helpful to test and refine PID algorithms before the equipment is commissioned or while the system is being designed. Fortunately, a PID controller can be created using instruments that most engineers have on their workbench. This article explains how to build a PID controller out of a digital multimeter (to measure the process variable) and a power source (to drive an actuator). It is based on a real-world application: a PID temperature control scheme using a Peltier device (also known as a thermoelectric cooler), a thermocouple, a digital multimeter (DMM), a DC power supply (20V, 5A), and a PID algorithm created and run in the LabVIEW development environment. This example uses a Keithley Model 2000 6-1/2-digit DMM and a Keithley Model 2200-20-5 unipolar Programmable DC Power Supply. The temperature controller consists of the Peltier device sandwiched between an aluminum plate (bottom) and an insulator. The bottom plate is immersed in an ice-water bath and the PID loop controls the temperature of a small aluminum plate underneath the insulator (Figure 1). This example demonstrates the effectiveness of a PID loop even when it’s run from a PC. This control scheme allows controlling the temperature to within ±0.05°C under most operating conditions with no shielding of the signal line.