Grounding for Control Systems

Every control system starts with a measurement of whatever it is that needs to be controlled. And, whether you're measuring temperature, pressure, position, speed or anything else, most automated control systems use a voltage level analog of the process parameter the system's trying to keep under control. That means you're usually making voltage measurements.

Voltage measurements are always made with respect to a reference point, which is commonly called “ground”. Even differential measurements, which ostensibly measure the voltage difference between two points, actually use an analog-computer circuit that computes the difference between voltage readings at those two points, each measured with respect to ground . Even when doing floating (non-referenced) differential measurements, there is always a virtual high-impedance path to ground.

Whenever you make any voltage measurement, therefore, it is critical that you keep in mind where your ground reference is.

Suppose, for example, that the system is closed-loop control of temperature using a fast-acting thermocouple (see diagram). Everything looks okay, but the temperature never seems to stabilize. It's as if the setpoint keeps jumping around!

What's happening is the signal leads are acting as a big antenna to pick up ambient EMI, which appears as millivolt-level noise across the virtual high-impedance path to ground. The reference junction rectifies that high-frequency noise, making it appear hotter. Whenever the EMI level changes, the reference-junction temperature appears to change, upsetting the control loop.

All the methods for eliminating EMI, as well as many other measurement problems, start with establishing a solid ground point and maintaining it throughout the circuit. To avoid such problems in the first place, always know where your ground is.

Planning a ground system is as much an art as a science, and the best approach depends on the application..In general, the basic rules of thumb are:

- Use a strategy appropriate to the signal bandwidth;

- View signal paths as transmission lines;

- Use twisted pairs for all transmission lines;

- Enclose each transmission line in a shield;

- Ground each shield at one end only;

- Ground single-ended measurement equipment at the detector/signal-conditioner end, not at the sensor end.

These rules of thumb work for every frequency range up to about 100 MHz. For higher bandwidths, you have to start planning for standing waves in the transmission lines, and that's for another story.