Your task: To check the control circuits of items such as valve actuators, motor drives, SCRs and heaters; manipulate test signals; and analyze the results. In Part 2, I will be covering controllers, recorders, indicators, signal converters, temperature sensors and shaft speeds. Most important: None of your tests must endanger the plant or its product.
It is a given that you already own a good digital multimeter and a clamp-on ammeter. You also need a run-up box. What's that? A little device that puts a 4 to 20 mA signal into the signal input of say, a control valve or SCR unit. It simulates the output signal of a controller or transmitter.
Wasn't in the budget? Make one yourself, using the components in figure 1 mounted in a plastic hobby-project box. All can be found in any Radio Shack or similar store -- if not in the maintenance shop. If you can't find a cheap milliamp meter (mA in figure 1), use your multimeter set on a milliamp range. String together a 9 V battery, a 100 fixed resistor, a 100 and a 2,000 variable resistor, and your meter. Connect the ends into the signal input and run the valve, SCR, motor drive, etc., through its range as you vary the current between 4 and 20 mA.
If the valve has a feedback potentiometer or equivalent for transmitting valve position back to the control room, use your multimeter to check that this signal corresponds to the observed valve position.
Motorized Valves. For 4 to 20 mA actuation, check the valve travel as above. If you have access to the feedback signal, check it. It is often inaccessible, buried inside the servo drive of the motor. With the 4 to 20 mA system, if the feedback potentiometer fails, control fails. In this event, some controllers can be configured to make the valve go to one or other end of its stroke as a fail-safe position. It’s worth checking the consequences of potentiometer failure.
Another type of valve actuator uses boundless control. Here, the forward and reverse motor windings are connected together at a common terminal. The non-common (switched) ends are joined by a capacitor so that the winding not directly energized receives a phase-shifted current when its partner is energized.
The temperature or process controller used in boundless control has two output switches that energize one winding for forward and the other winding for reverse rotation. The switch closures cease only when the temperature controller sees no deviation from the setpoint. If a feedback potentiometer is fitted, it is usually only for indication and plays no part in control. However, some controllers can exploit the potentiometer for override control.
Here’s a Trap. When testing the switching action, it’s no use connecting lamps across the windings hoping to see which one is being energized. They both are, one via the capacitor when either one of the switches is closed. This applies until the controller sees no deviation from setpoint, then both switches open. To check the controller’s switching action, you would have to lift the forward and reverse controller wires off the windings and use two lamps to check the switching outputs directly off the controller.
In Part 2, I will deal with electric heaters, temperature sensor issues, calibration checks and speed measurement.