Position proportional control dates back many, many years. It used to be the “go to” control method for proportional/modulating control before voltage and current proportional control was available. Even though it’s an older, outdated mechanical type of control, there’s still a lot of position proportional control used on processes today. It is a desirable control method when using older style shaded pole control motors in hot applications, such as the Barber Colman EA57 (Schneider T/A now). The shaded pole control motors have a mechanical slide wire to feed a resistance value back to the position proportional control (typically 100-135 ohms). This is how the control knows what position the motor is in, by the resistance signal that is fed to it. The shaded pole motors do not have internal electronics that newer mod motors and direct mount actuators have. Internal electronics are very susceptible to heat. They will start to fail around 140 degrees F. When using the slide wire feedback for position proportional control, you may experience a failure because of a broken slide wire, broken wiper on the slide wire or a broken feedback control wire(s), or possibly a slide wire input failure in the control. You might also have a new application that calls for a shaded pole motor because of higher heat, but you do not want to run the three wires for the slide wire feedback. In either of these cases, you can use TPS (3 position step) in the Honeywell UDC process control. Three position step works by counting the steps in a motor from 0% open to 100% open. You set the control up with the motor in a simple calibration procedure. Once the control is calibrated with the motor, when the motor drives open and closed, the control know what percent open the motor is at by the step position the motor is in. The Honeywell UDC2500, 3200 and 3500 controls all have the TPS feature. You need at least 2 relays in the control, one to drive the motor open and one to drive the motor closed. Calibrating the TPS will take care of the positioning part of it.