As I noted in the first part of this two-part series, negative feedback reports back what is, compares it with what should be and makes the system work to minimize the difference. Positive feedback is found in howling and whistling PA systems and in unstable control loops. Apart from making oscillators and musical instruments work, positive feedback is rarely a benefit and is outside this discussion. In previous articles, I have dealt with the relatively complex subject of feedback in temperature control loops that have troublesome lags. Here, I will look at specific and simpler cases of feedback usage in components of the process.
Motor-shaft position does not guarantee a proportional valve position, much less proportional flow of heating medium. That limitation may be acceptable for the many processes that do not demand great accuracy and stability. More critical processes depend very much on the final control element linearly obeying the controller signal in respect of heat delivery. Motor linkage geometry and valve flow characteristics can defeat this objective.
Butterfly and globe valves commonly have a crank arm linked to another arm on the motor shaft. Energy flow in relation to motor rotation can be grossly nonlinear. You can vary linkage geometry and add an adjustable cam if you are trying to reduce nonlinearities. You still need to consider upstream pressure and changes introduced when you replace a valve or motor. You can get around these problems by feeding back a fuel flow signal and balancing it against controller output to achieve linear heat delivery and tight, stable control (figure 1).
Pneumatic Actuators. For speed and muscle, you will be looking to the air-operated diaphragm valve incorporating a positioner. This positioner is a type of air relay used between the controller output and the valve diaphragm. It acts to overcome hysteresis (deadband), packing-box friction, and valve plug unbalance due to pressure drop. It uses position feedback for exact positioning of the valve stem in accordance with the controller output coming in the form of a pneumatic signal.
Digital Valve Positioners. Micro-processor-based positioners working from a digital input and digital position feedback offer benefits such as:
- Adjustment of valve characteristics from stored lookup tables or your custom curve.
- Provision of information on hysteresis, stem friction, speed, response characteristic, number of cycles and wear and tear, which aid predictive maintenance.
SCR Power Control. Say you are setting power by a time-proportioning circuit or by varying the firing-angle of the load current. You cannot guarantee stability or precise knowledge of the power level. You are up against, to name just a few factors, variable line voltage, age- or temperature-dependent resistance, load power factor and, in the case of induction heating, magnetic-coupling variations. Often, the only dependable solution is to feed back a true load-power signal and balance this against your control signal that is calling for power.
This small control loop can be used as a stable manual power controller, or it can lie within an overall temperature control loop to obtain a stable gain and optimize control response and accuracy.
Current and voltage feedback can separately enable protection against overcurrent and overvoltage being applied to the load.