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. In this series, I will look at specific and simpler cases of feedback usage in components of the process.
V4 = R1 / (R1 + R2) is fed back to a second input of the amplifier so that it subtracts from the input V1. So
V2 = V1 - V4
The ratio V3/V4, determined by the values of R1 and R2, is called the feedback factor (B). A bit of algebra shows that the gain with feedback is
H = A / (1 + AB)
If you make the open loop gain (A) very high so that AB is a lot greater than one, H becomes very close to 1/B. So, the gain with feedback is very stable and predictable, being determined almost entirely by two stable resistors and largely unaffected by internal noise or changes in A. Input impedance, bandwidth and linearity are greatly increased. Output impedance and distortion are greatly decreased. These benefits increase in proportion to B.
This is the kind of amplifier found at the input of virtually every electronic, analog input process indicator, recorder, controller, signal converter and multimeter currently on the market. The reason is its excellent stability and low zero drift when handling millivolt-size signals from such sources as thermocouples, RTDs, strain gauges and pH cells.
Other Application ExamplesChart Recorders.An engineer dropped in on a group busy designing a chart recorder. He was polled for ideas. It happened that he was not long back from a visit to the instrument repair shop of a steelworks. There, he had seen a 10 by 20' wall of pigeonholes containing many different recorder roll charts. They covered different sensors, ranges,oF andoC, square root, linear, overprinted scales and so on. A dramatic reduction of chart and scale variety looked within reach if only you could linearize and range pen travel for these signals. So why not apply a cam-correction mechanism to the servo pen drive?
Not good enough. Too much custom machining. Why not linearize and range the input amplifier to compensate for the various spans and nonlinearities of the process signals?
The amplifier turned out to be much like that in figure 1, with R1 and R2 replaced by a nonlinear electronic circuit. This circuit could be adjustable or plug-in replaceable to match the input type.
Soon, chart suppliers were relieved of a tedious, costly and labor-intensive part of their process while chart users gained a wider choice with reduced variety and inventory cost.
Later, a printing pen was added to put engineering units, numbers and time markers on the chart.
The above ranging techniques have long been replaced by stored lookup tables, digital circuitry and displays. Paper and ink designs are very much alive and being complemented by LCD color displays and archived data, evolving into the paperless recorder.
Electric Actuators. The constant speed, single-phase, reversing, induction motor is widely used to position valves and dampers that control delivery of heat. The stroke (rotation) of such motors is usually limited to 160o, adjustable down to 60o. The motor is geared down to give a stroke travel time of typically 15 to 60 sec, with torque about 150 lb-in (17 N-m). This apparently slow response usually is adequate for temperature control applications.
Adjustable switches on the motor are used to limit the travel at each end of the stroke.
Position Mode. In this mode, a feedback potentiometer puts out a position signal representing shaft rotation. This signal is compared with the controller's output signal. The difference drives the motor shaft to a position that is proportional to the control signal in the face of mechanical and valve stem friction. This makes a local servo control loop within the overall temperature control loop. The feedback signal also can drive a meter or digital display that shows shaft position. This avoids the risky practice of watching the controller output signal and hoping that the servo system is working and the valve travel is obeying the signal.
Next time, I will continue my discussion on position mode and negative feedback. I will also look at pneumatic actuators, digital valve positioners and SCR power controls.