How Does a Machine Know It Has Moved Far Enough? It Checks

A plain motor turns while power is applied and stops somewhere when it is removed, and where it stops depends on the load, on friction, on momentum and on the supply voltage, none of which are known precisely. Commanding a plain motor to move something to a particular position is therefore impossible with any accuracy. Adding a sensor that continuously reports where the output actually is, and a circuit that compares that report against the commanded position and drives the motor in whichever direction reduces the difference, converts the arrangement into one that reaches the target regardless of load and holds it against disturbance. The correction loop rather than the motor is what makes it work.

The same three elements appear in every such arrangement:

• •A command specifying the position, speed or force wanted
• •A sensor measuring what is actually being achieved
• •A comparison producing the difference between the two
• •A controller deciding what drive to apply given that difference
• •An actuator, which is usually a motor but may be hydraulic or pneumatic
• •A mechanical connection from the actuator to whatever is being moved

Deciding how hard to push given a particular error is the whole engineering problem, and getting it wrong produces characteristic failures. Responding too weakly leaves the output arriving slowly or stopping short permanently. Responding too strongly makes it overshoot, correct back, overshoot again and oscillate, which in a large machine is destructive. The standard approach combines three responses, one proportional to the current error, one to how long the error has persisted, and one to how fast it is changing, and choosing the relative weights of those three is the tuning. Methods for doing it range from formal calculation through published procedures to adjusting by hand until the behaviour looks right, and the last remains extremely common.

The device reporting position determines what the whole arrangement can achieve, and several kinds are used. A potentiometer is a variable resistor turned by the output shaft, which is cheap, simple and limited in precision and lifetime, and it is what small hobby units contain. An optical encoder counts marks on a disc as it turns, which gives high resolution and reports changes rather than absolute position unless a reference mark is included. An absolute encoder reports a unique code for each position, so the system knows where it is at switch-on without moving. A resolver uses coils and works reliably in heat, vibration and dirt, which is why aircraft and heavy industry use them. Choosing badly limits everything downstream.

The arrangement is so widespread that its absence is more notable than its presence. Industrial robots use one per joint, with position and speed loops running many thousands of times a second. Machine tools position a cutting head to micrometres. Aircraft control surfaces are moved by them, with the pilot commanding a position rather than applying force directly. Vehicle steering and throttle systems use them. Radio-controlled models use small self-contained units combining motor, gears, sensor and circuit in one package, which is what most people picture. Hard drives position read heads. Camera lenses focus. And a domestic thermostat is exactly the same arrangement applied to temperature rather than to position.

A motor alone stops wherever load and friction leave it, so accurate positioning requires a sensor reporting the actual position and a circuit driving the motor to reduce the difference from the target. The loop rather than the motor is what makes it work. Deciding how hard to push given an error is the hard part, with weak response stopping short and strong response oscillating. A domestic thermostat is the same arrangement applied to temperature.