This list is does not claim to be academically correct. It is simply a way of explaining the subject of measurement and control better. It is compiled in order of measure or sense, inspect and control.

  

Sensor

Typically a device that reacts to a physical property within its surrounds and provides typically an electrical output. This output can be a switch, a solid-state signal, an analogue output of sorts or a serial or bus type communication. Typically no display is provided.

 

Display

A display is usually a device that accepts an electronic input from a sensor and displays sensor output in a meaningful manner. It can usually scale, convert and offset to reflect a particular unit .

 

Gauge

A gauge is basically a combination of sensor and display.

 

Actual Value

The measurement of a physical property as provided by a sensor or gauge.

 

Limit switch

A sensor that has some sort of a set-point adjustment. It enables the device to signal if the physical property being measured has reached a certain threshold level (limit) or not. A simple process control method to switch something on or off when the threshold is reached.

 

Tolerance

Similar to the limit switch but used in conjunction with a gauge. The threshold value is typically set on the display device and may be easily altered for different products or process conditions. It is also provides the most simple form of control.

 

Nominal value /Set-point

The value that a physical property should maintain to achieve the optimal production outcome.

 

Control  (Closed-Loop)

A continuous loop of measuring, comparison to set-point and issue of a control signal to eliminate any deviation between the actual and the nominal.

 

Process Control

Typically a control loop that controls a physical dimension that is not measured on the final product. For example the temperature control of an extruder.

 

Quality Control A control-loop that involves measuring a physical aspect of the end-product and adjusting the manufacturing device to maintain that physical aspect as close to nominal as possible.

It is called on-line if the measurement and control takes place real-time and automatically on the production line.

Due to cost of the equipment or physical restrictions  only some aspects can be measured or controlled on-line. Therefore most quality control is still performed off-line in a manual or semi-automatic fashion.