Sensor

A sensor is a device for detecting and signalling a changing condition. And what is this “changing condition”? Often this is simply the presence or absence of an object or material (discrete sensing). It can also be a measurable quantity like a change in distance, size or color (analog sensing). This information, or the sensor’s output, is the basis for the monitoring and control of a manufacturing process.

Sensor Application Basics

Industry continually strives to develop product faster and more cost effectively. By automating processes, manufacturers can realize these goals while maintaining higher levels of quality and reliability. Presence sensing technology is used to monitor, regulate and control these processes. More specifically, presence sensors help verify that critical process steps are completed as intended.

Types of Sensors

• Photoelectric/Photoswitch Sensors
• Inductive Proximity Sensors
• Ultrasonic Proximity Sensors
• Capacitive Proximity Sensors

Discrete and Analog Detection of Sensors

• Discrete Sensing: The sensor produces an On/Off (digital) signal as output, based on the presence or absence of the target. 
• Analog Sensing: The output is proportional to the target’s effect on the sensor, either in relation to its position within the sensing range or the relative strength of signal it returns to the sensor.

Sensor Characteristics/Specifications

Sensing Distance: When applying a sensor to an application nominal sensing distance and effective sensing distance must be evaluated.

Nominal Sensing Distance: Nominal sensing distance is the rated operating distance for which a sensor is designed. This rating is achieved using standardized criteria under average conditions.

Effective Sensing Distance: The effective sensing distance is the actual “out of the box” sensing distance achieved in an installed application. This distance is somewhere between the ideal nominal sensing distance and the worst case sensing distance.

Hysteresis: Hysteresis or differential travel is the difference between the operate (switch on) and release (switch off) points when the target is moving away from the sensor face. It is expressed as a percentage of the sensing distance. Without sufficient hysteresis a proximity sensor will continuously switch on and off, or “chatter,” when there is excessive vibration applied to the target or sensor. It can also be made adjustable through added circuitry.

Repeatability: Repeatability is the ability of a sensor to detect the same object at the same distance time after time. Expressed as a percentage of the nominal sensing distance, this figure is based on a constant ambient temperature and supply voltage.

Switching Frequency: Switching frequency is the number of switching operations per second achievable under standardized conditions. In more general terms, it is the relative speed of the sensor.

Response Time

The response time of a sensor is the amount of time that elapses between the detection of a target and the change of state of the output device (ON to OFF or OFF to ON). It is also the amount of time it takes for the output device to change state once the target is no longer detected by the sensor.

The response time required for a particular application is a function of target size and the velocity at which it passes the sensor.