What is a sensor and How it works?
A sensor is a device that detects and responds to some type of input from the physical environment. The specific input could be light, heat, movement, humidity, pressure,flow etc.The purpose of sensors is to acquire information and to forward it in an evaluable format to the signal processing system. They are found in diverse tasks in technology, with different designs and operating principles. That is why it is important to categorise them. Sensors can be classified according to
• operating principle (optical, inductive, mechanical, fluid, etc.),
• output signal (analogue, digital, binary, etc.), to name just a few methods.
The sensors used most frequently in automation technology are those with digital outputs as they are much more immune to interference than those with analogue outputs. Digital controllers can also use the signals from these sensors directly without first having to convert them into digital signals by means of so-called analogue-digital converters as is the case with analogue signals. The sensors used most frequently in industrial automation are the so-called proximity sensors that determine the presence (or approach) of a work piece.
Proximity sensors are non-contacting and therefore have no external mechanical actuating force. As a result they have a long service life and are very reliable. A distinction is made between the following types of proximity sensor:
• Sensors with mechanical switch contact
– Reed switches
• Sensors with electronic switch output
– Inductive proximity sensors
– Capacitive proximity sensors
– Optical proximity sensors
Reed switches are magnetically-actuated proximity sensors. They consist of two contact blades in a small glass tube filled with protective gas. The action of a magnet causes the contact between the two blades to close so that an electrical current can flow. In the case of reed switches that work as N/C contacts, the contact blades are preloaded using small magnets. This preload is overcome by the then much stronger switching magnet. Reed switches have a long service life and a short switching time (approx. 0.2 ms). They are maintenance free, but must not be used in areas with strong magnetic fields (e.g. in the vicinity of resistance welders or CAT scanners).
Electronic sensors include inductive, optical and capacitive proximity sensors. They generally have three electrical connections for:
• supply voltage,
• output signal.
In the case of electronic sensors, no movable contact is switched over. Instead the output is either electrically connected with the supply voltage or to earth (= output voltage 0 V). When it comes to the polarity of the output signal, there are two different designs of electronic proximity sensor:
• In the case of positive-switching electronic sensors, the output has a voltage of zero (OFF) when there is no part within the sensor’s response range. The approach of a workpiece results in the output being switched over (ON) so that supply voltage is applied.
• In the case of negative-switching sensors, supply voltage is applied to the output when there is no part within the sensor’s response range. The approach of a workpiece results in the output being switched over to a voltage of 0 V.
Inductive proximity sensors
Inductive proximity sensors consist of an electrical resonant circuit (1), a flip-flop (2) and an amplifier (3). When voltage is applied to the connections, the resonant circuit generates a (high-frequency) magnetic alternating field that escapes from the front side of the sensor. Bringing an electrical conductor into this alternating field “attenuates” the resonant circuit. The downstream electronic unit, consisting of a flip-flop and amplifier, evaluates the resonant circuit’s behaviour and actuates the output. Inductive proximity sensors can be used to detect all materials with good electrical conductivity, for example graphite as well as metals
Capacitive proximity sensors
Capacitive proximity sensors consist of an electrical resistor (R) and a capacitor (C) that together form an RC resonant circuit as well as an electronic circuit for evaluating the oscillation. An electrostatic field is generated between the active electrode and the ground electrode of the capacitor. A stray field forms on the front side of the sensor. When an object is brought into this stray field, the capacitance of the capacitor changes. The resonant circuit is attenuated and the downstream electronic unit actuates the output. Capacitive proximity sensors not only respond to materials with a high electrical conductivity (e.g. metals), but also to all insulators with a high dielectric constant (e.g. plastics, glass, ceramic, liquids and wood).
Optical proximity sensors
Optical proximity sensors always have a transmitter and a receiver. They use optical (red or infrared light) and electronic components and modules to detect an object located between the transmitter and receiver. Particularly reliable transmitters of red and infrared light are semiconductor light emitting diodes (LEDs). They are small, robust, inexpensive, reliable, durable and easy to install. Red light has the advantage that it can be seen with the naked eye when aligning (adjusting) the optical axes of the proximity sensors. Photodiodes or phototransistors are used as the receiver component in optical proximity sensors. A distinction is made between three types of optical proximity sensor:
• through-beam sensors,
• retro-reflective sensors,
• diffuse sensors.
Through-beam sensors have transmitter and receiver units that are set apart. The components are mounted in such a way that the beam of light emitted by the transmitter hits the receiver (e.g. phototransistor) directly. If an object, work piece or even a person enters the path between the transmitter and receiver, the light beam is interrupted and a signal is triggered that initiates a switching operation at the output (ON/OFF).
In retro-reflective sensors the transmitter and receiver are arranged side-by-side in a housing. The reflector reflects the light beam from the transmitter to the receiver. It is mounted in such a way that the light beam emitted by the transmitter impinges almost entirely on the receiver. If an object, workpiece or even a person enters the path between the transmitter and reflector, the light beam is interrupted and a signal triggered that initiates a switching operation at the output (ON/OFF).
The transmitter and receiver in diffuse sensors are arranged side-by-side in a component. In contrast to the retro-reflective sensor, a diffuse sensor does not have its own reflector. Instead it uses the reflective property of the object or workpiece that enters its transmission range. If the light hits a reflective body, it is redirected to the receiver and the sensor output is switched. This operational principle means diffuse sensors can only be used if the workpiece or machine part to be detected is highly reflective (e.g. metallic surfaces, light colours).
Pressure-sensitive sensors come in different designs:
• mechanical pressure switches with binary output signal,
• electronic pressure switches with binary output signal,
• electronic pressure sensors with analogue output signal.
Mechanical pressure switches with binary output signal
In a mechanical pressure switch, the pressure acts on a piston area. If the force exerted by the pressure exceeds the spring force, the piston moves and actuates the contacts of the switching elements.
Electronic pressure switches with binary output signal
Typical examples of electronic pressure switches with binary output signal are diaphragm pressure switches that switch the output electronically instead of actuating a contact mechanically. Pressure or force-sensitive sensors are attached to a diaphragm for this purpose. The sensor signal is evaluated by an electronic circuit. As soon as the pressure exceeds a previously defined value, the output switches.
There are a lot of pressure sensing elements out there to see check the link below