Sensors are devices that respond to a physical stimulus and react by producing a signal that is used for measurement or control. Sensors that are frequently used in power transmission are speed, position, physical presence, liquid level, pressure, and force.
View a chart of the most common types of sensors used in automation systems; plus their function, advantages, and disadvantages.
Tachometer generators, also known as a tachogenerators, measure the rate of a rotating shaft using an internally generated electrical signal. They output a voltage proportional to the shaft speed, which is read by the tachometer and displayed on a readout or fed to an external device.
Tachometer generators fall into three basic categories: direct current (DC), alternating current (AC), and permanent magnet types. A majority of modern tachogenerators are permanent magnet types. They use a rotating armature, which rotates within a fixed magnetic field, so its rotation generates a voltage proportional to the shaft’s speed.
Tachometer generators are not the same as tachometers, they differ in form and function. Tachogenerators can be used to power tachometers, which are gauges that display RPM.
Tachogenerators are frequently used to measure the speeds of electric motors, engines, and the equipment they power: conveyor belts, machine tools, mixers, fans, etc.
Rotary encoders, also called shaft encoders convert angular position or speed of a shaft or axle to an analog or digital signal. They produce a pulse every time a shaft rotates through a specified angle. Main types include optical and magnetic. Magnetic encoders are less expensive and simpler, but their resolution is limited to about 1 degree or 360 pulses per revolution. Optical encoders can produce up to 5,000 pulses per revolution.
Incremental: Provide cyclical outputs (only) when the encoder is rotated. They can be either mechanical, optical, or magnetic. Incremental encoders are inexpensive, resistant to vibration, and provide position and speed data down to zero rpm. The use a rotating disk with etched gratings and a stationary mask with etched slots. A light passes through the mask and disk slots to produce pulses that are detected by phototransistors. One limitation is that position information will be lost upon a power interruption.
Absolute: Produce a unique digital code for each distinct angle of the shaft. The two basic types are optical and mechanical. They provide multiple tracks and each have a light source. The alignment of light pulses is unique for each increment of shaft position. Therefore, position data is not lost due to a power interruption.
Other advantages of absolute encoders over incremental encoders are: they provide better position data, they can be used in higher speed applications, and they don’t need a referenced starting point. However, they are more expensive.
A type of rotary electrical transformer that can be used in a variety of position and velocity feedback applications. The most common type is the brushless transmitter. It’s like an electrical motor because it has a stator and rotor. However, the rotor contains a primary winding that has a specified AC signal, called a reference signal, applied to it. The stator contains two secondary windings that are arranged 90 degrees from each other. As the rotor rotates, output voltages create signals to provide shaft position data. Resolvers are very rugged and are a good option for harsh environments.
Proximity sensors can detect the presence of objects without any physical contact. They can operate at higher speeds than mechanical limit switches, they can sense objects coming from any direction, and are typically not dependent on the weight or size of the object to produce a signal output.
Some of the different proximity sensing technologies include capacitive, inductive, eddy-current, hall effect, Weigand effect, photoelectric, ultrasonic, radar, field effect, radio absorption, and magnetic.
A type of sensor that detects presence and absence of objects. Mechanical limit switches are mechanically activated, meaning they have some type of arm, lever, knob, or plunger, which is physically activated when it makes contact with an object. Eventually the object pushes the actuator to its limit, which is where the contacts change state.
- Detecting presence and absence
- Detecting speed
- Detecting range of movement
- Detecting positioning and travel limit
- Breaking a circuit when unsafe situations arise
- Can be used in most industrial environments
- Very precise in accuracy and repeatability
- Consume little electrical energy
- Can switch loads with high inductance
- Can control multiple loads
- Generally restricted to equipment operating at relatively low speeds
- Must make direct contact with target
- Moving mechanical parts will wear out
Liquid Level Sensors
Commonly used in continuous and batch control process systems. Some indicate when liquid levels have reached a certain point such as magnetic and mechanical float switches, which is used to automatically control pump motors that regulate levels.
Another type is the bubbler sensor, which contains no moving parts, making them suitable for measuring the level of sewage, drainage water, sludge, or water with suspended solids. This system uses a tube with an opening below the surface of the liquid level. A fixed flow of air is passed through the tube. Pressure in the tube is proportional to the depth (and density) of the liquid over the outlet of the tube.
More sophisticated level sensing devices are available that use the following methods; capacitance/admittance, differential pressure, radar, electromechanical, ultrasonic, and optical technologies.
Measure pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding. A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed.
The three most common pressure sensor technologies are strain gauge, piezoresistive, and capacitive type sensors. These types of electronic pressure sensors generally use a force collector (such a diaphragm, piston, or bellows) to measure strain due to applied force over an area.
Both strain gauges and piezoresistive are sensors that are bonded to a movable diaphragm. An unbalanced pressure alters the resistance directly proportional to the pressure difference between the two sides. This is the most commonly used sensing technology for general pressure measurement.
Capacitive sensors use two plates that are electrically charged and movement of the diaphragm changes the capacitance values of both plates.
Two devices are used in the measurement of force, weight, and tension including linear variable differential transformers (LVDTs) and strain gauges. Strain gauges are also used for measuring torque, which LVDTs are not suited.
At the core of force sensors are load cells, which are transducers that convert force into measurable electrical outputs. Strain gauge-based load cells are most common.
A photoelectric sensor, or photo eye, is a type of proximity sensor used to detect the presence of a moving target by using a light transmitter (often infrared) and a photoelectric receiver. There are two techniques. In one technique, the target interrupts light going from a source to a sensor. And in the other, the object reflects or absorbs light via a reflective path. There are three commonly used types: opposed (thru beam), diffused (proximity), and retroreflective.
A thru beam sensor uses two devices (a light source and a detector) that face each other. Detection occurs when a target blocks or breaks the beam of light passing between them.
A diffused sensor emits a light beam that must be reflected back to it by the target object itself for detection to occur.
A retroflective sensor emits a light beam that is reflected back to the sensor from a retroreflector. Detection occurs when an object blocks the beam between the sensor and the retroreflector.
Content on this page was created using excerpts from the Power Transmission Handbook (5th Edition), which is written and sold by the Power Transmission Distributor’s Association (PTDA). The Power Transmission Handbook is just under 400 pages and is a valuable resource for anyone involved or interested in the power transmission industry.