Home
TYPES OF TRANSDUCERS IN AUTOMATION — OVERVIEW AND OPERATING PRINCIPLES
A measuring transducer is a device that converts a measured physical quantity—such as position, displacement, force, torque or a magnetic field—into an electrical signal that can be read by a controller. Transducer types differ in their operating principle, measuring range and output signal.
What is a measuring transducer?
A measuring transducer connects a mechanical or physical process with an automation system. It records a change in a specified quantity and converts it into a signal that can be sent to a PLC, motion controller, regulator or data-acquisition system.
Depending on its construction, the output may be:
- Voltage, for example 0–10 V
- Current, most commonly 4–20 mA
- A change in resistance
- A pulse train
- A digital signal transmitted via SSI, CANopen or another interface
- A frequency proportional to the measured quantity
The terms “sensor” and “transducer” are often used interchangeably, but they do not always mean exactly the same thing. A sensor may be the element that responds to a physical change, while a transducer also includes the mechanism that converts it into a usable output signal.
Overview of transducer types
| Transducer type | Measured quantity | Typical signal | Example application |
| Incremental | Relative position, rotational speed | A/B/Z pulses | Motor control, rotation measurement |
| Draw-wire | Linear displacement | Encoder-dependent, 4–20 mA or potentiometric | Booms, linear tables, overhead cranes |
| Torque | Torque on a shaft | mV/V, voltage, frequency or digital signal | Testing drives and gearboxes |
| Strain-gauge | Force, pressure or strain | Usually mV/V | Presses, weighing, force control |
| Hall-effect | Magnetic field, position or speed | Analog or digital | Commutation, position detection |
| Photoelectric | Light, presence or movement of a marker | Binary or pulse | Optical encoders, part detection |
| Electromagnetic | Position, speed or field variation | Voltage, current or digital | Contactless measurement |
| Signal converter | Electrical signal | 0–10 V, 4–20 mA or another standard | Signal matching and circuit isolation |
Incremental transducer – how does it work?
An incremental transducer, most commonly an encoder, generates a defined number of pulses as a shaft rotates. The number of recorded pulses indicates the change in position, while their frequency can be used to calculate speed.
A typical incremental encoder provides:
- Channel A
- Channel B, phase-shifted relative to channel A
- An optional reference pulse Z
- Complementary signals in versions designed for greater noise immunity
The phase shift between channels A and B makes it possible to determine the direction of rotation. Resolution is specified as pulses per revolution, and the controller determines relative position by counting them.
After a loss of power, a standard incremental transducer does not retain absolute position information. The machine may therefore need to perform a homing procedure after restart. This does not apply to absolute encoders, which encode a specific shaft position.
AMG incremental encoders can be used to measure speed, direction and relative position in drives, linear modules and production machinery.
How do draw-wire and travel transducers work?
A draw-wire transducer measures linear motion using a steel cable wound around a measuring drum. Cable movement rotates the drum and the connected encoder or potentiometer. A return spring keeps the cable tensioned and rewinds it during the return movement.
This solution converts long linear travel into rotary motion that is easy to measure. It is used where direct installation of a linear sensor would be difficult, expensive or impossible.
Typical applications include measuring actuator or boom extension, trolley position, platform height, gate movement, linear-table stroke, crane position, and level or sag.
The TWK draw-wire transducers available from AMG cover measurements from approximately 1 metre to several dozen metres. Cable movement can be recorded by an incremental encoder, an absolute encoder or an analog system.
What should be considered when installing a draw-wire transducer?
The cable should be pulled as perpendicular to the drum as possible, without excessive lateral deviation. It must not rub against edges, snap back into the housing or operate with slack.
Also check:
- Maximum extension speed
- Permitted acceleration
- Return-spring force
- Temperature range
- Mechanism service life
- Ingress-protection rating of the housing and cable entry
- Compatibility with the selected encoder
What is a torque transducer?
A torque transducer measures the twisting of a sensing element caused by torque acting on a shaft. It most commonly uses strain gauges bonded to a specially designed body.
Under load, the shaft undergoes a very small deformation. This changes the resistance of the strain gauges; the change is then amplified and converted into a torque value in Nm.
Torque transducers are used in motor test benches, gearbox testing, assembly tools, tightening-process monitoring, servo-drive tests, quality-control systems and machine-efficiency testing.
The transducer measures the actual mechanical torque on the shaft. This may differ from torque estimated from motor current because of friction, gearbox losses, temperature and changing operating conditions.
Strain-gauge transducer – operating principle
A strain-gauge transducer uses the change in resistance of a strain gauge caused by stretching or compression. The gauges are attached to an elastic element that deforms under force.
They are most commonly connected in a Wheatstone bridge. This circuit converts a very small resistance change into a measurable output voltage. A full bridge usually uses four strain gauges arranged to increase sensitivity and partially compensate for temperature effects.
Strain-gauge transducers measure compressive force, tensile force, pressure, mass, strain, torque and loads acting on structures.
The bridge signal usually has a low amplitude, so it requires a measurement amplifier, stable excitation and suitable cable shielding.
How does a Hall-effect transducer work?
A Hall-effect transducer converts a magnetic field into an electrical signal. The Hall element generates a voltage depending on the magnitude and direction of the field, and integrated electronics amplify and process the signal.
A transducer may operate as a switch triggered when a field threshold is exceeded, a magnetic latch, a linear magnetic-field sensor, a single- or multi-axis position sensor, or part of a speed-measurement system.
In automation, it is used for contactless detection of position, rotation and magnet presence, and for motor commutation. Linear sensors can generate a voltage proportional to the measured magnetic field.
A key advantage is the absence of mechanical contact between the sensor and the measured component. Measurement accuracy also depends on magnet geometry and quality, the working gap, temperature and external magnetic fields.
What is a photoelectric transducer?
A photoelectric transducer responds to a change in light intensity and converts it into an electrical signal. It may use an LED or laser as the emitter and a photodiode or phototransistor as the receiver.
In an optical encoder, a disk with transparent and opaque segments rotates between the emitter and receiver. Periodic interruption of the beam generates pulses corresponding to a change in position.
Photoelectric technology is also used in retroreflective sensors, optical barriers, reflective sensors, fiber-optic systems and marker-reading systems.
Its advantages may include high resolution and fast response. However, the effects of contamination, condensation, dust and alignment of the optical components must be considered.
What are electromagnetic transducers?
Electromagnetic transducers use changes in a magnetic field, inductance or magnetic flux to generate a signal corresponding to the measured quantity.
Examples include inductive travel transducers, sensors with a moving core, tachogenerators, resolvers and magneto-inductive transducers.
In a linear inductive sensor, movement of a metal core changes the inductance of the coils. Electronics convert this change into an analog signal or digital position value. AMG’s inductive position transducers operate on this principle.
What is a signal converter?
A signal converter does not necessarily measure a mechanical quantity directly. Its task is to convert one electrical signal into another that is compatible with the input of the target device.
For example, it may convert 0–10 V to 4–20 mA, 4–20 mA to 0–10 V, frequency to an analog signal, encoder pulses to a current value, or a resistive signal to an industrial standard.
Additional functions may include galvanic isolation, filtering, scaling and open-circuit diagnostics. A converter can therefore connect devices that are not directly electrically compatible.
AMG draw-wire transducers — SWG, SWP, SWF and SWH compared
| Model | Measuring range | Signal generation | Selected parameters | Typical application |
| SWP | 1 or 2 m | Potentiometer or 0–20/4–20 mA | ±0.2% linearity, up to 10 m/s, IP54 | Actuators, sliders, test benches |
| SWH | 1–3 m | Absolute or incremental encoder | ±0.05% mechanical accuracy, up to 8 m/s, IP65 housing | Linear tables, trolleys, machine retrofits |
| SWG/SWE | 5–15 m | Absolute or incremental encoder | ±1 or ±1.5 mm linearity, depending on range | Overhead cranes, gates, long axes |
| SWF | 5–30 m | Absolute or incremental encoder | Up to 6–12 m/s depending on version, IP65 housing | Lifts, warehouses, heavy transport systems |
The SWP model provides a direct analog or potentiometric signal. SWH, SWG and SWF work with a suitably selected encoder, allowing the output type to be matched to the control system.
How do you select a measuring transducer for an application?
Begin by identifying the quantity to be measured and how the result will be used. A different transducer is needed to monitor an axis position, measure force or assess torque on a shaft.
1. Identify the measured quantity
Determine whether the system must measure linear position, angular position, speed, torque, force, object presence or a magnetic field.
2. Select the measuring range
The range should cover the full motion or load, with an appropriate margin. It should not be unnecessarily large, as this may limit usable resolution.
3. Check accuracy and resolution
Distinguish accuracy, linearity, repeatability, resolution and hysteresis. A high encoder pulse count alone does not guarantee high machine accuracy; backlash, deformation, installation and mechanical-component quality also affect the result.
4. Choose the output signal
The signal must be compatible with the controller input. 4–20 mA is useful over longer cables; 0–10 V supports simple analog integration; A/B pulses measure relative motion; an absolute encoder reports a specific position; and a digital interface may also provide diagnostics.
5. Consider dynamics and environment
Check maximum speed and acceleration, operating temperature, IP rating, vibration and shock, humidity and dust, exposure to oil or coolant, electromagnetic compatibility and mechanical durability.
The entire measurement chain should be considered
Correct selection does not end with choosing the measurement technology. Installation, cables, controller input, readout frequency, filtering and system behavior after a power failure must also be considered.
For applications requiring measurement of long linear travel, consider draw-wire transducers. When a pulse signal describing shaft movement is needed, incremental encoders may be appropriate.
Providing the supplier with the range, speed, required accuracy, interface and operating conditions helps select not only the transducer itself, but a complete and compatible measurement chain.
FAQ
What is an incremental transducer and how does it work?
It generates pulses as a shaft moves. The pulse count corresponds to a position change, while pulse frequency can be used to calculate speed. Channels A and B also make it possible to identify direction.
How does a torque transducer differ from a torque sensor?
In practice, the terms often refer to the same device. “Transducer” emphasizes that shaft deformation is converted into a defined electrical signal for use in a measurement system.
How does a strain-gauge transducer work?
Force deforms an elastic element carrying bonded strain gauges. Their resistance changes, and a Wheatstone bridge converts the change into a voltage proportional to the load.
How do you select a measuring transducer for an application?
Identify the measured quantity, range, accuracy, resolution, dynamics, signal type and environmental conditions. Also check mechanical and electrical compatibility with the machine.
What types of transducers are used in industrial automation?
Common types include position, travel, speed, force, torque, pressure and temperature transducers; Hall-effect, photoelectric and inductive devices; and analog and digital signal converters.