The terms displacement sensor and position sensor are often used synonymously on these pages, frequently also as (part of one) displacement sensor system. You will find explanations concerning the mode of operation of inductive displacement sensors here.
(Displacement) probe sensors are equivalent to the displacement sensor as concerns their internal structure.. As opposed to displacement sensors with a free core, these are controlled by a probe and pressed against the movable object by a spring. You will find explanations concerning the operation of inductive displacement sensors and probe sensors here.
The abbreviation LVDT stands for Linear Variable Differential Transformer. You can learn more about the operation of the LVDT here.
The differential inductor is a displacement sensor consisting of two coils. More information about the operation of the differential inductor can be found here.
Long-stroke sensor or single inductor
Long-stroke sensors (or single inductors) have only one real measuring coil, the second is formed by a spare circuit. In contrast to the LVDT and to the inductor, they work according to the eddy current principle. More detailed information can be found under Mode of operation of long-stroke sensors .
Eddy current sensor
“Eddy current sensors” are used predominantly in the sense of a contact-free gap measurement proximity sensor (also called a distance sensor) when the measuring principle is based on the eddy current process (in contrast to contact-free distance sensors with capacitive, ultra-sound or optical measuring processes). More information about the operation of eddy current sensors can be found here.
Draw wire sensors
Draw wire sensors are also known as displacement sensors based on the measuring wire principle, string potentiometer, draw wire transducer, yo-yo sensor, pull wire sensor, wire transducer, or cable extension transducer.
Terms from the field of displacement sensors
The nominal stroke determines the measurement range for which the sensor is designed. In other words, the technical data is observed within this range. Depending on the measuring principle, the sensor zero point in the case of differential transformers and differential inductors lies in the middle. Shifting of the zero point in an end position of the measurement range is possible with certain amplifiers. In the case of single-choke sensors, the zero point is at the beginning of the nominal stroke distance. (MESSOTRON displacement sensor WP)
The mechanical stroke defines the maximum displacement which the displacement sensor can register. The technical data, in particular the maximum linearity error, is guaranteed, however, only for the nominal stroke.
The nominal output defines the measurement output per supply volt for the nominal stroke. Displacement sensors can be calibrated to a specific nominal output, e.g. to 80 V. This permits easy sensor replacement without extensive changes to the settings of the measurement electronics and thus creates a high level of flexibility in the experimental setup.
(Messotron offers the most extensive spectrum of 80mV/V sensor technology)
The sensitivity defines the measurement voltage per supply volt and per millimeter of displacement.
The supply voltage of the displacement sensor can be selected freely within a wide range. It defines how great the excitation voltage of the oscillator can be at maximum without interfering thermal effects occurring in the sensor.
The carrier frequency is the frequency of the supply voltage at which the displacement sensor should be operated. Common values are 5 or 10 kHz.
Inductive displacement sensors deliver a constant display in the case of static measurements; jumps in resolution are practically unrecognizable. In the case of dynamic measurements, the resolution is limited by the electrical noise voltages of the sensor and amplifier. In the case of a transmitted frequency band of up to 500 Hz, a resolution of 1/10000 of the nominal stroke can be calculated as a reference point.
The linearity error defines how great the deviation of the measurement output from the best straight line through the zero point is permitted to be and is specified relative to the entire displacement distance. MESSOTRON displacement sensors normally have a maximum linearity error of ±0.5%. The displacement sensors can also be delivered with low linearity error levels of ±0.25% or ±0.1% depending on the version.
Measuring devices fundamentally exhibit errors due to temperature fluctuations. In the case of inductive displacement sensors, one can differentiate between two effects caused by temperature changes: the temperature error of zero point shifts the sensor zero point and thus all measurement values by the same amount. It is relative to the nominal stroke. The temperature error of span determines the maximum temperature-dependent deviation of the measurement output from the value which corresponds to the displacement. It is relative to the actual value.
DIN 40050 protection
The inductive displacement sensors from MESSOTRON are generally dust-proof in accordance with DIN 40050, IP65, and can be had in a waterproof (IP66) and pressure-resistant version, if necessary. If there is a danger of foreign bodies penetrating the gap between the sensor and the movable core, the location must be secured by additional measures.
Operating and test pressure
The encapsulation of the resistant displacement sensor from MESSOTRON is designed for operating pressures between 120 bar and 450 bar. Generally, however, in a technical system, brief pressure peaks occur which significantly exceed the operating pressure. Therefore, the displacement sensors are capable of withstanding pressure as high as the test pressure. The displacement sensor should not, however, be permanently stressed with the test pressure. Special versions can be fabricated for greater operating pressures than listed.
Terms from the field of eddy current sensors
Conditioning electronics converter
Eddy current sensors require an electronic component for operation. This can be integrated into the sensor or separately and then connected. The separate unit is known as a converter.
The electronic component supplies the coil in the sensor with a high-frequency alternating current and transforms the sensor’s output signal into a signal proportional to the distance.
Because the sensor, cable and electronics represent a coordinated unit, you cannot simply replace the electronics or shorten the cable.
You can find more information under Mode of operation of the eddy current sensors.
The separate electronic component for operation of the eddy current sensors without integrated electronics is called a converter.
Measurement range of the sensor
The sensor’s measurement range defines the valid range of distance of the sensor to the component surface.
In the case of some conditioning electronics, switching thresholds can be defined in order to, for instance, trigger an alarm.
The gap between the sensor head and the next point of the measurement object is called the minimum gap. The minimum gap must be adhered to during mounting to avoid damaging the sensor and to ensure a flawless measurement function
The specification of the linearity always refers to the coordinated eddy current sensor system consisting of the sensor and the electronic component.
With eddy current sensors, the output values measured depend heavily on the type, condition (inner structure), size and geometry of the material to be recorded. Every sensor is adapted to the object to be measured.
Low temperature sensors MNS:
The MNS low temperature sensors with integrated electronics are adjusted to a precisely defined object at the factory. Steel of the type ST37 is used as the reference material in this case. A square measuring slab of steel (ST37) with a thickness of 1 mm and a polished surface serves as the standard measuring slab. The length of the square is equal to the diameter of the active surface.
The parameters of the sensors apply to the cited reference material, other materials or object dimensions may lead to deviations in the parameters (e.g. measurement range, linearity) and must be compensated with correction factors. Some typical correction factors for inductive devices include: St37 = 1; V2A approx. 0.7; Ms approx. 0.4; Al approx. 0.3; Cu approx. 0.2
MNH high-temperature sensors:
In the case of the MNH high-temperature sensors, the sensor, cable and electronics make up a unit.
In this case, the adaptation to other object materials or geometries is optional;
for MNHCON, a special adjustment can be made at the factory, or
for MNHµCON, adjustment can be made on site via software.
Length of the connection cable
The length of the connection cable for separate conditioning electronics has a non-negligible influence on the measuring result because the cable resistance influences the overall attenuation.
Eddy current measuring chain
The eddy current measuring chain from Messotron consists of an eddy current sensor, a cable and a converter. Because all three components have already been attuned to each other at the factory and the linearization curve has already been saved to the converter, the measuring chain is immediately ready to use.
Voltage range within which the proximity sensor operates safely.
Eddy current sensors with integrated electronics operate on DC voltage. In this case, the maximum and minimum values must not be exceeded, even by the residual ripple.
The active surface of the sensor can be mounted flush in metal.
The active surface of the sensor must be surrounded by a metallic free space.
The sensor frequency is the frequency of the supply voltage of the eddy current sensor.
Quickest (sinewave) change of gap (maximum number of signal changes) at the output within one second. The values specified are determined by a standardized measurement procedure in accordance with IEC 947-5-2 .
Other terms for dynamic range.
Other terms for dynamic range.
Other terms for dynamic range.
Protected against reverse polarity
Internal protection guarding the switch against destruction when switching the connecting cable.
Terms from the field of measuring amplifiers
A measuring amplifier for sensors has three primary functions:
- it provides the sensor with the power supply needed
- it amplifies the output signal of the sensor and
- it converts the output to a standardized analog or digital level.
Carrier frequency measuring amplifier
CF or carrier frequency measuring amplifiers deliver the bridge supply voltage required for operation of inductive displacement sensors in the form of an AC voltage.
DC measuring amplifier
DC measuring amplifiers are direct current measuring amplifiers. They deliver the bridge supply voltage required for operation of resistive sensors (e.g. expansion measuring strips) in the form of a DC voltage.
Carrier frequency (is also called bridge frequency)
Inductive displacement sensors are passive sensors which require a measuring amplifier to operate. The amplifier delivers the supply voltage required for generation of the electromagnetic field in the form of an AC voltage (typically with a frequency of 5 or 10 kHz at Messotron). This frequency is referred to as carrier frequency.
Bridge supply voltage or bridge voltage
The bridge supply voltage is the voltage which must be present on the sensor in order for it to operate correctly. In the case of inductive sensors, the bridge supply voltage is an AC voltage with a frequency corresponding to the carrier frequency. In the case of resistive sensors, the bridge voltage is a DC voltage.
Current with which the sensor is supplied.
The sensitivity represents the relationship of the output voltage of the sensor to the input voltage of the sensor. The measuring amplifier can evaluate the sensitivity in the specified range.
Dynamic bandwidth or cut-off frequency
The dynamic bandwidth specifies the maximum frequency at which an output signal is permitted to change (e.g. through position change of the immersion core) in order for the measuring amplifier to still process it.
The accuracy class of a measuring device specifies the magnitude by which the value measured may deviate from the actual value if the operating conditions (temperature, frequency, position) are to be maintained. The accuracy class corresponds to the percentage of deviation: An accuracy class of 2.5 means, for example, a maximum deviation of 2.5%.
The linearity error in measuring devices corresponds to the maximum deviation between the target characteristic line (straight lines) and the actual characteristic line of the measuring device. The error information is related to the measurement range and is labelled with e.v. (for end value). The linearity error is calculated according to the fixed point, the minimum, or the tolerance range method. How is it done at Messotron?
The calibrator serves to generate the defined mV/V signals in order, for example, to verify the measurement range setting of the measuring amplifiers or to calculate the input sensitivity.
The simulator provides for the continuous simulation of mV/V signals. This can be used, for example, to verify the measurement range setting of the measuring amplifiers or to calculate the input sensitivity.
The temperature coefficient describes the relative change of a physical magnitude dependent upon the change in temperature in contrast to a reference temperature. Generally, there is a linear relationship only within a limited temperature interval.