Worth knowing about displacement sensors with potentiometric technology
Guide for Linear Potentiometers
The simplest conversion of a longitudinal movement into a proportional electrical quantity can still be performed with potentiometric measuring systems. These are absolute measuring systems with a resolution better than 0.01 mm and the well-known advantage that the signals are immediately displayed again in the correct position if the power supply fails. Our linear potentiometers cover mechanical measuring distances up to 2000 mm.
Whether as probe with spring return, with ball joints to compensate lateral misalignment, as slide-guided version or for installation in hydraulic applications, they all have a very high-quality resistive element made of conductive plastic (exceptions are the oil-filled with wirewound element). And despite the variety and diversity of the products, some demanding applications require sensor adaptation.
We at MEGATRON are your partner for this adaptation process and accompany you during the product selection, up to the end of the life cycle of your application. Particularly with potentiometric sensors, special adaptations of the measuring paths, the assemblies and improvements in component precision are often required, because in demanding special applications such as in medical technology, the standard variants often do not meet all requirements.
Basic questions about linear sensors? Here you will find the answers
The measuring principle of the potentiometer was already described by Johann Christian Poggendorff in 1841 and has been used since the beginning of the use of electricity. A potentiometer has a resistance track on the surface of which a movable sliding contact is guided which taps a voltage potential. This resistance element can be circular or elongated.
However, the term "potentiometer" or "poti" for short is used today in technical practice mostly for angle sensors, i.e. for the round version.
Potentiometric displacement transducers, on the other hand, are sensors used for length measurement. These sensors are therefore more likely to be found under the name "linear potentiometers", or under the more general term "linear displacement transducers", when a resistance value is specified, or when it is obvious that potentiometric technology is involved.
For the operation of such a sensor a power supply is connected to the displacement transducer. A wiper is moved on the resistor track by means of mechanics (probe, slide, depending on the version), whose potential changes due to the displacement: The output voltage at the wiper is proportional to the position of the wiper on the resistor track. The resistor track is made of high-quality conductive plastic, which generates a continuously variable output signal and allows actuation speeds of up to 10 m/sec.
- The measuring principle is an absolute procedure, i.e. the same measured value is immediately available again when switching on or after a power failure.
- It is a long proven technology and is easy to handle.
- Only a voltage source with low power is required, because the measured value is taken with low power.
- With potentiometric displacement sensors, displacements in the range from 0 to 10 mm, but also lengths of up to 2000 mm can be measured with a cost-effective sensor and further processed in analogue form.
- Linear potentiometers are very temperature stable in the voltage divider circuit
- Potentiometric displacement transducers are robust against EMC and ESD influences
- We recommend the overhead mounting, so that abrasion does not remain on the resistance element.
- The displacement sensor should not be subjected to excessive vibration, as there is a risk that the slider will briefly lift off the track and the time curve of the measured value will be interrupted.
- The sensors are not suitable for strongly oscillating applications. High frequency movements at one and the same point must be avoided at all costs! This leads to punctual abrasion, loss of signal quality of the resistor track, and the slider can also be permanently damaged.
- Potentiometric displacement transducers with conductive plastic track may only be operated in voltage divider circuit, as the measured value must be taken with low power. In the rheostat circuit the resistor track with conductive plastic is permanently damaged. This means that linear potentiometers should not be used as variable resistors in a circuit.
Potentiometric displacement transducers always have three connections in the standard version, e.g. pins A, B and C. The displacement transducer is supplied with a DC voltage between points A and C. As an example, 10 V is assumed here. The connection A is at 0 V, the connection B thus at +10 V. The measurement of the displacement is based on the principle of the voltage divider circuit, i.e. the moving wiper (connection at pin B) has a certain potential relative to the reference point (A), depending on the position of the wiper between the start and end of the resistance track.
For the example shown (simplified):
- If the displacement path is at the first end position, the wiper is at the beginning of the resistance track, so the voltage at pin B is approx. 0 V
- The displacement is in the middle, the voltage at B is about 5 V
- The displacement path is at the second end position, the wiper is at the end of the resistance track, the voltage at pin B is about +10 V
If the sensors are correctly connected in voltage divider circuitry, the voltage value at point B is independent of the absolute resistance value of the displacement transducer. However, please note here that this illustration is simplified, as different values for electrical and mechanical displacement travel still apply for the operation of the sensor (see mechanical and electrical travel). Furthermore, no sensor works without measurement inaccuracy. The deviation of the measurement result from the ideal straight line is specified by means of linearity specifications.
|Example||Voltage U||Resistance R||Power P||Application possible?|
|1||10 V||1 kΩ||0.1 Watt||yes|
|2||20 V||1 kΩ||0.4 Watt||no|
|3||25 V||5 kΩ||0.125 Watt||yes|
|4||60 V||10 kΩ||0.36 Watt||no|
|5||60 V||20 kΩ||0.18 Watt||yes|
Please take into account the maximum load capacity of the wiper and the resistance element when connecting. This information can be found in the respective data sheet. A too high current through the wiper leads to the immediate destruction of the wiper and/or the resistor track at the current position of the wiper.
The resistance value and the applied voltage are important for determining the power loss with which the displacement transducer is loaded. Here are some calculations using the example of the MM10 displacement transducer:
- max. load capacity 0.2 Watt
- available resistance values 1, 2, 5, 10, 20 or 50 kΩ
The following applies to the power loss:
P = U² / R (power P = voltage U² divided by resistance R)
On the basis of this calculation example it is immediately apparent that at an operating voltage of 20 V the displacement transducer with 1 kΩ cannot be used, because otherwise the power dissipation with which the resistance element is loaded becomes too great.
Please also note to use a very high input resistance when further processing the signal in an electronic system, so that the current via the slider (pin B) is as low as possible. To record the measured values, the voltage difference between pin B and pin A is measured.
The output characteristic curve of most potentiometric displacement sensors is not suitable for sensor operation over the entire available displacement range. A basic distinction must be made between the following displacement ranges:
- Effective electrical limits: This is the actual travel to be used for the measurement. The linearity values specified in the data sheet apply to this travel and continuous operation should only take place within these limits.
- Total electrical limits: Within this range the wiper provides an output signal, but this signal usually does not change at the ends of the travel, but remains constant. This is a kind of "dead zone", which is designed in this way for technical or design reasons and is not available with all sensors.
- Mechanical limits: The actual displacement that the position sensor can perform. Mechanical stops prevent the slider from moving beyond the resistance path on most models.
For many models, the mechanical travel corresponds to the complete electrical travel. This leaves only two of the three ranges described above. Although this simplifies the consideration, the effective electrical travel must still be adjusted to the application when installing the sensor. Otherwise, the described dead range falsifies the measurement results. Please observe the notes on the data sheets of the model you have selected.
Potentiometric displacement sensors with oil filling are available for applications with special requirements on the durability of the sensor. They are the right choice especially for recording measured values at measuring points in filthy, moist or even corrosive environments (salts, corrosive gases). Applications for these sensors are found in heavy industry such as shipbuilding, mines, steelworks, chemical plants and many more.