- Linear taper potentiometer: In this the device has a resistive element of constant cross-section. It does not describe the geometrical characteristics of the resistive element however it explains the electrical characteristics of the device. In this type of sensor the resistance between the wiper and any one end of the terminal is proportional to the distance between them.
- Logarithmic potentiometer: The resistive element that it has is either tapered in from one end to the other or the potentiometer is made of a material with varying resistivity from one end to the other. It is called as a logarithmic potentiometer because in this the output voltage is a logarithmic function of the slider position. They can also be simulated with a linear potentiometer using an external resistor, however this simulation would not result in a very accurate device. These type of sensors are more expensive.
Finally, they can be classified on their working style-contact and non-contact. All the types of potentiometers discussed above can be classified under these categories:
- Contact type: These potentiometers consist a moveable contact on an electrically resistive component. All the earlier versions of this sensor were made from a resistive wire (like Nichrome) that was wound around an insulating former. The moveable contact that acts as a wiper, makes some contact with the wire that is wound and slides along it. The wiper acts as a variable resistor also known as Rheostat. Thus this sensor works on the principle of changing resistance of the wire with respect to its length. The resistance of the wire is considered to be directly proportional to its length, so if the length increases, resistance does too. In order to measure the angular displacement, only two terminals of a potentiometer are used. It works on the principle that resistance changes with change in angle. The variable value of resistance helps us to find the actual angular displacement.
“Due to its working principle it is also sometimes referred to as Resistive Potentiometer. It is a very special resistor with three terminals”.
- Non Contact type: These potentiometers have a pair of planar magnets that are disposed opposite yet parallely to each other on a magnetic yoke. The polarity of these magnets are arranges in such a way that they form parallel magnetic fields around magnetic detection element. This detection element is retained on a support shaft that connects to a rotary shaft on an actuator and can rotate freely relative to the stationary one. As soon as this rotary shaft is rotated, there is a change in the output of the magnetic detection element which allows the detection of the rotation angle of the shaft. The non-contact types of potentiometers are considered over the contact type due to their long life and reliability. These sensors are extremely small in number due to which Hall effect position sensors are used as their replacement.
How to Select?
Now let us suppose a situation where we enter a shop to buy a potentiometric position sensor and find ourselves in a dilemma of how to select one, as the shop has so many options and we do not want to buy something that would not be accurate as per our requirement. This is a real difficult situation because the shopkeepers cannot help us unless we tell them our requirements. In such situations a good understanding of the different parameters of the device helps a lot. In order to select a potentiometric position sensor following parameters should be considered are as follows.
Linear or Rotary?
This defines the motion of the sensor in order to identify it’s track of movement. It helps to identify if the sensor would be useful in calculating the angular displacement or the linear displacement.