Rotary Position Sensors: Basic Guide and Specifications

Rotary position sensors are used in a variety of industrial and scientific applications. These devices vary by signal output and angle measurement range, among other things. These factors determine the operational success of a rotary sensor.

A rotary position sensor is an electromagnetic device emitting electrical signals corresponding to the angular rotation of an object along a shaft. These instruments are used in various applications such as the following:

• Position sensing for automobiles like determining the steering wheel position and the pedal position.
• Detecting gear position in different machines
• Damper control for ventilation systems
• Industrial process control systems

In many ways, a rotary position sensor is similar to a rotary encoder, but the primary difference is that an encoder measures complete shaft rotations while a rotary position sensor, on the other hand, measures the angular movement of an object regarding an incomplete rotation around the axis.

Different types of rotary position sensors

These are the different kinds of rotary position sensors used today:

• Rotary potentiometer. A rotary potentiometer is a contacting device consisting of a circular track with a wiper attached to the centre shaft. The device receives a consistent amount of voltage, and as the shaft rotates, the wiper remains in contact with the track resulting in an output voltage variance which is proportional to the angular displacement. Rotary potentiometers are also referred to as voltage dividers which provides a complete 360 degrees rotation measuring all angles during this full turn.

• Hall-effect sensor. The working principle of a hall-effect sensor involves the voltage difference with a magnetic field to produce output about the object's angular position. The element of this sensor is a magnet, Hall element, and a rotating axis. The angular displacement of any of the aspects of the sensor will result in magnetic field changes; hence the corresponding difference in output voltage. A hall-effect sensor is more suitable for applications in rugged environment because it does not require contacting technology, unlike a potentiometer. However, one disadvantage of a hall-effect sensor is the possible interference it receives if there are magnets close to the device.

• Resolver. This device consists of three coils. One of the coils rotates, while the other two remains stationary at a 90-degree distance. The two stationary coils receive current in the same manner as the rotating coil. Comparing the current flow received by the fixed coil and the rotating coil determines the angular displacement of the object. A resolver also works using a magnetic field; hence also subject to sensitivity and interference from nearby magnetic devices.

By weighing the advantages and disadvantages of each type of rotary position sensor, you can determine which device best suits the requirements of the application.

Measurement specifications for rotary position sensors

Apart from comparing the advantages and disadvantages of each type of rotary sensor, the following specifications also help in selecting the ideal sensor to use.

• Measurement accuracy. Each rotary position sensor is given an accuracy rating which is expressed in positive or negative percentile with reference to a full rotational scale. The lower the accuracy percentage of a rotary position sensor, the more efficient it is.

• The maximum angle of measurement. This specification pertains to the highest angle the device is capable of measuring. In selecting a rotary position sensor, the maximum angle refers to the desired angle measurement range while the assigned maximum angle value is the mechanical range of the device.

In addition to the above specifications, rotary position sensors may also vary according to the type of electrical output used to produce data relative to the object's position.

• Analog current rotary position sensor. Analog input may be used to relay the object's angular displacement. Sensors using analogue signal are ideal for applications that require sending signals over a long distance.

• Analogue voltage rotary position sensor. In contrast, with an analogue current output, a rotary position sensor emits signals in voltage range.• Analogue frequency rotary position sensor. The output for this type of sensor is relayed using a modulation scheme like frequency modulation or amplitude modulation.

• Digital output rotary position sensor. This type of sensor transmits signals using parallel and serial signs. Sending signals via serial signs correspond to data sent one bit or byte at a time while parallel signs send multiple bits of data at once.The selection process for rotary position sensors largely influences the success of the installation. To avoid measurement errors and costly replacements, it is essential to choose the instrument that best suits the intended use while also taking into consideration the external elements that will impact the sensor's performance.