Position Sensors

Position sensors

Position sensors are used in robotics for sensing and controlling the arm position. These types of devices are available in plenty and the three most widely used devices are:

  • Encoders
  • Potentiometers
  • Resolvers

Encoders:

Encoders are used for converting the angular or linear displacement into the digital signals. Some types of encoders are:

  • Linear encoders: It is used to calculate the directions and positions that are in the linear form. Example: Linear motors.
  • Rotary encoders: It helps to calculate the directions and positions that are in the angular form. Examples: Motor.
  • Incremental encoders: It is used in robots for sensing the position from its last position.
  • Absolute encoders: It brings a definite position that is proportional to a fixed reference position.

The incremental encoders have a glass disk, which is noticeable with discontinuous stripes. The disk includes a phototransmitter on one surface and a photoreceiver on the other surface. As soon as the disk starts rotating, the light beams are finished alternately and broken down. The photoreceiver gets the output as a pulse train, and its frequency is proportional to the rotational speed of the disk.

The construction of absolute encoders is almost same as the incremental encoders. The difference is that it adds many stripes and as well as photo transmitters and receivers. It is mostly used to determine the absolute position of a part. The stripes are set to give a binary number that is relative to the shaft angle. The resolution of this type of encoder can be given by:

Resolution = 2n

The above equation describes n as the number of the tracks on the glass disk.

Potentiometers:

Potentiometers produce an output voltage that is proportional to the position of the wiper. They are the analog devices used for calculating the linear or rotary movements based on the design. It consists of a rotating wiper, which is in contact with a resistive element. This wiper is connected with an object that is motion. An AC or DC voltage is supplied to the resistive element. The wiper and ground voltage is proportional to the ratio of the wiper’s one side resistance to the resistive element’s total resistance.

Potentiometers are basically a voltage divider system. The wiper separates the voltage of the resistive element into two parts. By measuring the voltage obtained, it is possible to find out the position of the wiper.

Resolvers:

A resolver is also an analog device as like potentiometers, and it is a rotary electrical transformer basically implemented for calculating the degrees of rotation. It requires only AC signal for excitation because the use of DC current will not produce any output signal. The output signal of a resolver is proportional to the angle of rotating element with respect to the fixed element.

The resolvers have a stator with a pair of windings and a rotor with one winding. The sinusoidal electric current excites the rotor winding, and hence the electromagnetic induction makes the current to pass through the stator windings. By this process, it generates a cosine and sine feedback current with the stator windings, which are fixed at 90o. The magnitudes are measured to find out the angle of the rotor that is proportional to the stator windings.

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