An encoder is a sensor that is fixed to a rotating object such as wheel or motor to measure the rotation of the object. This measurement can help robots to determine the angle of a rotating sensor, velocity, acceleration and displacement.
A typical encoder employs one or more optical sensors - a special reflector for producing electrical pulses that are transmitted to a microcontroller. The electrical pulses can be used in PID feedback control system so that the position of the moving robot can be determined.
Functional principle of a rotary encoder by Richard Hewitt. Video made for the LearnCNC project at the University of British Columbia
Types of Shaft Encoders
Listed below are different types of shaft encoders used in the robotic system:
- Absolute shaft encoder – This encoder can determine the absolute position of an encoder shaft. This encoder employs magnetic, optical or mechanical sensors along with a rotating disk to evaluate the shaft position. The position data can be output in either an analogue or digital format.
- Quadrature encoders – This encoder, also known as incremental rotary encoder, measures the relative shaft movement with the help of two optical or mechanical sensors that sense the rotation of the shaft from one angle to the next. It also includes an external circuit to count shaft movements from a reference point in order to keep track of the current position of the shaft.
- Optical rotary shaft encoder – This encoder can rotate at high speeds up to 30000 rpm. It consists of a disc, a light source and a photo detector array that reads the optical pattern code resulted from the disc. This code is read by a microcontroller or microprocessor to evaluate the shaft angle.
The encoder that is used to transform angular movement into digital impulses includes a shaft, flange, output connector and cylindrical body.
During operation, an internal transparent disk provided with photographically printed radial markings is rotated which interfers with an infra red light source produced by an LED component.
However, a phototransistor present on the other side of the disk converts light beam into electrical pulses.
The pulses are amplified and used for determining the speed/direction of rotation or angular displacement of the robot.
The following are the major applications of shaft encoders in robotic systems:
- Determination of shaft rotational direction
- Navigational control
- Measurement of angular travel
Products on Market
Some of the shaft encoders that are currently used in robotic systems include:
Optical Shaft Encoder (2-pack)
Vex Robotics’ Optical Shaft Encoder is a continuous mechanical rotation type encoder consisting of an infrared light sensor and encoder wheel having 90 slits. It can be used for measuring both relative position of a shaft and rotational distance traveled by the shaft.
MA3 Miniature Absolute Magnetic Shaft Encoder
Developed by US Digital, this encoder measures the shaft position over 360° without any gaps.
This instrument is available in three shaft torque versions, and the standard torque version includes a sleeve bushing lubricated with a viscous motion control gel for providing torque.
Sources and Further Reading