What Is a Rotary Encoder?
A rotary encoder is a device that measures the amount of movement or angle of rotation of an object to be measured.
Generally, it is attached to the shaft of a motor or reduction gear. They are sometimes attached to servo motors and stepping motors. Rotary encoders are mainly used for rotary devices that require precision control.
Uses of Rotary Encoders
Rotary encoders are widely used in a variety of products driven by motors. The following are examples of rotary encoder applications:
Feedback control of industrial robots
Control of stage equipment in semiconductor manufacturing equipment
Position control of elevators
Speed and position control of self-propelled cranes
Rotary encoders are used for rotational position control and rotational speed control. Simple speed control can be achieved with an inverter only. Encoders are useful for precise speed control or for controlling a motor in the middle of rotation.
Principle of Rotary Encoder
A typical rotary encoder uses light to make measurements. It consists of components such as a light-emitting diode, a slit disk, and a phototransistor.
1. Light-Emitting Diode
The light-emitting diode receives power and emits light constantly. The light is focused by a lens and then directed to the slit disk.
2. Slit Disks
The slit disk is a rotating disk with evenly spaced holes and is fixed to the encoders axis of rotation.
3. Phototransistor
A phototransistor is installed at the end of the light that passes through the holes and emits a pulse wave when the light is received. By measuring this pulse wave, the rotational speed is measured. In addition to light, products that use changes in magnetic force or electrostatic capacitance are also available.
Types of Rotary Encoders
Optical rotary encoders are divided into two types of measurement methods: incremental and absolute. The former measures the relative value of the rotational position, while the latter measures the absolute value of the rotational position.
1. Incremental Type
The incremental type rotary encoder is similar to the above principle, converting light passing through a slit in a rotating disk into a pulse signal and transmitting it. Two types of signals are used to detect the light passing through the slit.
They are generally called A-phase and B-phase. Encoders with a Z-phase signal for home position detection are also available. If there is a malfunction in waveform capturing, the counts will be missed, which is a characteristic that causes errors.
The disadvantage of this method is that the absolute position cannot be determined. However, it is possible to determine the direction of rotation because of the built-in 2-phase signal.
2. Absolute Type
An absolute rotary encoder has a groove on the rotating disk for position information determination. When light passes through this groove, it is detected by the light-receiving element and the absolute position can be measured. Therefore, since absolute position is detected, the direction of rotation can be detected according to the order of the grooves.
In the absolute type, a gray code is generally used for the code of each position. Gray codes, also called alternating binary codes, are a coding method in which adjacent bits change by only one bit. Gray codes are highly resistant to noise and errors and provide high accuracy because they minimize position mis-detection.
How to Select a Rotary Encoders
When selecting a rotary encoder, the measurement method, resolution, and load capacity should be considered.
1. Magnetic Type and Optical Type
There are two types of rotary encoders: magnetic and optical. The magnetic type has excellent weather resistance, while the optical type features high measurement accuracy. Among the optical types, the absolute type has higher measurement accuracy and can detect absolute position.
2. Resolution
Resolution is the minimum phase that can be measured. The higher the resolution, the higher the measurement accuracy, but it is also more expensive, and the signal may be more complex and less sensitive to noise. Select a resolution that is sufficient to control the machine to which the rotary encoder is to be installed.
3. Load Capacity
Load is the weight that can be applied to the rotary shaft. If a load greater than the allowable load is applied, the shaft or bearings of the rotary encoder will be damaged. Therefore, select a product with an allowable load that is greater than the maximum load that can be assumed.
