Classification of Common Encoders and Working Principles of Each Type

A rotary encoder is a type of sensor primarily used to detect parameters of mechanical motion, including speed, position, angle and distance. The application of encoders for signal detection has been widely adopted across various industries.

The operating principle of the rotary encoder is as follows: when the shaft of the rotary encoder drives the grating disc to rotate, the light emitted by the light-emitting element is cut into intermittent light beams by the slits on the grating disc, and is received by the receiving element to generate an initial signal. After being processed by the subsequent circuit, the signal is output as a pulse or code signal.

Rotary encoders feature small size, light weight, diverse types, complete functions, high frequency response, high resolution, low torque, low power consumption, stable performance and long reliable service life.

Common categories of rotary encoders comprise incremental encoders, absolute encoders and sine wave encoders. What are the differences among these three?

1. Incremental Encoder

When the shaft of an incremental encoder rotates, corresponding phase signals are output. The determination of rotation direction and the increment and decrement of the pulse count are implemented by the subsequent direction discrimination circuit and counter. Its counting starting point can be set arbitrarily, supporting unlimited pulse accumulation and measurement across multiple revolutions. In addition, the Z signal — which generates one pulse per revolution — can be used as the reference mechanical zero position. When the base pulse count is fixed and higher resolution is required, frequency multiplication can be applied to the original pulse count using the two channels of signals A and B with a 90° phase difference.

2. Absolute Encoder

When the shaft of an absolute encoder rotates, it outputs position-corresponding codes (binary, BCD, etc.) in a one-to-one mapping. The rotation direction (forward/reverse) and current displacement position can be identified directly from changes in code values, with no need for a direction discrimination circuit. It is equipped with an absolute zero-position code. When measurement restarts after a power outage or equipment shutdown, the position code at the moment of power failure or shutdown can still be read accurately, and the zero-position code can be reliably located. In general, the measuring range of a standard absolute encoder is 0° to 360°, while special models are capable of multi-turn measurement.

3. Sine Wave Encoder

The sine wave encoder falls into the category of incremental encoders. Its core difference from standard models lies in that its output signals are sine wave analog signals rather than digital signals. It was developed primarily to meet the demands of the electrical industry, serving as a feedback detection element for electric motors. Compared with other encoder systems, this type of encoder is suitable for scenarios where improved dynamic characteristics are required.

How to select a rotary encoder in practical applications?

To ensure reliable motor control performance, the feedback signal from an encoder must deliver a high volume of pulses. Particularly at very low rotational speeds, generating a large number of pulses with a conventional incremental encoder presents challenges in multiple aspects. When the motor operates at high speed (6000 rpm), transmitting and processing digital signals becomes difficult.

In this scenario, the bandwidth required for signals supplied to the servo motor — for instance, when using an encoder with 10,000 pulses per revolution — can easily exceed the MHz threshold.

On the other hand, adopting analog signals greatly alleviates the above issues and can achieve the equivalent of a high pulse count from encoders. This is enabled by the interpolation of sine and cosine signals, which provides a calculation method for rotation angles. This method supports high-rate multiplication of the base sine wave signal: for example, a sine wave encoder with 1024 pulses per revolution can deliver over 1,000,000 pulses per revolution. The bandwidth required to receive such signals only needs to be slightly higher than 100 kHz. The interpolation and frequency multiplication are completed by a secondary processing system.

For end users, selecting an encoder with matching performance is essential. It is advisable to fully understand the characteristics of each type of rotary encoder to identify the most suitable option for actual use.