Encoders For Servo Motor Overview


There are two types of encoders: absolute and incremental. An absolute encoder produces a pulse train, which indicates the change from one step to the next. The advantage of an absolute encoder is that the signal does not matter, only whether it increases or decreases in its rate of signal output. Incremental encoders encode displacement by producing a pulse train that moves in a clockwise or counterclockwise direction with each revolution. This helps eliminate sideways movement errors caused by acceleration, deceleration, vibration, and shock. With this type of encoder, information is also encoded into position as well as velocity. For example, when the encoder changes direction, it sends a signal that indicates that it has turned 90 degrees. The part of the motor is then calculated as a function of time. It is possible to send feedback information from the engine back to the encoder so that no position reference is needed. However, in most cases, this type of feedback is not used because it offers no advantage over accurate rotary potentiometer or tachometer feedback used on the encoder input to determine position.

What is a servo motor?

A servo motor is identical to a DC brush or permanent split-capacitor (PSC) motor. The only difference is in its electrical characteristics. A servo motor contains a built-in feedback circuit that lets it respond to commands from its controller to keep the shaft at a precise angular position while avoiding mechanical interference between the moving and stationary parts of the system.

Servo motors have been used for decades in applications such as robotics, machine tools, and various types of automation because they maintain position without regard for load or speed variations. Their use has increased as manufacturers have begun making them smaller, lighter, and more energy efficient.

What is an encoder?

An encoder translates relative movement into an electronic signal that can precisely measure the angular position of a spindle or shaft. An encoder for a servo motor uses an optical sensor to detect a pattern of marks on the surface of a rotating drum attached to the engine. The effects are typically evenly spaced and appear as a series of bright and dark lines when viewed through the sensor. As the drum rotates, a “clock” edge sweeps across the sensor, generating pulses proportional to angular position. In some configurations, multiple sensors provide full 360° coverage with only one mark required per revolution. Position feedback information is then sent out using an RS-422 serial interface.

How are encoders used on servo motors?

An encoder attached to a servo motor is used with a controller, computer, and special software that knows the mechanical setup of the system and the relation between the actual position and the position commanded by the controller. This “closed loop” control method gives precise position feedback to control motors and other electromechanical systems for accuracy and precision.

A PC or personal computer can run the software that controls these servo systems. Still, more specialized controllers are now available from companies that perform this function with greater reliability.

The closed loop concept maintains continuous two-way communication between the controller and encoder. The controller gets feedback on how far it has moved from its zero point at all times as it moves toward the desired position.

Choosing the right encoder for your servo motor

The right choice of the encoder is very important for normal operation. It always combines accuracy, ease of use, and price.

In the following table, we present some basic characteristics of encoders (patents and descriptions), such as encoder types, resolution, and other parameters that affect the quality of signals when measuring position. These parameters are extremely important for correct operation in servo systems:

General information about Encoders for Servo Motor

When specifying an encoder for a servo system, it should be clear whether it is absolute or incremental.

An absolute encoder changes the output values of the motor when an end stop or a limit switch is activated.

An incremental encoder provides position feedback by measuring the rotary shaft and converting it into an electronic signal with the help of a microcontroller. The controller then uses this output to adjust position, velocity, or both.

In some applications, one member of a pair of actuators may be controlled by one encoder while another owns another member.

These are important characteristics because they determine how accurate a position measurement will be:

Quality (resolution) is especially important when operating on microcentering servo motors. This fine servo motor moves to the center of its operating range when power is applied, resulting in more accurate positioning.

The simplest incremental encoder is a summing box. This type of encoder has an internal circuit that converts the signal from each sensor into an analog voltage for use with either hardware or software controllers. Each position resolution value has its analog signal from the encoder. The summing box combines the signals from all sensors and generates only one output signal to simplify wiring and minimize noise sensitivity. The output signal is usually low pass filtered and amplified but can be amplified “naturally” from the motor’s mechanical properties.

Most modern encoders offer high resolution over a wide range of speeds and loads. One sensor type allows measurement at any speed and load, while another is suitable only at a specific rate or load.

An encoder with two sensors provides a faster response to changes in position and eliminates the need to switch gears or speeds. This type of encoder allows an absolute position measurement even when the encoder is stationary, as long as no change in speed or load occurs, and the system requires continuous measurements for positioning accuracy.

Servo Motor Encoder Resolution

The resolution of an encoder is how many position values it will measure in the same time it takes to complete one motor revolution. The higher the resolution, the more accurately a given motor can be controlled by a given controller.

An encoder’s resolution rating is usually expressed as a multiple of either inch per foot (.0002″/ft) or degrees per second (0.00001°/s). The higher the number, the higher the accuracy required by a given application.

The accuracy of an absolute position measurement depends on what gear or gear train is used and how fast and at what speed it turns.


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