By Josh Cosford, Contributing Editor
Encoders are electronic devices manufactured to relay movement and position. For example, for fluid power applications, the decoder relays an electronic signal to a PLC or other electronic device for the positional measurement of hydraulic cylinders or motors.
In most cases, encoders use a light shone through a coded plate with gaps allowing a sensor on the opposite side to read the light flashes. There are two primary types of encoders; the incremental style and the absolute version.
With the incremental encoder, the output signal depends only on the rate of flashes to provide the PLC or controller with a velocity signal but only relays movement and not position. Hydraulic motors best take advantage of incremental encoders to provide an output of speed in rpm.
Any motor with a specific speed requirement will use an encoder to ensure precise angular velocity, such as machine tools to control spindles for grinders, mills or boring machines. Although currently somewhat rare, encoders make perfect wheel speed sensors for on-or-off-highway construction, agriculture, and mining equipment. Wheel speed is essential for sophisticated vehicle dynamics, not just road speed. Controls such as anti-skid and traction control require accurate rpm feedback from each drive wheel, helping maintain safe on-road operation as well as off-road traction and stability.
For repeatable and precise control over movement, absolute type encoders are a must. The absolute encoder uses a different code plate, multiple lights, and sensors to create a unique light pattern to precisely show the actuator’s position. As a result, the PLC always knows the actuator’s exact position. Even when the machine is turned off or disconnected entirely, the position will be again known when powered up.
Linear encoders for hydraulic cylinders come in two ways — slides with a measurement unit that moves the length of the actuator or the string potentiometer that retracts into its own body. The slide-style linear encoder is challenging to install into a hydraulic cylinder, so it must be mounted on the machine and attached to the moving part of the rig. However, the string potentiometer could be mounted to the back of the cylinder and fixed on the rod attachment. The spring keeps the wound potentiometer taut and only extends when the cylinder does, keeping the assembly compact.
An absolute encoder works well to measure the angle of a rotary actuator because the code plates may be manufactured as discs or rings. Hydraulic rotary actuators offer high torque over a limited range (typically less than 360 degrees of side-to-side movement). In addition, precise control of the position for robots, material handling or gate valves may require precise action, making the encoder the ideal choice to guarantee accuracy.
Although encoders are still relatively rare in fluid power applications, technological advancements produce cheaper and more reliable solutions to traditionally more expensive or complicated options. However, when combined with traditional hydraulic sensors like pressure transducers, both force and velocity/position feedback encourage sophisticated and powerful machine control.