Encoders – Absolute Vs. Incremental
Positioning tasks require precise position values to monitor or control motion activity. Metal cutting machinery uses encoders to assist in positioning the machine. In many applications position sensing is done using rotary encoders, also called shaft encoders or simply encoders. These sensors transform a mechanical angular position of a shaft into an electronic signal that can be processed by a control system.
Incremental Rotary Encoders
For decades CNC plasma and oxy-fuel cutting machines have been configured with incremental encoders. Incremental encoders are the most commonly used encoders in metal cutting and have the potential to be less expensive. Incremental encoders lose positional information whenever power is removed from the CNC control system or the servo drives. As a result, the machine must be re-homed to a reference point whenever power is turned off. It is impossible to simply press “resume”. This is a big negative, as it does take up additional time for the operator to re-home the machine. After completing the homing process, with most CNC controls the machine operator can press a “resume” key to return to a previous position.
Absolute Rotary Encoders
Absolute encoders are homed by the installing technician when the machine is initially commissioned. From then on, the machine does not require homing – even if power to the CNC or servo drives is interrupted. Absolute encoders provide positional information even for movements that occur while the system is without power. If the power is turned off, it is no problem because accurate positional values are available once the encoder is powered up again.
Park Industries configures all CNC controlled machines with absolute encoders and has been doing so for more than a decade. Park Industries uses absolute encoders on the more than 500 machines shipped each year.
Absolute encoders have advantages from a time-saving, efficiency perspective. There are no limit switches, or shoes because they are unnecessary. You do not have to ‘square’ the machine, as you do with machines configured with incremental encoders.
BENEFITS OF ABSOLUTE ENCODERS:
- No homing necessary after power loss or E-stop
- No interruption of work position on power loss
- No need to re-home the machine daily
- No need to remember or follow unnecessary shut-down or power-up procedures