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INSTALLING DATUM & LIMIT SWITCHES

When using linear drive systems, it is nearly always necessary to use limit and datum switches which are connected to the motion controller. Limit switches are placed at the ends of the axis and prevent overtravel that may cause mechanical damage or injury. A limit- switch prevents overtravel in the negative direction and a limit+ switch prevents overtravel in the positive direction. These switches are usually normally closed for failsafe operation. When the load hits a limit switch, the motor will stop immediately and motion will then only be permitted in the opposite direction. There are numerous switches on the market that will suit many applications. When using a rotary axis which allows 360 degree motion, limit switches are normally unnecessary.


   


A datum switch is normally placed near the limit- switch but between the two limit switches. This switch is used to define the datum or zero position. Most motion controllers have a datum function on startup that runs a motor in the negative direction until the load hits the datum switch. The motor will then decellerate to rest, overshooting the datum switch. The motor can then creep back in the reverse direction at slow speed until the switch is hit again, accurately reaching the exact position where the switch changes state.

When fitting switches to a machine, it is suggested to allow overshoot and position the datum switch a safe distance from the limit- switch to prevent a limit switch from being hit during a datuming procedure . This is usually achieved using a mechanical ramp.

Cables for limit and datum switches should be rugged enough to avoid damage. Switches should be located in a safe position (usually under a protective cover) and to prevent damage and accidental tripping during operation.

AUTOMATED MOTION SYSTEMS PTY. LTD. - WESTERN AUSTRALIA