About homing and switches...

[paulus.v]

Hi JW,

Let me try to clarify what the homing on index function on a servo motor is.

The incremental encoder on a servo motor generates pulses when the motor moves. The motor driver is capable of counting the pulses and store them on power off but if, for example, you move the motor with the power off or by a power failure where the axis moves from inertia, that axis position is lost. So the absolute position of a cnc machine cannot be determined reliable from the servo incremental encoders data and homing sensors are needed.

Most incremental encoders have an additional "index" signal that provides a reference position of the rotor. It is output at each motor revolution. This signal can be used to increase the accuracy of the homing to an almost perfect and repeatable position. The homing on index routine moves the axis until the home sensor is triggered then rotates the motor to the index position. In this case the sensors accuracy need only to be grater than the axis movement of half a motor revolution. And the sensor be mounted so that when triggers the motor shaft is at about 180 degrees from the index position. Depending on the actual setup, the amount of axis movement per motor revolution, switches could have enough accuracy for homing on index.

Why is homing important?

First of all because if you have homing and set each axis maximum movement, the machine knows its limits! And will never hit a limit switch. If you load a g-code that exceeds the machine working area you get a warning. Otherwise you will only find out that you made a mistake when the axis hits the limit and ruins your work because...

Secondly, if something bad happens and you have to cut power to the machine you loose the workpiece position. If you have accurate homing, the workpiece zero position is referenced to the machine zero (home). If those numbers are stored you can easily resume the work after a power failure or if you want to pause and continue the next morning... just home the machine and go to the stored workpiece coordinates. And continue cutting where you left off.

Paul.