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4. CONTROL OF VIX DRIVES 79
Homing
The term ‘homing’ refers to an automatic return to a mechanical reference position which is
usually performed when the system is first powered up. All subsequent moves will then be
relative to this reference position. The home position is usually determined by an optical or
proximity switch, though a mechanical switch can also be used.
Definition Of Terms
To aid the description of homing operations the following terms are defined:
Positive motion - is motion towards the positive limit
Home switch positive edge - is the edge of the home switch on the positive limit side
Home switch negative edge - is the edge of the home switch on the negative limit side
Home switch operating range - is the distance moved whilst the switch is operated
Four of these terms are illustrated in Figure 4-7.
Positive
edge
+CW
Negative
edge
-CCW
NEGATIVE
LIMIT
POSITIVE
LIMIT
HOME SWITCH
OPERATING RANGE
DISTANCE
POSITIVE
MOTION
Figure 4-7. Home Switch Operation
Switch Considerations
Any type of switch will have an operating range that may correspond with a significant
motion of the motor shaft, depending upon the gear ratio between motor and load.
Consequently, just detecting the home switch voltage level will not give a well defined home
position. To improve the accuracy it is possible to stop on either the positive or negative
edge of the home switch.
Switches generally exhibit a hysteresis characteristic when operated from opposite
directions, therefore homing moves always make the final approach to the home switch from
the same direction.