Appendix D DC Motor Operation - Background
D.2
Positioning a Stage
D.2.1
General
Whenever a command is received to move a stage, the movement is specified in
motion units, (e.g. millimetres). This motion unit value is converted to encoder counts
before it is sent to the stage by the APT software.
Each motor in the system has an associated electronic counter in the controller, which
keeps a record of the net number of encoder counts moved. If a request is received
to report the position, the value of this counter is converted back into motion units.
D.2.2
Home position
When the system is powered up, the position counters in the controller are all set to
zero and consequently, the system has no way of knowing the position of the stage
in relation to any physical datum.
A datum can be established by sending all the motors to their ‘Home’ positions. The
‘Home’ position is set during manufacture and is determined by driving the motor until
the negative limit switch is reached and then driving positively a fixed distance (zero
offset). When at the Home position, the counters are reset to zero thereby
establishing a fixed datum that can be found even after the system has been switched
off.
See Section 4.4. for details on performing a Home move.
D.2.3
Limit Switches
A linear stage moves between two stops, and movement outside these limits is
physically impossible. Linear stages can include stages that control the angle of a
platform within a certain range, although the movement of the platform is not really
linear but angular. Rotary stages can rotate indefinitely, like a wheel.
Linear and rotary stages can contain microswitches that detect certain positions of the
stage, but they differ in the way these switches are used.
All linear stages have a –ve limit switch, to prevent the stage from accidentally being
moved too far in the –ve direction - see Fig. D.2. Once this switch is activated,
movement stops. The switch also provides a physical datum used to find the Home
position. Some linear stages and actuators also have a +ve limit switch (such as the
ZST range of actuators), whereas others rely on a physical stop to halt the motion in
the positive direction. A rotary stage has only one switch, used to provide a datum so
that the Home position can be found. Movement is allowed right through the switch
position in either direction
