Specifications

Program Example 3: Automatic Trapezoidal Profile with Home Search

One of the more challenging aspects of motion control is the establishment of actual position

at powerup. This is especially true for open-loop systems which do not have a position

feedback device. However, a simple limit switch located at an exact location on the

positioning mechanism can provide “position feedback” at one point. For most stepper

control systems, this method is a good and economical solution.

In the drawing above, the load moves left or right depending on the CW/CCW direction of

motor rotation. The PLC ladder program senses the CW and CCW limit switches to stop the

motor, before the load moves out-of-bounds and damages the machine. The home limit

switch is used at powerup to establish the actual position. The numbering system is arbitrary,

depending on a machine’s engineering units.

At powerup, we do not know whether the load is located to the left or to the right of the

home limit switch. Therefore, we will initiate a home search profile, using the registration

mode. The home limit switch is wired to X1, causing the interrupt. The example, beginning

on the next page, preferentially starts in one direction only (CW), and pulses until it reaches

the first Overtravel Limit (CW Limit). It will ignore the Home Switch if it passes it. This

means that Homing is always accomplished from the same direction for better consistency.

The CPU will then reverse the direction of pulses (loads 80000200 into V3631) and travel

away from the CW Limit until it hits the Home Switch, then travels past it and reverse slowly

back to the Home Switch. A value of 0 will be loaded to V1174 (CTA174) to represent the

Home position.

DL06 Micro PLC User Manual, 3rd Edition, Rev. C

E–53

Appendix E: High-speed Input and Pulse Output Features

Motion

Numbering System

Motor

Load

CCW limit (X0) CW limit (X2) Home limit (X1)

0 1000 2000 3000-1000 -2000 -3000

Limit Switches

Positioning System