SS4544 SyncroStep Motion Control System
Application Note 111 - Dual Axis Drive Setup
Testra Corporation  1201 N. Stadem Drive Tempe, AZ 85281 Ph:(480)560-6141 Fax:(480)907-2876
Updated 2/2/09


    Often with larger gantry style machines, one would like to use two separate motors to drive each side of the gantry rail.  This has advantages in eliminating either a drive shaft or belt and gear pulleys between the two sides.   It can make your mechanics simpler and easier to maintain, while giving improved performance.  The difficulty that arises though is keeping the two sides aligned so that your x-y area does not become a parallelogram.  I have seen some machines in which both motors were connected either in series, or parallel and driven with one driver.  They used adjustable mechanical stops, and the operator would have to manually push the gantry assembly against both stops before powering up the machine.  This is somewhat a kludge, but might be satisfactory for some engraving systems where accuracy of alignment would not be critical.

    With the ss4544 system, both motors may be driven independently during the alignment and homing phases, then electronically locked in exact relationship with each other.  This makes the alignment function easily implemented with software control, simplifying setup and maintenance, and allowing very good alignment between the two sides.  .

1. Mechanical Considerations

    The two motors may be mechanically connected to drive the two sides of the gantry system in any convenient fashion.  Some factors to consider in determining if the resultant system will accurate enough are:

    a.  Backlash
        The current software does not treat the two sides independently.  Once the two axes are locked, the motors will move exactly the same.  Therefore you need a very low backlash drive system, or you will have to use the average backlash of the two sides for compensation.  If a customer seriously needed independent backlash compensation, it could be included but there would be a non reoccurring engineering (NRE) fee involved.

    b.  Lead Screw Compensation
        The current software does not treat the two axes independently.  Once the two axes are locked the motors will move exactly the same.  Therefore you will need well matched lead screws for the two sides if that is what you are using. .

2. Homing Switches and Flags

    Both sides need to be independently homed.  It is recommended that homing switches be placed near either end of axis travel, and that they both be relatively matched.  The switch/flag setup needs to be such that once the switch is engaged, it is not mechanically possible to dis-engage without reversing travel direction.  In addition the flag width allowing overtravel needs to be large enough to allow for the deceleration from homing speed to a stop.  The flag width is also an axis setup parameter.

    This method allows for rapid homing. When properly designed, the system cannot be trapped by letting the flag get behind the home sensor, and there will be room to decelerate from a rapid homing speed. Upon receipt of a home command, if the home slot is not covered, the axis is accelerated toward the home sensor at the homing speed, and when detected is decelerated to a stop. The homing speed is one of the axis setup parameters. The home sensor will now be covered. The axis is now moved slowly away from the home sensor until the home sensor is not covered.  This speed is currently hard coded and there is no parameter setting. The system then moves the motor to the center of a pole on the step motor for the actual home position. The axis is then moved to a user defined starting position which is also one of the axis setup parameters.

3. Homing Routine for Dual Axis Drive 

  When the homing routine is properly setup the following will then occur on the dual drive axis, if for example the R-axis of the ss4544 is used for the second motor, and is locked with the Y-axis motor.  This may be set in ssDEMO by going to <Motion Setup>/<Axis Locking>/<Y-Locking with> and select (R-axis).  Make sure on this menu that all of the logical axes match with their physical axes. Next set up normal homing parameters for all of the axes used on your machine.  Make sure that the R-axis homes just after the Y-axis.  

   a. Y&R will both be locked at their current position, and homed using the Y-lim home flag.  This will cause both sides to move together and align with the specified Y starting position.  The Y starting position will be specified as the same as the expected flag position of the R-lim home flag.

   b. R will next be homed independently of Y, causing the other side to be aligned with the R-lim home flag.  R will then be moved to an exact square alignment position, at which time both the Y and R axes become locked with each other again.

4. Aligning the two sides
    You can accomplish the alignment either mechanically or by software.  Each has its advantage or disadvantage.  You can make the second flag (R-axis for this example) mechanically adjustable.  The advantage is that you can use the same MPM file on any machine of the same type.   If you chose to make the adjustment in software, a different MPM will be needed for each machine.

    In any event once you have the flags positioned and the homing sequence is working properly, you should check for how much your machine is out of square.   One method would be to draw a rectangle using the flag  position as one of the extents of the rectangle.  You could also drill holes at the four corners of the square.  Next measure the diagonal distance between opposite corners.  The two measurements should be identical if your machine is square.  Otherwise you can use these measurements to calculate how far to offset the R-axis position to square it up.  This offset should then be added to the start position parameter of the R-axis.  If you are making a mechanical flag adjustment, you need to move the actual flag position by this amount.

5. Diagnostic Routine

    A built in diagnostic routine in the ss4544 allows the flag position in relationship to a motor pole to be checked.  If the ssDEMO program is used to setup all of the motion parameters for the ss4544, the flag positions may be tested in the <Hardware Diagnostics>/<Evaluations>/<Evaluate-?>/<?-Flag Check> where ? is the particular axis you are going to test.
A motor pole is 4 steps wide.  Ideally we would like the mechanical position of the flag to exactly line up with the first step  position of a motor pole.  This would give +- 2 steps margin for error.  If the flag was mechanically adjusted to be lined up exactly with  the third step position of a motor pole there would be no margin for error. In fact with a tiny error you wouldn't know which pole would be moved to for the home position.
    The Flag Check routine tests to see that the flag is within +-1 step of the pole position.  If it is, a green indicator is set. If not the indicator is red.  If red, the flag position should be mechanically adjusted to land within +-1 step of a motor pole position.  With fine pitch lead screws, making the step size very small, this adjustment may be somewhat difficult.  If metal flags are used along with an optical interrupter, this can usually be done by bending the flag a small amount.  If you want to use the ssCNC feature of checking for lost steps at the end of a job, the flags must be adjusted properly, otherwise you might get a false indication of lost steps (will always be 4) when in actuality everything was fine.
    In testing the dual axis drive system flags, for this example you must first test the y-axis flag and get it adjusted.  At this point you must leave the y-position where it is, then go to the r-axis evaluations and test the r-flag.  When in the R-axis Evaluations you must be careful, because any commands given here are totally independent of the y-axis, so the possibility exists that you could try to move too far away from the proper relationship of the two axes and they would be fighting each other.

This page was last updated 8/17/01
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Testra Corporation   1201 N. Stadem Drive  Tempe, AZ 85281  Ph. 480-560-6141  Fax: 480-907-2876