SS4544 SyncroStep Motion Control System
Application Note 102 - Homing Systems
Testra Corporation  1201 N. Stadem Drive Tempe, AZ 85281 Ph:(480)560-6141 Fax:(480)907-2876
Updated 2/3/09


    The SS4544 firmware has a built in parameterized homing system that is very generalized and can handle most homing situations without further programming.  Utilizing opto-interruptors and a simple metal flag that passes through the opto during deceleration, a homing system may be constructed that allows homing at a rapid rate, and will be repeatable to the accuracy of the machine.  Mechanical homing switches may also be used.  It is recommended that switches with considerable overtravel be used, otherwise you will have to home at a very slow rate.  Your motion system must decelerate within the overtravel distance or something will break.

1. Recommended - Flag with Over-Travel

    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 rapid homing. 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. A diagnostic routine allows the flag to be tweaked, in order to give the greatest margin for getting to the proper home position. This system only needs one flag input.

    The flag may be placed anywhere within the mechanical travel limits of the axis, but to minimize homing time, it should be placed near either end of the travel limits.  The width of the flag only needs to be greater than the deceleration distance at the desired homing speed.   If the width of the flag is made large, the situation arises that if you try to home when near the end of an axis while under the flag, the system will move slowly to try to find the edge of the flag with the standard firmware.  Otherwise custom software would have to be written to accelerate out from under the flag, stop, then do the standard homing as described above.  Note that the flag has to be long enough so that the opto cannot get behind it and get trapped, otherwise you will have situation 3 below.

2. Flag with Over-Travel + Index

    This is the same as above except back-off is until an index pulse (usually from an encoder) is detected. There is no real advantage to this system in home positioning, and has a disadvantage in that homing speed is restricted to that in which deceleration can occur within the home flag position and the second index pulse.. There is extra complexity due to the extra flag input and index sensor.   This method is not implemented as a standard routine, so would have to be either written by the user or implemented by special request.

3. Flag that can be trapped and hard limits

    Hard limit sensors may be placed at both ends of the axis and logically wired in parallel (one extra I/O plus two sensors). Operation is pretty much the same as the prior methods except that if the hard limit is hit, then the homing direction is reversed to get the flag under the home position. The hard limits will still need some over travel, otherwise the homing will have to be done at a slow crawl. Therefore you are really back to case 1 with a bunch of extra junk, unless you just don't "trust" software limits.  This method is not implemented as a standard routine, so would have to be either written by the user or implemented by special request.

4. 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  positiion 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 interruptor, 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.

If you have comments or suggestions, email us at
Testra Corporation   1201 N. Stadem Drive  Tempe, AZ 85281  Ph. 480-560-6141  Fax: 480-907-2876