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Did You Know? New Sensorless Homing Commands

Oct
20
by Matt Cole

Applied Motion Products stepper and servo drives have supported sensorless homing for several years. Also known as “hard-stop homing”, there is no need for a hard-wired home sensor (hence the name sensorless) when using this method because a physical hard stop is being used to stall the motor, create a position following error, then automatically recover from the fault by moving off the hard stop and setting the home position. Historically, this procedure required two Q segments to accomplish: one segment to set the initial parameters and execute the move into the hard stop and the other segment to recover from the fault and declare the home position.

Now with the addition of our StepSERVO™ closed loop integrated motors and SV200 series servo drives, a set of new homing commands simplifies the process of sensorless homing with the following commands: HA (Homing Acceleration), HL (Homing Deceleration), HV (Homing Velocity), HC (Hard Stop Homing Current limit), HO (Home Offset), HS (Hard Homing execute). This new HS command allows for hard-stop homing to be completed in one Q segment with fewer commands.

The Home Offset (HO) command is not only used to indicate the offset distance from the initial home position, but also to define the direction of travel when homing starts; a negative HO value causes the motor to start in the CCW direction. Additionally, the HC command is used to set the maximum motor current (controlling the applied torque) that will be used as an indicator for when the hard stop has been reached.

The following command sequence and homing diagram show an example of hard-stop homing to an index pulse using the new Hard Stop Homing command with a parameter of 1 (HS1):

Homing diagramHA1100
HL1100
HA2100
HL2100
HA3100
HL3100
HV15
HV25
HV30.5
HC0.5
HO25000
HS1

 

In the example above, the search for the mechanical end-stop begins once the HS1 command is executed. The HV1 speed of 5 rps is used along with the HA1 and HL1 acceleration and deceleration rates of 100 rev/sec2. The start direction comes from the sign of the HO command (“-” is CCW, no sign is CW); in this case the Home Offset is 25000, so the starting direction will be clockwise as viewed from the shaft end of the motor.The motor will stop when the actual current equals the preset Hard Stop Current (HC) of 0.5 amps once the actuator presses against the end-stop. The parameter for HS is 1, which means that the motor will seek the first encoder index pulse after the hard stop has been reached and direction reverses; the HV3 speed of 0.5 rps is used when seeking the index pulse in this example. Finally, the offset move of 25,000 counts will be made using the HV2, HA2, and HL2 values. Once the command sequence has completed, the same commands as before can be used to set the home position (EP and SP).

Other homing-related commands have also been added: FH (Find Home) and EH (Extended Homing). For more information on these commands, take a look at the built-in Q Programmer in the new Step-Servo Quick Tuner software and the SVX Servo Suite software, which are both free for download. Also, keep an eye out for an updated version of the Host Command Reference, which will contain these homing commands and other new commands that are available in Applied Motion’s new servo-based products.

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