Introduction to Backlash

Backlash error is inherent in mechanically driven systems, such as ballscrew/leadscrew stages, systems with gearboxes, and belt-driven stages. In newer stages from Aerotech, the effects of backlash may not be large enough to measure or notice. However, as a system ages, backlash generally increases. Some stages also inherently have greater backlash than others. Backlash compensation is available in the A3200, Ensemble, and Soloist controller series.

This guide is most useful for older stages that require backlash compensation to remain in working order.

Measuring Backlash

The most accurate method for calculating backlash requires a dial indicator/dial gauge. In summary, the process will be to "zero out" the backlash in one direction, then move in the other direction and measure how far the stage moves in software before the dial gauge updates.

Take the following steps to measure backlash:

1. Connect to the controller and run Motion Composer.
2. Move or jog the stage in the negative direction. Note the direction of motion and the final position of the axis. We'll call this value P₁.
3. Place your dial gauge against the stage tabletop, i.e. the moving carriage, on the plane facing the positive direction of travel.
4. Zero the dial gauge.
5. Slowly jog the stage in the positive direction using Motion Composer's jog feature.
6. Stop moving as soon as the dial gauge begins to indicate motion.
7. Note the distance moved on the dial gauge, we'll call this value D.
8. Note the new Position Command. We'll call this value P₂.
9. Calculate the backlash as follows:
   
   BacklashPositive = (P₂ - P₁) - D
   
   
10. Repeat this process to confirm that the measurement is accurate.

Measure the backlash using the primary units (usually mm or in) of the axis to be compensated. The distance measured should be relatively small You can also measure in the reverse direction, jogging positive in step 2, placing the dial gauge on the negative-facing plane of the carriage in step 3, then jogging negative in step 4. If you do, calculate the backlash as follows:

BacklashNegative = (P₁ - P₂) - D

BacklashPositive and BacklashNegative should be approximately equal.

A3200

To add the backlash distance to the parameter file, take the following steps:

1. Connect to the controller in Configuration Manager.
2. Open the Active parameter file.
3. Expand the Parameters node and click on Axis.
   
   
   
   
4. The BacklashDistance parameter will be available in the main pane. Enter the value you calculated here.
5. In the Parameters node, navigate to Task > Motion.
   
   
   
   
6. Verify that the parameter MotionUpdateRate is 1KHz at minimum. This is the default MotionUpdateRate value for most systems.
7. In the Parameters node, navigate to Axis > Motion > Filters.
   
   
   
   
8. It may be necessary to set the BacklashIIRFilter if the backlash compensation accelerations are too great on this axis. This is left to the discretion of the user. See the documentation below.
   
   
9. Save the parameter file and Reset the controller as normal.
10. Repeat the backlash measurement process and confirm that the correction is appropriate.

Backlash compensation is only active after the axis has been Homed, and is zero at the end of the Home cycle. Backlash direction is dependent on the direction of the final move of the Home cycle; for example, if the final Home move is in the negative direction, only positive moves will use the BacklashDistance parameter.

Ensemble and Soloist

To add the backlash distance to the parameter file, take the following steps:

1. Connect to the controller in Configuration Manager.
2. Right-click on the controller and select Retrieve Parameters.
   
   
   
   
3. Expand the Parameters node on the controller and click on Axis.
   
   
   
   
4. The BacklashDistance parameter will be available in the main pane. Enter the value you calculated here.
5. Right-click on the controller and select Send Parameters.
   
   
   
   
6. Send and commit the parameters as normal, then Reset the controller to initialize with the new value.
7. Repeat the backlash measurement process and confirm that the correction is appropriate.

For full instructions on working with your parameter file, please refer to the Working with Parameters article in the Help documents.

Backlash compensation is only active after the axis has been Homed, and is zero at the end of the Home cycle. Backlash direction is dependent on the direction of the final move of the Home cycle; for example, if the final Home move is in the negative direction, only positive moves will use the BacklashDistance parameter.

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