Chapter 4 - Calibrating a Rotary

The Best Fit Calibration Frame tool can be used to calibrate external axes.

This article will show how to calibrate a rotary axis using the Intercam Utilities.

The required files for this tutorial can be downloaded here:

https://intercamsa-my.sharepoint.com/:f:/p/portal/EtT1xj8AiVtCqe3QJJjFuuQBknVe4hL6qv_oTss-jKgUyQ?e=o3h0fQ

It is recommended that the user first reads https://intercam.atlassian.net/l/cp/bwbThrHV to fully understand the concept of calibration.

Getting Started

Capturing the points

First, we will define one point on the rotary.

We would need to define a point in the CAD that can be touched, or replicated on the real rotary.

Usually, a rotary will have a marking that can be used for this purpose.

If the rotary does not have such marking, the end user should mark an “X” on the edge of the rotary at its zero position. See Figure 1:

Figure 1: CAD Reference point.

Once this point is located. The user should copy transform(rotate) that point along the Rotary's rotational axis, in the positive and in the negative directions.

The rotational increments between each point should be fixed. So, if the user chooses to do a step of 30 degrees, the locations for the points would be the following:

-90, -60, -30, 0, 30, 60, 90

A calibration will be more accurate if more points are used. Acceptable steps range from 45degrees to 20degrees.

Figure 2: CAD Points - Step of 45 degrees.

With the points in their positions. We can now record their coordinates.

Figure 3: CAD Points

Now, we will use the robot to touch these same points in the real part and write down their coordinates.

The virtual points and real-world points must be the same number of points otherwise an error will occur.

The virtual points and real-world points must be in the same order otherwise a large deviation will occur, that will break the best bit frame positioning.

It is important that, when touching the points, the rotary is only rotated in one direction. This will help improve the calibration results.

Thus, we would begin with -90 degrees, and work our way to +90 degrees.

Figure 4: Measured Points

Creating the frame using virtual and real world points.

Now we must:

Launch Robotmaster
Import the virtual CAD model
Open the Best fit calibration frame command:

Figure 5: Best Fit Calibration Frame

We can now load both sets of points:

First set of points will be the Virtual Points (CADPoints_Utilities_Example.txt)
Second set of points will be the Real-world Points as measured with the robot. (MeasuredPoints_Utilities_Example.txt)
We can decide if we want to import the points, or just the calibration frame

Figure 6: Calibrated Frame Calculator

The data will be imported:

Figure 7: Calibrated Frame on Robotmaster.

When importing a calibrated frame, a pop-up message will be displayed indicating the maximum point deviation after calibration, and the point number where this maximum deviation occurs. Just as Figure 8 shows.

Figure 8: Point 5 - Maximum deviation point.

Aligning the virtual Rotary with the real-world Rotary

Next, transform the rotary from the origin to the calibrated frame:

Figure 9: Transforming the Rotary

Move the gnommon to the calibrated frame:

Figure 10: Moving the rotary to the calibrated frame

Align the gnommon’s X axis to the calibrated frame’s X axis:

Figure 11: Aligning X axes.

Align the gnommon’s Y axis to the calibrated frame’s Y axis:

Figure 12: Aligning the Y axes.

Done! The Rotary will move to the calibrated position.

Figure 13: Rotary placed at its calibrated location.