Non-Linear Compensation

Coordinate Measuring Machines utilize scales that are mounted to each axis of motion will require compensation. This compensation will correct for any linear deviations identified during CMM calibration.

A certified Master Scale or Laser system is used to compare against the reported position from the CMM Digital Readout. This deviation can then be mathematically corrected. This information must be supplied to your CMM software to properly measure. There are two main types of scale comparison technologies. These are Linear and Non-Linear Compensation.

For additional information refer to Technical Note #2.

Scales that are mounted on you CMM often required compensation to correct for inaccuracies. These inaccuracies are the cause of scale manufacturing and scale assembly to the CMM superstructure. To correct these inaccuracies, there are two methods the Geomet applies, depending on conditions.

• Linear Error versus Non-Linear Error Overview
  • Linear Error
  • Non-Linear Error

Linear Error

Some scales have an error that can be identified as linear error. To identify these errors, laser readings are done at predetermined positions along the full travel and compared to the CMM reported position. The table below shows a condition of a pure linear error:

The Compensation column is the value used to convert the CMM Value to read correctly as shown by the Laser.

CMM Value * Compensation = Laser Value.

In a linear error condition, we can supply one compensation value to correct the entire length of the CMM axis travel.

Non Linear Error

This form of compensation provides a more accurate solution for determining scale corrections using many segments over the axis of travel. A typical scale when mounted to the CMM can stretch and shrink over its length. The compensation required between 10.000” - 11.000” may be different than between 20.000” – 21.000”.

The method to determine the compensation is done by taking position readings along the full travel, for example, every 1.000”. A table is built using the Master Scale or Laser position and the reported CMM position.

Non-Linear Setup Tool

Non-Linear Setup Tool

Activating the Non-Linear Setup Tool requires that the Scale Compensation Mode setting in System Options is set to Multi-Position Linear Compensation.

Activating the Non-Linear Setup Tool

Keyboard: <Alt + Ctrl + shift + F2>

Taking a Tour of the Linear Setup Tool

Calibration Data List

The list of Master Scale or Laser readings along with its corresponding DRO positions are shown under Calibration Data. This list does not show the compensation value, which is calculated as:

X_{Reference Value} / X_{DRO Reported Value} = X_{Compensation Value}

Selecting an entry in the data list will bring the position values down into edit controls labeled DRO Value -> and Laser Value ->. From the edit controls changes can be made when performing or editing a calibration.

When building the data list, new entries are added by entering the reference and DRO positions in the Step Value Group. Pressing <Append> will then transfer that data into the list. This list performs a sort to keep the list in linear order.

Removes the selected position from the data list.

When a position record is selected, changes can be made to the laser and DRO position values. Once the changes have been completed, press <Update> to transfer the changes into the data list.

Displays a graph of the position difference values. This graph can be printed for the current axis or for all three axes.

When <Disable Comp> is active, Non-Linear compensation is deactivated and the CMM will use Linear Calibration. When pressing, the Non-Linear tables are saved to the data file.

Moves the current position displayed in the Active DRO Group into the edit control labeled DRO Value ->.

Motion Jogger Group

The Motion Jogger Group is available on CMMs that have a motion controller.

Positions the CMM under DCC to the absolute position entered in Jog Value.

Positions the CMM under DCC to the relative step position entered in Jog Value.

Toggles the state of the motors between On or Off

Related Procedures:

Scale Factors, Manual CMM Home, DCC CMM Home