Link Type - Skew Detection

Note: The Skew on Actual Position link type is available only in RMC100 CPU firmware version 20000504 and later. The Skew on Following Error link type is available only in RMC100 CPU firmware version 20030916 and later.

What is Skew?

Skew is calculated over a group of axes. The minimum and maximum actual positions (or following errors) in that group are found, and the difference is calculated. The difference between the highest and lowest actual position (or following error) is the skew. The maximum detectable skew is 32,767 position units. This link type can also detect differences between pressure or force in selected axis, in which case the maximum detectable difference is 32,767 pressure or force units.

The RMC100 has two skew link types: Skew on Actual Position and Skew on Following Error.

Skew on Actual Position

This link type compares the Actual Positions of each axis to detect skew. For example, if four axes have positions 1000, 1010, 998, 1005, then the minimum and maximum positions would be 998 and 1010, so the skew would be 12 position units. For the skew on actual position to be meaningful, the axes must be configured to have the same position units for the same positions. For example, each of the four corners may be configured to have 0 be fully closed, 1000 be 1.000 inch above fully closed, etc.

Skew on Following Error

This link type compares the Following Errors of each axis to detect skew. In other words, each axis must be just as far ahead or behind its respective Target Position. This allows the target positions to be different on each axis.

This type of skew is useful if the axes cannot be configured to have the same position units for the same positions, or if they are offset.

Detecting Excessive Skew

This link type is used to detect excessive skew in a group of axes. This link type must be used in conjunction with the Set Extended Link Value (l) command. The skew limit is set with the Set Extended Link Value command, and then this link type is used with its link value defining the axes to be included in the skew calculation. When the skew among these axes exceeds the skew limit set in the Extended Link Value, this step sequence will jump to the next step in the sequence. Otherwise, the step sequence will continue to wait for the condition to be true.

Because this link type waits until the skew is excessive, it is usually used to detect an error condition, and therefore probably is running in a step sequence on an axis other than your primary control step sequence is running. For example, axis 0 may be running the standard step sequence, while axis 1 is waiting to see if the skew gets too large. When it detects this condition it will stop axis 0's step sequence and put the machine in a safe state.

Example:

This example assume that axes 0 and 1 are moving together with axis 0 running the primary control step sequence and axis 1 running this skew-detection sequence. Axis 1 executes step 10 which sets the Extended Link Value to 100 and then waits until the difference between actual positions on axis 0 and 1 is greater to or equal the Extended Link Value. Should this limit be reached or exceed then step 11 is executed by the slave which issues a Kill Axis (K) command to stop both axes.

Using with the Link Type and Link Value Dialog Box

1. Under Link Type Category, select System-wide (Basic, non-axis).

2. Under Link Type, select Skew Detection.

3. Under Link Condition, check the boxes next to each axis that you want to be included in the skew detection.

4. Under Link Condition, check the type of comparison to determine the skew.

5. Click OK.

6. Ensure that the Extended Link Value has been set up using the Set Extended Link Value command.

Using without the Link Type and Link Value Dialog Box

1. Enter '<' into the Link Type field for Skew on Actual Position, or '0x7f' for Skew on Following Error.

2. Enter the Link Value.
   You must understand binary to enter the Link Value manually. Each bit of the Link Value corresponds to an axis. Bit 0 corresponds to axis 0, bit 1 corresponds to axis 1, etc. Bits 8-15 are always unused. Each axis's bit that is on means that you wish to include that axis in the skew detection.

3. Ensure that the Extended Link Value has been set up using the Set Extended Link Value command.

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