Set Bias Drive Command

Character: B
Decimal: 66
Hexadecimal: 0x42
Command Value: Requested Bias Drive in Millivolts

At times it may be necessary to apply an extra amount of drive if you know the system tends to lag at a particular time in a pressure ramp. This command can be used to add an additional amount of drive while regulating pressure.

Note: This command should only be used as a last resort. In most cases the Feed Forward or Drive Transfer Percent fields will give the desired results with greater consistency.

This command uses the Mode, Pressure Set A, Pressure Set B and Command Value fields different from their usual uses:

Pressure Set A and Pressure Set B:

These fields are used to give the rate that the Bias Drive is increased and decreased. Pressure Set A is used when the Bias Drive is being moved away from 0 volts. Pressure Set B is used to control the rate that the Bias Drive moves back toward 0 volts. See the description of the Mode field below for details on the units used by these fields.

Mode:

The Mode field is used to specify the units of the Pressure Set A and Pressure Set B fields during the Bias Drive command. The possible values are:

Mode 1
The Pressure Set A and Pressure Set B fields use units of millivolts per millisecond. For example, if the current Bias Drive is 0 volts, the Command Value is 1000 millivolts, and the Pressure Set A field is set to 50, it will take 20 milliseconds for the Bias Drive to reach 1000 millivolts.

Mode 3
The Pressure Set A and Pressure Set B fields are given in milliseconds. For example, if the current drive output is 0 millivolts, the Command Value is 1000 millivolts, and the Pressure Set A field is 50, then it will take 50 milliseconds for the Bias Drive to reach 1000 millivolts.

Command Value (Requested Drive):

This value gives the number of millivolts to ramp the Bias Drive up or down to. The ramping of the Bias Drive will begin as soon as the axis is in pressure monitoring mode, however this Bias Drive will not actually be added to the output drive until the axis begins regulating pressure.

Note: Using this command requires overwriting the Mode, Pressure Set A and Pressure Set B fields. If this is done using a full command profile from the Event Step table, then the current pressure fields will not be messed up. However, these fields must be restored to their intended values when the next Set Pressure command is issued.