Using Analog Channels as Velocity Inputs

Analog channels may be configured to be used as one of two velocity input types:

Velocity Control
The input is used with the corresponding drive output (analog modules without drive outputs cannot be used for velocity control) for closed loop control. As described below, the position for the axis changes at a speed determined by the transducer input. See Controlling Speed from a Tachometer Feedback for details.
Velocity Reference
The input is used without the drive output. As described below, the position for the axis changes at a speed determined by the transducer input. Because no drive output is used, the axis cannot do closed-loop control. This is most often used for joysticks, in which case another axis will be geared to the movements of the joystick on a velocity reference axis. See Using Position and Velocity Joysticks or Using an External Target Generator for details.

For the most part, all fields are used as described in the position status, command, and parameter field topics. However, the following fields are used differently:

Transducer Counts. The counts are signed, as they are for pressure and force. See Counts for a mapping.
Actual Position. The position is not derived directly from the Transducer Counts as it is with all other operation modes. Instead, the position is changed at the velocity given in the Actual Speed field.
Actual Speed. The actual speed is derived directly from the Transducer Counts. To convert the Transducer Counts to the Actual Speed, do the following:

Actual Speed = (COUNTS - 325) x SCALE / ( 32768 x Prescale Divisor)

The 325 subtracted from the counts are a 1% dead-band. Therefore, if the input generates between -325 and +325 counts, the axis will have 0 velocity. Notice that this value is 1% of the full input value (e.g. 10V, 5V, 20mA), which will be represented by 32500 counts, as described in the Counts topic.

Scale. This parameter is used to compute actual speed (as described above). A scale of 0 is treated like 32,768.
Offset. This parameter sets the usable range of positions, as with standard position axes. It has no effect on the mapping of the input voltage/current and the speed generated.

When using Velocity Reference, the only parameters that are used are Configuration, Scale, Offset, Extend Limit, and Retract Limit. All others are ignored.

Use the following steps to configure an analog velocity input:

Step 1: Assign Analog Channel(s) to be Velocity Input(s)

Because only channels 0 and 2 have drive outputs associated with them on the analog modules with drive outputs, it is only these two channels that can be configured as Velocity Control; any channel may be configured as Velocity Reference. Use the following steps:

On the Tools menu, click Module Configuration.
In the Slots list, click an analog module.
Click Slot options.
In the Analog Channel Assignment dialog box, click the tab of the channel pair (0-1 or 2-3) you wish to reassign.
Click the option button of a channel assignment that designates one or more channels as Velocity Control and/or Velocity Reference. The assignment selected depends on how you intend to use the channels.
Click Update RMC.
The Update Module Configuration dialog box will be displayed to indicate the progress. If the RMC could not be reset automatically, you may be prompted to reset it manually.
In the RMC Configuration dialog box, click Close.

Step 2: Configure the Analog Transducer Type for the Velocity Input

Refer to Configuring the Analog Transducer Type for details on selecting between a voltage or current input type.

Step 3: Set the Scale, Extend Limit, and Retract Limit Parameters for the Velocity Input

Refer to the individual field sections for details on setting each parameter.

Step 4: Set the Count Offset for the Velocity Input

Refer to the Set Count Offset (35) command.

Analog Input Ranges

See the COUNTS topic for detailed information on the maximum and minimum limits of the various analog input ranges.