Speed Control

The RMC100 provides two modes of closed loop speed control:

Speed Control with Position Loop
This mode performs closed loop control on the axis position (not speed). To achieve speed control, the RMC100 generates a target that moves at the commanded speed. This means that if the Actual Position of the axis falls behind the Target Position, the RMC100 will attempt to catch up to the moving Target Position, requiring the axis to move at a speed much higher than the commanded speed. This may be undesirable in certain applications.

Speed Control with Velocity Loop
This mode performs closed loop control on the axis speed. If the Actual Speed of the axis falls behind the Target Speed, the RMC100 will attempt to catch up to the Target Speed.

Note: Speed Control with Velocity Loop is supported in RMC100 CPU firmware dated 20030515 or later.

Using Speed Control with Position Loop

Speed control with Position Loop is used in the same way as position control with the following exceptions:

The Rotational bit must be set in the Mode word.
The Command Value field for the Go command indicates the direction of the move, rather than the Requested Position.

Using Speed Control with Velocity Loop

Speed control with Velocity Loop is used in the same way as Speed Control with Position Loop with the following addition:

The Velocity Loop bit must be set in the Mode word.

Speed Control Commands

There are three main commands to use in speed control:

Go Command

When a Go command is issued with the Rotational bit in the Mode word set, a Speed Control with Position Loop move is initiated. The Command Value holds the requested direction. For further details, see Go Command.

When a Go command is issued with the Rotational bit and the Velocity loop bits in the Mode word set, a Speed Control with Position Loop move is initiated. The Command Value holds the requested direction. For further details, see Go Command.

Advantages:

This command can start a speed control move. All command fields can be set with a single command.

Disadvantages:

When used with a communication type that allows only a Command and Command Value each command cycle (PROFIBUS-DP in Compact Mode and Communication DI/O using Command Mode or Parallel Position Mode), the speeds and accelerations are limited to those in the Motion Profile Table.

Set Speed (Signed) Command

This command takes a number between –32,768 and 32,767 as the Command Value and sets both the requested direction (positive indicates an extend or clockwise move, and negative indicates a retract or counter-clockwise move) and the requested speed. The Rotational bit in the Mode word must be set prior to issuing this command.

Advantages:

Both the speed and direction can be changed with a single command.

Disadvantages:

Only speeds between 0 and 32,767 can be set with this command. Also, this command cannot set the Rotational bit in the Mode word, so another command must do this.

Set Speed (Unsigned) Command

This command works the same as the Set Speed (Signed) command except that only the requested speed is changed; the requested direction cannot be changed with this command. Therefore, the Command Value is an unsigned number between 0 and 65,535 and holds the requested speed. The Rotational bit in the Mode word must be set prior to issuing this command.

Advantages:

Allows the speed to be set to any valid value (0 to 65,535 position units/second).

Disadvantages:

The direction cannot be changed with this command. Also, this command cannot set the Rotational bit in the Mode word, so another command must do this.

Speed Control with Cyclic Operations

In speed control the positions will increment or decrement until the position wraps. However, position can be reset on the fly. This is desired if there is a machine cycle that is less than 65536 position units long. If a machine cycle is 30000 units long it would be nice to reset the position to 0 when the axis gets above 29999. This way the same positions will occur at the same place in the cycle. This can be accomplished using either a Home Input to reset the positions or an Offset Positions command. The Home Input must be used if the absolute positions must always be the same or if the cycle distance is not an integer number of position units. For other cases, the Offset Positions command provides a cheaper and easier to implement way of maintaining the relative distance between events. In either case the Event Step Table must be used to either re-arm the Home Input or to reset the positions.