Loop Time
Loop Time
The RMC is a deterministic controller. It reads the inputs, computes the control algorithms and updates the outputs at a specific interval. This interval is called the controller loop time, in reference to the way the controller repeatedly "loops" through its code.
The RMC will always run at its loop time setting. When it finishes all the calculations for one loop, it waits until the next loop time before doing its calculations again.
Setting the Loop Time
To set the loop time of the RMC, use the Control Loop Time Settings Page.
Monitoring the Loop Time
Use the Control Loop Time Settings Page to monitor how much of the control loop the RMC is actually using. It is important that the RMC never exceeds its loop time. If it does, unexpected behavior may result.
Available Loop Times
The loop time can be set by the user. The loop times available on an RMC may depend on the number of axes configured on the RMC:
|
Loop Time |
||||
CPU |
4 ms |
2 ms |
1 ms |
500 µs |
250 µs |
RMC150E
|
(note 1) |
|
|
(note 2) |
(note 3) |
RMC75E
|
|
|
|
|
(note 4) |
RMC75S
|
|
|
|
(note 5) (note 8) |
- |
RMC75P |
|
|
|
(note 6) (note 8) |
- |
RMC75P |
|
|
(note 7) (note 8) |
- (note 8) |
- |
Note 1: RMC150E, 4ms loop time: This loop time requires firmware 3.33.0 or newer. It is not supported by RMC150E/RMC151E CPU revisions 1.0A, 1.1A, 1.1B, 1.2A, and 1.2B. MDT (-M) modules must be revision 7 or newer to support 4ms.
Note 2: RMC150E, 500us loop time: Supported only for 1 or 2 axes. This loop time is not supported by SSI (-S) modules revision 5.0 or older, or Analog Input (-A, -H, and -G) modules revision 6.0 or older.
Note 3: RMC150E, 250us loop time: Supported only for 1 axis. This loop time is not supported by SSI (-S) modules revision 5.0 or older, or any Analog Input (-A, -H, and -G) modules.
Note 4: RMC75E, 250us loop time: Supported only for 1 axis.
Note 5: RMC75S, 500us loop time: Supported only for 1 control axis with up to one reference input.
Note 6: RMC75P (2.1F or newer), 500us loop time: Supported only for 1 control axis with up to one reference input.
Note 7: RMC75P (2.1E or older), 1000us loop time: Supported only for 1 control axis with up to one reference input.
Note 8: RMC75S/RMC75P, various loop times: These loop times are allowed for all RMC75S and RMC75P configurations with pre-3.31.0 firmware in order to ensure backward compatibility. However, using these loop times with unsupported configurations is not recommended. If you have an application using an unsupported loop time, you must check your Maximum Loop Time to ensure that it never exceeds your selected loop time.
_LoopTime tag
The _LoopTime tag can be used to access the loop time when using expressions. _LoopTime is a REAL, and is specified in seconds.
See Also
Copyright (c) 2005-2012 by Delta Computer Systems, Inc.