Red Lion MDC - Manual (Page 9)

THEORY OF OPERATION
In Master Mode, the MDC uses actual motor speed (Feedback Input) and an
analog control voltage (Drive Output) to precisely control the motor’s speed.
The MDC operates on a 10 msec control cycle. Actual motor speed is read and
drive output is corrected every 10 msec. The Drive Output voltage is
generated by a 12 bit DAC (digital to analog converter). Motor speed is
controlled by adjusting the Output Value (in bits) to the DAC.
The Output Value (0 to 4095 bits) is made up of two parts, the Open Loop
term, and the correction term or Trim Value. The Open Loop term is the ratio
of the Reference Speed to Maximum Speed multiplied by 4095. This is the
uncorrected “Feed Forward” part of the Output Value. The Reference Speed
is the current Speed setpoint taking into account any acceleration or
deceleration which may be in progress. The Maximum Speed is Max RPM
Feedback which is set when scaling the unit. (See the Program Scaling section
of the manual for more information.)
The correction term or Trim Value is derived from actual motor speed
(Feedback Input) and the Reference Speed. The Trim Value is added to the
Open Loop term when the MDC is running Closed Loop to provide precise
control. The Error Gain determines the rate at which the Trim Value changes
in response to a motor loading or a sudden speed change. The Trim Value is
updated every 10 msec. It may be viewed as one of the programmable display
options (see Program Displays Menu).
The Drive Output voltage is then the ratio of Output Value (in bits) to 4095
multiplied by the Full Scale Reference voltage. The internal Full Scale
Reference is factory calibrated to 10 VDC but may be adjusted to any value
within a 5 to 15 VDC range. An external reference option also exists. (See
Isolated Drive Output section of manual for more information.)
In Follower Mode, the basic operation is the same except that the
Reference Speed is generated as a ratio to the Lead Input speed. Also, the
MDC operates on a 20 msec control cycle in order to process both Lead and
Feedback input frequencies.
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