MKS 651C, Instruction Manual, Page: 44 / 172

Front Panel Chapter Three: Overview
30
When used in the PID control mode, the 651 unit employs a Proportional, Integral, and
Derivative (PID) algorithm for control. PID control requires the input of user-defined lead and
gain values. Each set point uses its own lead and gain values to optimize response from set point
to set point. Although there are default values for lead and gain, you should enter the values for
optimum control. To enter lead and gain parameters from the front panel, the Key Lock switch
must be set to Local. Select PID Pressure control from the Setup menu to set the lead and gain
values. (To enter the Setup menu, hold down both scrolling keys simultaneously for
approximately three seconds. Scroll through the Setup menu until the Control mode entry
appears. To change from Self-Tuning to PID, turn the Adjust knob clockwise.) Once the lead
and gain parameters have been entered, the 651 unit can operate with the Key Lock switch set to
either Local or Remote. Refer to How To Set the Lead and Gain Parameters, page 81, for
instructions on setting lead and gain parameters.
The control mode selection applies to the five internal set points (A through E) and the analog set
point.
Softstart Control
The softstart feature is used to reduce the rate at which a control valve moves toward set point.
The rate is given as a percent of full speed and can be used on either a pressure or position set
point. Once set point is achieved under softstart control, the valve is free to move at full speed.
Softstart control can be applied to set points A through E, analog set point, valve open, and valve
close. Refer to Chapter Four: System Setup, page 39, for instructions on how to set softstart
rates from the front panel.
If a set point is established via RS-232 input, the softstart rate for that set point is selected
through use of the I6 value RS-232 command. If an analog set point is established via digital
logic input, the softstart rate for the analog set point is controlled by digital logic on pin 7 of the
I/O connector. To achieve softstart control of digital logic or analog set points, the softstart line
must be held low. If the line is not held low, the valve will move at 100% full speed. Refer to
Chapter Six: Remote Operation, page 61, for additional information about softstart control via
the rear panel.
Battery Backups
There are two types of batteries used in the 651 instrument: a lithium battery and an optional
lead-acid battery. The lithium battery is included inside each 651 unit and is used to power
memory for storage of configuration and learned system information while power is off. The
optional lead-acid battery allows user-configuration of the 651 instrument to drive a valve open
or closed upon an AC power failure.