MKS 937B Operation Manual
55
creating a large positive deviation). The valid range for Integral gain is
0.0001 to 10 1/seconds. The default setting is 0.02 for 937B.
Derivative: This is the derivative control parameter for PID control of system
pressure. The derivative function creates a valve control signal based on the rate
of change in the sensor’s pressure reading (i.e. the first order derivative of the
pressure reading with respect to time).
The derivative term functions to slow the rate of change in the
pressure. It reduces the magnitude of the overshoot produced by the
integral component, and improves the combined control stability. However,
differentiation of a signal amplifies noise and this can cause instability in
the process control if the noise and the derivative gain are sufficiently
large. The range for Derivative gain is 0.001 to 1000 seconds and the
default value is 1.5 seconds.
Base: Base is a control parameter that sets the lower limit of the control signal
output. This is expressed as the true flow rate for a MFC, or as the percent of
valve opening for a valve. For example, a Base setting of 200 for a MFC means
the flow rate should not go below 200 sccm (this value must be less than the full
scale flow rate), while a Base setting of 20 for a valve means the valve control
signal output should not go below 20% of its opening.
The valid range is from 0 to full scale for a MFC, and 0 to 100 for a valve. The
default setting is 0 and it must be either equal to less than the Start parameter
value.
A non-zero setting for Base will override the Softstart process and Start setting.
Preset: The Preset value is used to set the flow rate for a MFC or the position (as
a percent of the valve control signal output) of the valve when the Static control
mode is invoked.
The valid range for Preset is from 0 to full scale for a MFC, and 0 to 100% of full
open for a valve. The default value is the full scale for a MFC, or 100 for a valve.
Once a PID control is terminated, the control mode switches to Setpoint control
mode, and the setpoint value is changed to Preset value automatically.
Start: The Start value is used to
i. If the Softstart parameter is set to 0 or the Base parameter is less than the
Start value (non-zero Base parameter overrides the Softstart), this parameter
sets the initial position of the valve or initializes a MFC setpoint when the PID
control is switched on,
ii. If the Softstart parameter is set to a non-zero number, this parameter sets
the position of the valve, or a MFC setpoint when the Softstart process ends
(or when the normal control process starts).
The range is from 0 to full scale for a MFC, or from 0 to 100 % for a control
valve. The default setting is 0, and it must be equal or larger than Base
parameter value.
Softstart: The Softstart value is the time period over which the valve drive signal
(thus, the valve position) or the MFC flow drive signal goes from 0 to the value
defined by Start (flow rate for a MFC or position for a valve). This is normally
used to prevent turbulence, reducing the particles in the process chamber during
the initial pump down process. The valid range for Softstart is 0 to 1000 seconds,
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