Setpoint Ramping - Lakeshore 335 User Manual

4.5.1 Heater Outputs 63

4.5.1.7.7 Setpoint Ramping

The Model 335 can generate a smooth setpoint ramp when the setpoint units are
expressed in temperature. You can set a ramp rate in degrees per minute with a range
of 0 to 100 and a resolution of 0.1. Once the ramping feature is turned on, its action is
initiated by a setpoint change. When a new setpoint is entered, the instrument
changes the setpoint temperature from the old value to the new value at the ramp
rate. A positive ramp rate is always entered; it is used by the instrument to ramp
either up or down in temperature.
Always use the ramping feature to minimize temperature overshoot and undershoot.
When ramping is not used, a setpoint change can cause the error used by the PID
equation to become very large, which causes the I contribution of the control output
equation to become larger the longer the error exists. This will result in a large over-
shoot or undershoot once the setpoint temperature is reached, since the I contribu-
tion will only decrease when the error polarity is reversed. Using a ramp rate that
keeps the control output from reaching the extremes of 100% or 0% while ramping is
optimal.
The ramping feature is useful by itself, but it is even more powerful when used with
other features. Setpoint ramps are often used with zone control mode. As tempera-
ture is ramped through different temperature zones, control parameters are auto-
matically selected for best control. Ramps can be initiated and status read back using
a computer interface. During computer-controlled experiments, the instrument gen-
erates the setpoint ramp while the computer is busy taking necessary data.
When an incomplete ramp is shut off, the setpoint will remain on the most current setting
(the reading will not jump to the end of the ramp).
If the input type or input curve is changed while a ramp is in progress, both ramping and
the heater are turned off.
If Ramp is on and the setpoint is set to sensor units, the ramping function will remain on,
but when another setpoint is entered, the setpoint goes directly to the new setpoint
value.
To bypass ramping and load the setpoint with the current temperature, with the control
loop displayed, press and hold the Setpoint button for 3 s.
Menu Navigation:
Output SetupQOutput (1 or 2)Q QSetpoint Ramping (Off or On)
ENTER
Default: Off
Ramp RateQ (0.1 K/min to 100 K/min)
Default: 0.1 K/min
Interface Command: RAMP
To stop a ramp, when the desired control loop is displayed, press Setpoint then imme-
diately press Enter. This stops the ramp at the current setpoint, but it leaves the
ramping function activated. To continue the ramp, enter a new setpoint. Refer to
section 4.5.1.7.6 for details on setting the Setpoint parameter.
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