Lake Shore 370 User Manual
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Lake Shore Model 370 AC Resistance Bridge User’s Manual4.11.2.3 Setpoint UnitsClosed-loop temperature control will operate with a setpoint in resistance units for any input channel and a setpoint inKelvin for any channel that has a temperature response curve assigned to it. Setpoint units will default to ohms if nocurve is selected for the control channel. If the setpoint units are changed during control, the instrument will convert thesetpoint value to the new units when possible to minimize disturbance to the load.The next control setup screen appears as a prompt for setpoint units.Control SetupSelect With °®Setpoint Units: KelvinUse the s or t key to select setpoint units of ohms or Kelvin. Press Enter to accept the new selection and continue tothe next setting screen. Press Escape to cancel the new selection and return to the normal display.4.11.2.4 Setpoint Change Pause TimeThe temperature control loop needs time to settle after any setpoint change. If temperature control is active whilescanning channels, the loop may not get enough time to settle into the new setpoint before the scanner moves away fromthe control channel. The instrument can be forced to read only the control channel for a period of time after any setpointchange to guarantee settling. This time period is set with the setpoint change pause parameter. Manually changingscanner channel will override the pause and force a reading.The next control setup screen appears as a prompt for the setpoint change pause time.Control SetupEnter A Value ForSetpoint Change Pause Time: 25 sUse the data entry keys to enter a change pause time from 1 to 255 seconds. Press Enter to accept the new selection andcontinue to the setpoint ramp feature, Paragraph 4.11.7. Press Escape to restart the setting sequence and enter a newvalue. Press Escape again to cancel the sequence and return to the normal display.4.11.2.5 Control FilterDuring normal operation the PID control algorithm uses the unfiltered input readings from the control channel as itsfeedback. This method offers the best control stability and some noise immunity because of the natural averaging thattakes place in the integrator (I) part of the control equation. The control algorithm can be configured to use filteredreadings instead of unfiltered readings. This configuration can improve noise rejection but it is sometimes at the expenseof more difficult tuning or even control instability. The filter creates time lag that destabilizes the control loop. Controluses the same firmware filter parameters set for the control input channel. Any control application should be tuned firstwith the control filter off. Filter action should be added gradually, starting with a very short settling time, andadjustments to P and I made be required along the way (most often P will decrease and I will increase). The filter timeconstant should always remain shorter than the I setting in seconds.The next control setup screen appears as a prompt for control filter On/Off.Control SetupSelect With °®Control Filter: OffUse the s or t key to select control filter on or off. Press Enter to accept the new selection and continue to the nextsetting screen. Press Escape to cancel the new selection and return to the normal display.Operation 4-17 |
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