Open and Closed-Loop Feedback Systems:
Bumpless Transfer

When a controller is running in manual mode and then switched to auto mode, the controller will immediately begin to compare the SP to the PV and determine the amount of error. When this occurs, it's important for the PV to be as near to the SP as possible so that the error is as small as possible. E.g., in a temperature controller this means that the operator must be very careful and ensure that the temperature is increased in manual mode to a point where it's very near the SP. In some industries where the operator must tend to more than one machine, it's not practical for the operator to stay with the system until the process temperature is raised to the SP. In these systems where the controller must be placed in auto mode when the process temperature is well below the SP, a function called bumpless transfer is used to ensure that the SP and the PV are exactly the same when the controller is changed from manual to auto mode so that error is zero.

Bumpless transfer in its simplest form changes the SP to match the PV anytime the processor is changed from manual mode to auto mode. This ensures that the error is zero when the switch is made, and the controller will not be allowed to go into a condition that may cause the temperature to oscillate to extremes. E.g., when a plastic injection molding machine is first turned on, its barrel will be at room temperature (approximately 75°F). The operating temperature for the barrel heaters is approximately 600°F. If the operator placed a setpoint of 600°F into the controller when the unit's first turned on and the barrel temperature is at room temperature, the amount of error would be 525°F (E = SP - PV). This would be a large error and it would cause the controller to have problems keeping the system under control until the barrel temperature reaches the SP.

The condition of excessive error can be controlled by bumpless transfer. When the operator first starts up the injection molding machine, the controller is set to manual mode and the output is set to 50%. This means the electric heating elements called band heaters receive 50% of the total possible energy and the temperature of the barrel begins to increase to approximately 400°F. At some point the operator changes the controller from manual mode to automatic mode. When this occurs, the controller changes the SP to match the actual temperature at that time. The actual temperature is provided to the controller from the thermocouple as a feedback signal.

Since the controller changes the SP to match the actual temperature from the sensor (PV) at the time when the controller is switched to auto mode, the SP will need to be adjusted later to the final control temperature. When the operator is ready, the SP can be changed to 600°F and the controller will add heat to increase the barrel temperature to 600°F, which is the normal operating temperature. If the controller would have been set in auto mode when the barrel temperature was room temperature (75°F), the 525°F error between room temperature and 600°F may have been too large and caused the controller to become unstable. If the process temperature was raised to 400°F while the controller is in manual mode, the error would be smaller and the controller stands a better chance of bringing the system to 600°F without going out of control.

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