Series 988 Enhanced Software Features Includes 986, 987, 988 and 989 1/8 DIN Microprocessor-Based Temperature/Process Controller Watlow Controls 1241 Bundy Boulevard, P.O. Box 5580, Winona, Minnesota USA 55987-5580 Phone: (507) 454-5300, Fax: (507) 452-4507 http://www.watlow.
Table of Contents Cascade . . . . . . . . . . . . . . . . . . . . . . . .3 Differential . . . . . . . . . . . . . . . . . . . . . .7 Dual PID . . . . . . . . . . . . . . . . . . . . . . . .9 Duplex . . . . . . . . . . . . . . . . . . . . . . . .11 Ratio . . . . . . . . . . . . . . . . . . . . . . . . . .
Cascade Requirements Cascade control requires enhanced software and two analog inputs, input 1 to monitor the primary, or outer, loop and input 2 to monitor the secondary, or inner, loop. At least one control output is required to control the process. Overview Cascade control can handle a difficult process with minimal overshoot, while reaching the set point quickly. This minimizes damage to system components and allows for oversizing heaters for optimal heat-up rates.
difference between the set point and the process temperature generates an internal percent output value for the second, or inner loop, controller. This value cannot be seen by the operator. This internal percent (% int) output generates the internal set point for the secondary, or inner loop. The secondary loop uses this set point and the value of input 2 (typically attached to the heater sheath) to control the heat source temperature.
The outer loop can be auto-tuned by setting [`AUt] to [PidA]. While autotuning, the outer loop will be controlled in an on/off mode at a set point equal to [AtSP] x [`SP1]. In a heating application, make sure the set point is set at a value above ambient temperature. In most cases, the auto-tuning feature will tune [PidA] for acceptable control. If not, you must then manually tune the outer loop. Before beginning manual tuning, record the values of [Pb1A] and [rE1A] generated by the auto-tuning feature.
Figure 9.6 - Lube oil tank with cascade control. Sample Application A Series 988 controller is used to heat lube oil to 125°F with a screw-plugstyle heater. To protect the oil from breaking down and maximize its life, it is desirable to limit the maximum heater sheath temperature to 250° F. The Series 988 is ordered with two thermocouple inputs. Input 2, the inner loop in the cascade configuration, measures the heater sheath.
Differential Requirements Two inputs and the enhanced software option are required. Overview Differential control allows the Series 988 to control one process at a difference to another process. Input 2 acts as a remote set point input. However the displayed set point indicates the desired difference between input 1 and input 2.
Figure 9.8 - Freezer display case with differential control. Sample Application An application using differential control is to maintain glass temperature of a freezer display case above dew point at a differential to the ambient air temperature. A thermocouple at input 2 senses the outside air temperature and adjusts the internal set point to maintain the glass temperature 10 degrees higher. Substituting values we have: glass temperature = ambient + 10°.
Dual PID Sets Requirements The Series 988 controller needs the enhanced software option to use dual PID sets. Overview Standard software units have a single set of PID parameters. Units with enhanced software can use two independent sets of heat/cool PID parameters, PID A [PidA] and PID B [Pidb]. To enable dual PID, enter the Global Menu and set the algorithm prompt [ALgO] to dual PID [Pid2].
Figure 9.10 - Test chamber controlled with dual PID sets. Sample Application A test engineer needs to control the temperature in a test chamber that can be operated at normal atmosphere or under vacuum conditions. If he tunes the controller for normal atmospheric conditions, when he reaches the portion of his test that requires a vacuum, he must stop the test and enter new PID parameters to maintain stable temperatures.
Duplex Requirements The duplex control feature requires enhanced software and a process output. NOTE: Duplex applications require a special valve. Figure 9.11 - Fluid sample container with duplex control. Overview Certain systems require that a single process output control both heating and cooling outputs. A Series 988 controller configured with enhanced software and a process output can function as two separate outputs.
Ratio Requirements Ratio control requires enhanced software. Two analog inputs are required to monitor the process, and at least one output adjusts the controlled part of the process. Overview This feature allows the Series 988 to control one process as a ratio of another process. This is especially useful in applications that mix two materials, whether steam, paint or food ingredients. Input 2 of the controller measures the part of the process that is either uncontrolled or controlled by another device.
