In this concluding section of my two-part look at ramp and soak controllers, I'll explain some of the options built into program controllers to anticipate every item on your wish list. Perhaps you'd like a few of the following capabilities:
- The ability to program the number of segments in a recipe so it can go from a single ramp up to 80 ramp/soak pairs. By using a number of straight-line segments, you can make a good approximation to a curve as a setpoint program.
- The ability to program the controller to execute a step only once, repeat it a specific number of times, or repeat it continuously.
- The ability to store up to 50 programs. This enables you to make quick changes from one setup to another and to recall previously used programs.
- The ability to string two or more stored programs together. This is particularly useful if you want more segments than the maximum limit for one program.
- The ability to configure the program end. For example, you can program the process to hold at the last dwell temperature, or reset to your choice of setpoint or fixed power level.
Those are just a few of the options available. Let's look more closely at a few other options.
Hold Back. As the setpoint ramps, the temperature may lag behind. If the deviation exceeds a preset value from setpoint (called hold back), the program can be automatically put on hold until the temperature returns to within that setting. This feature has many uses. For example, it can be used to guarantee that the soak segment does not begin timing until the rising temperature is acceptably close to the required soak temperature.
Configurable Relays and Outputs. With some controllers, it is possible to configure the internal relays or logic outputs to operate at fixed temperatures or deviations from setpoint. Alternatively, they can be configured as event outputs to operate as the program ends or enters one or more particular segments. These outputs enable alarms and peripheral equipment to be operated according to the process requirements.
Logic Inputs. Actuated by external signals or contact closures, logic inputs can be used to initiate functions such as run, hold, reset to a fixed setpoint, skip remainder of a segment, change to a new program, self-tune and adaptive-tune. An analog DC retransmission signal is an option. It can be used to represent values such as process temperature, output power, setpoint or deviation from setpoint. The setpoint retransmission signal from the master controller can command slave controllers on other zones (in a multi-zone process) to follow the same program. Another option is a digital signal, which can do the same job but with absolute transmission accuracy.
Recovery from Power InterruptionOccasionally, power interruptions halt the process mid-profile. Upon restoration of power, the controller memory will recall where the process was before interruption and be ready to continue (figure 1). Keep in mind, though, that the process will have cooled by an amount dependent on interruption time. You have three choices, any one of which can be called up automatically:
- Abandon the program and reset to a fixed setpoint or to some preset power level. You might do this if you have lost too much heat to guarantee proper completion of the program.
- Make the program continue from the last setpoint before power was interrupted (point A on figure 1). For a prolonged power failure, you will have to judge how losing some temperature and having it jerked back to the last setpoint will affect the work.
- Move the setpoint to match the current temperature (point B on figure 1) and continue the program. Figure 1 shows such a recovery from an interruption of a ramp segment. This mode of recovery from within a dwell segment gives a ramped return to the program, starting from current temperature.
As you can see, there are many options available with ramp and soak controllers. Evaluate your process to determine which features will serve you best.