Tony Berry, Sales Director with Hawco, explains how cascade control can provide precise and stable process heating without the problems of overshoot and the resultant wasted energy.
There is a great deal more to designing a process temperature control system than just selecting the appropriate control loop components. Just controlling the temperature of the heat source does not guarantee an accurate and stable process temperature.
The net result can at best mean excessive energy consumption and, at worst, the costs of heating element damage and product quality deficit.
With dual loop controllers now available at considerably lower cost than before, the way is open for process engineers to enjoy the many benefits and advantages that cascade control offers in comparison to conventional control loop configurations.
To put these advantages into perspective, consider the application where the heating source is remote from the process or the nature of the process exhibits an inherent lag in terms of thermal response. The conventional approach with a single controller is to locate the sensor within the process environment..
If the controller sensor is sited within a furnace, oven or autoclave, and not within the product in the process, then the PID parameters need to be adjusted to accommodate long response times resulting from thermal inertia of that product mass. Initially, the controller will call for maximum output from the heater element. This is not only energy inefficient but could significantly reduce the life of the heater unit itself.
What is more, thermal inertia of the heated product could cause a temperature overshoot that may be undesirable or indeed affect the quality of the processed product.Equally, if the sensor is positioned close to the heat source, the controller will not necessarily control the actual product temperature to the accuracy required. This is especially true when the temperature is affected by disturbances such as access door opening or unplanned fluctuations in energy input.
An offset may need to be Cascade control introduced into the controller to compensate for these variations but the procedure is only valid if the dynamics of the process, or of the product characteristics, remain constant throughout the complete cycle.
The answer to these imponderables is dual loop, or cascade, control – an ideal method for processes where sluggish response or long thermal lag times are inherent, but one that up to now has often been dismissed on the basis of its component costs.
Cascade control is generally employed to manage processes which otherwise would be difficult to control precisely. Typical examples of such processes might be a heat treatment furnace, an environmental chamber or a composite curing autoclave with large thermal capacity. The applications are characterised by long process lag times and/or with final control elements that are capable of letting the heater generate large amounts of energy into the process at a given time.
In these situations, especially when the workpiece parts have large mass, the workpiece temperature moves up very slowly with respect to the heat input to the process chamber. The process heat could reach damaging levels before the workpiece sensor could indicate the problem and the feedback signal calls for an energy reduction to the process.
Tighter control of the process can be achieved by monitoring both the temperature of the workpiece and the process environment with two controllers. The primary controller is linked to a sensor placed in or on the workpiece. The secondary controller receives input from the process chamber environment and from the primary control output .
Changes in the workpiece temperature are transferred through to the final control element and the output of one controller becomes the setpoint of the other, creating the cascade effect.
An example, which clearly demonstrates the working principle, is the heating of a workpiece in a furnace where the product temperature is critical. By attaching a sensor of a secondary control channel to measure the furnace interior temperature, a comparison can be made of the amount of energy sent to the system with the desired energy input or setpoint. This setpoint emanates from the primary controller that is measuring the workpiece temperature and will vary as the process temperature deviates from the setpoint.
Output from the primary controller is determined by the deviation of the workpiece temperature from the setpoint. If this deviation is large, the primary controller output will be high, the secondary controller setpoint will be correspondingly high and, in turn, will trigger the secondary controller to supply an increase in heating energy. Conversely, if the primary controller deviation is low, then its output — the setpoint of the secondary controller — is equally low and therefore less heat is applied.
Such a control system guarantees that the product reaches the actual required temperature and achieves this condition within optimum energy consumption limits.
Cascade control also offers additional features such as blended cascade control, a procedure that is applied in process situations where it is essential maximum temperature limits are not exceeded at any time. The system can also be employed to achieve maximum ramping for users operating temperature profiling applications.
Operators may well find that cascade control is worth consideration even if the process application does not involve critical temperature control. Not only is the greater level of processing precision desirable but the comparatively low cost investment in controller hardware is readily recovered in terms of energy savings payback.
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