A Look at Control System Fundamentals
Whether cooking or curing, drying or heat setting, every heat processing application requires controls to adequately manage the process and ensure that acceptable product is produced. A process control system consists of four key parts: feedback control, final control element, process heater and sensing devices (figure 1). A closer look at how these parts work together will help you understand the basics of process control.
Taking Measure of Your Process VariablesSensors are used to measure process variables and report those readings to the feedback control. Predominantly three types of sensors are used for process control: thermistors, RTDs and thermocouples. In addition, in processes where the sensor cannot contact the variable being measured, noncontact temperature measuring devices are used.
A thermistor is a nonlinear device whose resistance varies with temperature. Thermistors are used at temperatures under 500°F (260°C). Fragility limits their use in industrial applications.
With resistance temperature detectors (RTDs), changes in temperature vary the resistance of the RTD element, normally a thin platinum wire. Platinum RTDs are used in applications where high accuracy and low drift are required. Three-wire sensors are used where the distance between the process and controller is more than several feet. The third wire is used for leadwire resistance compensation.
The most commonly used industrial sensor, thermocouples usually are made by welding two dissimilar metals together to form a bead. Different thermocouple types are used for various temperature measurements. Placed at the point of sensing, the junction produces a millivolt signal whose amplitude is dependent on both the junction metals and the temperature under measurement.
Thermocouples require cold-end compensation. Connections between the thermocouple wire and copper at the controller's terminal block can produce voltages that are not related to the process temperature. Thermocouple voltage outputs are nonlinear with respect to the range of temperatures being measured and, therefore, require linearization for accuracy.
In addition to these devices, noncontact infrared pyrometers and thermopiles are used in industrial processes when the process is in motion or cannot be accessed with a fixed sensor.
Sensor Placement. Reducing the transfer lag is essential for accurate temperature control using simple temperature controllers. The sensor, heater and work load should be grouped as closely as possible. Sensors placed downstream in pipes, thermowells or loose-fitting platen holes will not yield optimum control. Gas and airflow processes must be sensed with an open element probe to minimize lag. Remember - the controller can only respond to the information it receives from its sensor.
Responding to Process Load ChangesThermal lag is the product of thermal resistance and thermal capacity. A single lag process has one resistance and one capacity. Take an example of a tank heating water. Thermal resistance is present at the heater/water interface. Capacity is the storage capacity of the water being heated. Sometimes the sensor location is distant from the heated process and this introduces dead time. Introduction of additional capacities and thermal resistance changes the process to multi-lag.
Depending on your desired level of control and the speed of change in you process, you may elect to employ one of several control modes.
On-Off. On-off control has two states: fully off and fully on. To prevent rapid cycling, some hysteresis is added to the switching function. In operation, the controller output is on from startup until temperature setpoint is achieved. After overshoot, the temperature then falls to the hysteresis limit, where power is reapplied (figure 2). On-off control can be used where:
- Where some temperature oscillation is permissible.
- The process is underpowered and the heater has little storage capacity.
- On electromechanical systems (compressors) where cycling must be minimized.
Proportional controllers can have two adjustments: manual reset and bandwidth (gain). Manual reset allows positioning the band with respect to the setpoint so that more or less power is applied at setpoint to eliminate the offset error inherent in proportional control (figure 4). Bandwidth (gain) permits changing the modulating bandwidth to accommodate various process characteristics. High gain, fast processes require a wide band for good control without oscillation. Low gain, slow moving processes can be managed well with narrow band to on-off control (figure 5).
The gain/bandwidth relationship is expressed inversely:
Gain = 100% DIVIDED BY Proportional Band (in %)
Proportional with Integral Automatic Reset. With this control mode, integral action moves the proportional band to increase or decrease power in response to temperature deviation from setpoint. The integrator slowly changes power output until zero deviation is achieved. Integral action cannot be faster than process response time or oscillation will occur.
Proportional with Derivative Rate Action. With this control mode, the derivative action moves the proportional band to provide more or less output power in response to rapidly changing temperature. Its effect is to add lead during temperature change. It also reduces overshoot on startup.
Proportional Integral Derivative (PID). This type of control is useful on difficult processes. Its integral action eliminates offset error while derivative action rapidly changes output in response to load changes.
What About Outputs?Load power can be switched by four different proportioning means. With current proportioning, a 4 to 20 mA signal is generated in response to the heating percentage requirement. This signal is used to drive SCR power controllers and motor-operated valve positioners. During phase-angle control, only a portion of an AC sine wave is applied to the load. The effect is similar to light dimmer function. If time-proportioning control is used, a clock produces pulses with a variable duty cycle. Outputs are either direct- (used for cooling) or reverse-acting (used for heating).
Keep in mind that in time-proportioning control, the cycle time normally is adjustable to accommodate various load sizes. A low mass radiant or air heater requires a fast cycle time to prevent temperature cycling. Larger heaters and heater load combinations can operate satisfactorily with longer cycle times. Use the longest cycle time consistent with ripple-free control.
Power Handling. Power is switched to an electric heating load through the final control element. Small, single-phase 120/240 V loads may be connected directly to the temperature controller. Larger, higher voltage heaters must be switched through an external power handler. Power handlers are either large relays (contactors), solid-state contactors or power controllers.
Probably the most widely used power handling devices, mechanical contactors are rugged and have fuses to protect against burnout due to shorts. But, they will wear out in time due to contact arcing and cannot be fast-cycled for low-mass loads. Also, they produce RF switching noise.
Used on loads requiring fast switching times, solid-state contactors need heat sinking and I2T fuse protection. SCR power controllers switch AC power by means of thyristors - solid-state devices that are turned on by gate pulses. SCRs have unlimited life, require no maintenance and are available for switching single- or three-phase loads in zero crossing/burst firing or phase-angle modes.
Power controls are connected to the control signal and load. The control signal to the power controller may originate from a manual potentiometer, PLC or temperature controller. This signal normally is 4 to 20 mA, but it can be other currents or voltages. An increase in the signal level produces an increase in output.