Process control refers to the methods used to maintain the output of process variables– such as temperature, pressure, flow, or level– within a desired range. Precise
control of these variables is critical in industrial settings as it improves the quality of products while enabling automation, allowing smaller staffs to monitor and
control complex processes from a central location.
Process control is part of a closed loop system in which a process variable is measured, compared to a setpoint, and action is taken to correct any deviation from the
setpoint. Closed loop control is feedback-dependent; receiving feedback from sensors monitoring the process variable and providing feedback to the final control element
that corrects any deviation from the setpoint. By carefully monitoring and correcting process variables, controllers greatly assist in reducing variability, increasing
efficiency, and ensuring safety. Any equipment that requires constant monitoring of a process variable can benefit from a process controller.
Let’s use the example of an automated production facility that makes cookies. Process controllers are responsible for delivering a specific ratio of ingredients, mixed
them together for an exact amount of time before being portioned into a consistent size and shape. A conveyor transports the raw cookies to the oven where they are baked
to a perfect consistency and counted out for packaging.
In the above example, controllers monitor and correct temperature, pressure, batching, humidity and other processes. If any of these were out of specification, the cookies
would be ruined. It is the process controllers that reduce variability in the product and guarantee a consistent cookie.
Years ago, workers would’ve handled all these processes manually-- checking temperatures, mixing ingredients, timing the baking. The process was much slower, less cost
effective, and output was lower because of that. Now production is highly automated. It is process controllers that are responsible for the increase in efficiency.
Process variables such as pressure and temperature are potentially dangerous. Many of the “ingredients” used in industry (though not necessarily in cookies) are harmful to
people and/or the environment. It is process controllers that maintain conditions to ensure safety.
Control system components
Process controllers are arranged into control systems (also known as control loops) that consist of the controller, any associated sensors, a power supply, the final control
element, as well as any necessary load handling devices.
As we know, controllers seek to maintain the measured process variable at a preset point.
Sensors provide the input signal to the controller. That signal is based upon a measurable physical property like temperature, pressure, pH, flow, level, etc. There are a
staggering array of sensors, transmitters, and transducers compatible with process controllers. Nearly the only limitation is the type of signal a controller is capable of
reading. More sophisticated controllers accept voltage, current, contacts, frequency, thermocouple/RTD, and other signal types.
The final control element refers to the device that acts upon orders from the controller. It can be a heater that is activated when the sensor finds a temperature lower than
the set point or a valve that opens when the pressure sensor measures a pressure higher than the set point.
Process controllers, many (though not all) sensors, and final control elements require power to operate. A power supply is an integral element to control loops
Control loops regularly feature additional instruments. Transmitters or signal conditioners are often used to isolate, filter, amplify, or convert a sensor input signal when
conditions dictate it. Control loops also frequently include data acquisition devices for archiving information related to the process.
Load handling devices are often needed when the final control element, such as heaters or solenoids, require more power to operate than can be supplied by the controller.
Types of Control Action
Depending upon the unit, process controllers are capable of providing multiple types of control which are suited to different applications and process variables.
On-Off Control
On-off control, also called hysteresis control, is the simplest type of control. As expected, on-off controllers switch abruptly between two states with no middle state.
They are for use with equipment that accepts binary input, for example a furnace that is either completely on or completely off.
On-off controllers only switch output when the set point has been crossed. In the case of heating control, the controller switches on when below the set point and off when
above the set point. To prevent rapid cycling of the system which can cause damage, hysteresis or on-off differential, is added to the controller operations. The differential
prevents cycling by exceeding the setpoint by a small amount before the controller switches on or off.
On-off controllers are often used in applications that don’t require precise control, in systems which cannot handle having the energy turned on and off frequently, where
the mass of the system is so great that temperatures change extremely slowly, or for temperature alarms.
PID Control
PID control uses three different control terms; proportional (P), integral (I), and derivative (D) to help the controller’s algorithms provide a more accurate response to
deviations from the set point.
When a controller receives input that a process variable has varied from the set point, instructions are sent to the final control element for correction. For example, a
controller receives a signal from a thermocouple that a process temperature is too low prompting the controller to turn on a heater to bring it back up to temperature.
Simple on-off control often leads the final control element to overshoot the set point, especially when the original deviation was small. Repeatedly overshooting the set point
causes the output to oscillate around the setpoint in either a constant, growing, or decaying sinusoid. The system is unstable if the amplitude of the oscillations continuously
increase with time.
PID controllers use the algorithm derived from their three control terms to maintain system stability by limiting overshoot and resulting oscillation. The proportional variable
controls the rate of correction so that it is proportional to the error. The integral and derivative variables are time-based and help the controller automatically compensate
to changes in the system. The derivative variable considers the rate at which the error is increasing or decreasing while the integral variable uses knowledge of accumulated
errors to the length of time the process is not at the set point. This information is used to correct the proportional value.
PID controllers are generally considered the most efficient type of controller. They are widely used in industrial settings. Though each of the variables must be tuned to a
particular system, PID controllers provide very accurate and stable control.
In order to make PID controllers even more responsive to real-world situations, many manufacturers have incorporated fuzzy logic (or fuzzy control)
into the instruments. Fuzzy logic is a mathematical system that attempts to emulate human reasoning. Rather than the binary logic of standard controllers, fuzzy logic introduces
continuous variables which provide an effective means of capturing the approximate, inexact nature of the real world.
This ability enables controllers with fuzzy logic to make quick, subtle changes that significantly improves response to fast-changing variables independent of the programming
done by the operator. For example, as heaters, valves and other final control elements age, they show signs of wear and no longer respond in the same way as they did when new.
Fuzzy logic recognizes this and automatically compensates.
Profile Control
Profile Control refers to controlling a changing process variable against time. Users input the desired time and process profile with the help of extensive instruction set like
jump, loop, loop with count apart from ramp and soak control.
Profile control is especially useful for cycling applications which require multiple temperature profiles as well as specific on and off periods.
Limit Control
Limit control involves an independent switch which will shut down the system if a process variable crosses a preset threshold. Limit controllers are for use in processes where
for safety or quality issues, a process variable must be kept within specified tolerance levels.
Limit controllers are designed to work in conjunction with another controller. These units also require a manual rest to acknowledge the limit relay has been activated.
Things to consider when selecting a process controller:
- What type of input is provided by the sensors?
- What type of control is needed?
- What power supply is available to power the controller?
- What amperage and voltage is required for the load?
- What type and number of outputs are needed to control load or load handling devices?
- What size controller is required?
- Are there any mounting requirements for the controller?
- What are the environmental conditions the controller will be exposed to?
- Are any auxiliary functions required such as communications, remoter setpoint, retransmission, etc.?
- What temperature range is required?
If you have any questions regarding process controllers, please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
