Process Control Methods: On/Off Control

Telmar Alarm/Limit Controller

Many athletes are familiar with the term “turning it on” or “turning it up,” typically in reference to a great performance or improvement in play. “They really turned it on in the second half” or “They turned it on in the third quarter.” The frustrating part for those athletes, though, is that they sometimes cannot control when the magic happens. Sure, they can train well to put themselves in positions to have great games, but very few athletes have the ability to flip a switch and ‘turn it on’ by command; those who possess anything close to controllable magic may go down as some of the best to ever play their respective sport.

Thankfully, in the world of process control, a system exists where it is possible for a control element to be determined by either being turned on or off: on/off control theory is based on the idea that there are two positions for a specific control element, i.e. open or closed. The lack of a middle-ground position may sound absolutist at first, and it’s not the most complex method employed by process controllers, but there are distinct advantages to the on/off system. For example, on/off control is often used industrially; however, basic home appliances such as fridges and ovens both utilize on/off control. The oven and the fridge are both used for straightforward purposes, matching the control method – such is why on/off control is not very popular for use in a commercial setting requiring a wide range of complexity. Another prime example of on/off control is a heating system, where, when the house gets cold, the heater turns on, and when the house reaches a certain temperature, the heater turns off. The process variable, in this case the temperature, is determined by the output, meaning that when the output crosses a certain threshold, the change occurs in the system being switched from either on to off, or off to on.

Due to the aforementioned nature of the controlled output being either 100% or 0%, this method of control is not the best for every application. One of the most common uses for on/off control relates to HVAC systems, where maximum output is being delivered or the system is off. To prevent nearly constant oscillation between the desired temperature and the range either above or below, a staple of on/off control oriented processes is the deadband. A deadband is essentially the process control equivalent to a demilitarized zone, a designed neutral space where no change in the output signal occurs. Let’s say the heating thermostat in a house has a setpoint of 65°F. If the deadband range of the thermostat is 4°F, the furnace will start when the measured air temperature is 61°F (65 minus 4). The furnace will run until the air temperature reaches 65°F. The deadband, depending on the capability of the controller, can sometimes be repositioned in relation to the setpoint, but the key function of deadband remains the same. No change in controller output state occurs while the process variable, in this case room temperature, is in the deadband. This keeps machinery from rapidly cycling on and off, with resulting excess wear and tear or other negative consequences. The deadband compensates for the disadvantage of such an on/off control system in terms of absolution. Another process which illustrates on/off control is a liquid tank filling operation. When liquid in a tank reaches a certain level, level sensors and switches exist which, upon sensing the liquid  at a predetermined level, will send a signal to a controller, causing a fill valve to close or pump to cease operation.

The same elements which make on/off control appealing and compatible with some processes also render some disadvantages when applied in other scenarios. The aforementioned rapid cycling may have been somewhat corrected thanks to the theory and implementation of the deadband, but issues relating to the black-and-white operational principle also provide a potential drawback in different systems. A delay with a time value greater than zero exists in a good amount of practical on/off control situations where ‘dead time’ impacts the time it takes for an on/off position to switch. This means the value thresholds established for each position may be crossed before the opposing position kicks in to bring the value back to within the intended control range. For processes requiring strict value limits to be maintained with little or no margin for error, this particular truth of on/off control is extremely hard to ignore. Also, systems which are working together to form a more complex, connected process are rarely used in conjunction with on/off control because when the oscillations in on/off controlled systems occur – especially in processes where deadbands are inapplicable – they could ricochet through the connected system and introduce unforeseen complications and process instability. The ‘overshooting’ of the element being controlled is a common risk to consider when evaluating the potential disadvantages of on/off systems, causing them to be typically incompatible with environments where precise regulation is required. That said, the relatively low cost and simplicity of this process control method makes on/off control a rugged and long-lasting choice for systems which can function well within the limitations of the controller.

Properly applied, on/off control methodology can provide an inexpensive and effective solution that is easy to apply and maintain. Analynk, under their Telmar brand name, manufactures numerous analog devices that can be applied in a process control application. Share your process control challenges with product application specialists, leveraging your own experience and knowledge with their product application expertise to develop effective solutions.

Attaining Close Temperature Control of Flowing Process Liquids

Selection and placement of temperature sensors is a
critical element of achieving close temperature control.

Temperature control is a common operation in the industrial arena. Its application can range across solids, liquids, and gases. The dynamics of a particular operation will influence the selection of instruments and equipment to meet the project requirements. In addition to general performance requirements, safety should always be a consideration in the design of a temperature control system involving enough energy to damage the system or create a hazardous condition.

Let's narrow the application range to non-flammable flowing fluids that require elevated temperatures. In the interest of clarity, this illustration is presented without any complicating factors that may be encountered in actual practice. Much of what is presented here, however, will apply universally to other scenarios.

What are the considerations for specifying the right equipment?

KNOW YOUR FLOW

First and foremost, you must have complete understanding of certain characteristics of the fluid.

• Specific Heat - The amount of heat input required to increase the temperature of a mass unit of the media by one degree.
• Minimum Inlet Temperature - The lowest media temperature entering the process and requiring heating to a setpoint. Use the worst (coldest) case anticipated.
• Mass Flow Rate - An element in the calculation for total heat requirement. If the flow rate will vary, use the maximum anticipated flow.Maximum Required Outlet Temperature - Used with minimum inlet temperature in the calculation of the maximum heat input required.

MATCH SYSTEM COMPONENT PERFORMANCE WITH APPLICATION

Heat Source - If temperature control with little deviation from a setpoint is your goal, electric heat will likely be your heating source of choice. It responds quickly to changes in a control signal and the output can be adjusted in very small increments to achieve a close balance between process heat requirement and actual heat input.

Sensor - Sensor selection is critical to attaining close temperature control. There are many factors to consider, well beyond the scope of this article, but the ability of the sensor to rapidly detect small changes in media temperature is a key element of a successful project. Attention should be given to the sensor containment, or sheath, the mass of the materials surrounding the sensor that are part of the assembly, along with the accuracy of the sensor.

Sensor Location - The location of the temperature sensor will be a key factor in control system performance. The sensing element should be placed where it will be exposed to the genuine process condition, avoiding effects of recently heated fluid that may have not completely mixed with the balance of the media. Locate too close to the heater and there may be anomalies caused by the heater. A sensor installed too distant from the heater may respond too slowly. Remember that the heating assembly, in whatever form it may take, is a source of disturbance to the process. It is important to detect the impact of the disturbance as early and accurately as possible.

Controller - The controller should provide an output that is compatible with the heater power controller and have the capability to provide a continuously varying signal or one that can be very rapidly cycled. There are many other features that can be incorporated into the controller for alarms, display, and other useful functions. These have little bearing on the actual control of the process, but can provide useful information to the operator.

Power Controller - A great advantage of electric heaters is their compatibility with very rapid cycling or other adjustments to their input power. A power controller that varies the total power to the heater in very small increments will allow for fine tuning the heat input to the process.

Performance Monitoring - Depending upon the critical nature of the heating activity to overall process performance, it may be useful to monitor not only the media temperature, but aspects of heater or controller performance that indicate the devices are working. Knowing something is not working sooner, rather than later, is generally beneficial. Controllers usually have some sort of sensor failure notification built in. Heater operation can be monitored my measurement of the circuit current.

SAFETY CONSIDERATIONS

Any industrial heater assembly is capable of producing surface temperatures hot enough to cause trouble. Monitoring process and heater performance and operation, providing backup safety controls, is necessary to reduce the probability of damage or catastrophe.

High Fluid Temperature - An independent sensor can monitor process fluid temperature, with instrumentation providing an alert and limit controllers taking action if unexpected limits are reached.

Heater Temperature - Monitoring the heater sheath temperature can provide warning of a number of failure conditions, such as low fluid flow, no fluid present, or power controller failure. A proper response activity should be automatically executed when unsafe or unanticipated conditions occur.

Media Present - There are a number of ways to directly or indirectly determine whether media is present. The media, whether gaseous or liquid, is necessary to maintain an operational connection between the heater assembly and the sensor.

Flow Present - Whether gaseous or liquid media, flow is necessary to keep most industrial heaters from burning out. Understand the limitations and operating requirements of the heating assembly employed and make sure those conditions are maintained.

Heater Immersion - Heaters intended for immersion in liquid may have watt density ratings that will produce excessive or damaging element temperatures if operated in air. Strategic location of a temperature sensor may be sufficient to detect whether a portion of the heater assembly is operating in air. An automatic protective response should be provided in the control scheme for this condition.

Each of the items mentioned above is due careful consideration for an industrial fluid heating application. Your particular process will present its own set of specific temperature sensing challenges with respect to performance and safety. Share your requirements with temperature measurement and control experts, combining your process knowledge with their expertise to develop safe and effective solutions.