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.

Overcoming Signal Attenuation in Industrial Wireless Transmission

Wireless signals facilitate mobile and remote connections
of industrial process measurement and control equipment

Wireless connections for process measurement and control devices continue to grow in their application range and adoption. The ease with which remote, mobile, even nearby devices and equipment can be connected to monitoring and control stations keeps wireless connectivity a considered option for many facility modifications and additions.

How do you know if the wireless connection you intend to establish will work? It's not a question of whether the gear will function properly, but one of whether the signal will be able to find its way from point A to point B. Signal attenuation is the reduction in signal strength that occurs along the path between two points. Too much attenuation and the signal is not effective in delivering data to the destination. Two main elements contribute to signal attenuation.

Distance

Radio signals deteriorate over the distance traveled. This attenuating factor can be overcome by boosting transmission power, but regulatory limits are in place that disallow much in the way of increasing transmission power. Keeping in mind that signal attenuation at a level that renders a transmission indecipherable, just caused by distance alone, requires a very substantial distance. That said, there are several effective solutions that can be put in place. One involves increasing the height of the transmission antenna. Another is to install a repeater at a point along the transmission path. A repeater, properly placed, will effectively receive the signal from the transmitter, then transmit a new signal of greater strength that replicates the original received data. The use of repeaters can greatly extend the distance spanned by a wireless transmission.

Obstructions

Many are familiar with the objective of establishing "line of sight" transmission paths between transmitter and receiver. Physical structures and materials of all types should be considered detrimental to the transmission of wireless signals. A basic understanding of Fresnel zones is helpful in overcoming the barriers presented by physical obstructions. As with distance, antenna height or location can be a significant factor in dealing with the challenge of obstructions. Repeaters, described earlier, can be instrumental in getting your signal over or around otherwise impassable obstructions.

Routing wireless signals presents different, but no more complicated, challenges than routing cable. The tools are different, the medium is different, but you are still just trying to find a way from point A to point B. With experience, wireless signal propagation becomes is mundane is routing conduit.

Share your industrial wireless ideas and challenges with the experts at Analynk Wireless. Analynk Wireless manufactures equipment used to establish wireless process connections across the room, across the plant site, across the highway, and around the globe.

Industrial Wireless Application: Remote Equipment Monitoring

The A16000 Expansion Module increases the I/O
capacity of a standard wireless transmitter or receiver
Image Analynk Wireless, LLC

Imagine yourself a newly hired facilities manager, the go-to person responsible for the proper and continuous performance every machine on site. One machine in particular, you are informed, is a large walk-in refrigerator that houses the primary raw material for the production operation. There is a lot riding on that machine because the stored material is useless if not kept cold. Oh, and by the way, the plant site is bisected by a public street and the refrigerator is located across the street from the main building where the facilities office is housed. Your assessment of the equipment reveals that a lone temperature alarm device monitors refrigerator temperature and sounds a loud horn if the refrigerator temperature reaches a high limit setpoint. There are no existing wire pathways between the main building and the walk-in refrigerator that are available for your use.

Clearly, the level of risk associated with the refrigerator is high. It merits implementation of an improved strategy to monitor refrigerator performance. Things under consideration include some the following items.

• A real time display of the current refrigerator temperature in the facilities management office. 
• Analysis of the temperature data for an upward or downward trend that might indicate the beginning of a malfunction of the cooling system or controls.
• Monitoring of refrigeration compressor motor current, which can be related to the temperature data to confirm that the compressor is operating when it should.
• Real time display of refrigerant suction and discharge pressures.
• Analysis of refrigerant suction and discharge pressure to identify trends or conditions that may indicate service is needed or malfunction is imminent.
• Verify the door to the refrigerator is closed.
• Monitor evaporator fan motor current to verify that all fans are operating.

The greatest challenge in this application is not the gathering of the information, nor its analysis. The difficulty, as well as a substantial cost and time constraint, is delivering the information from the point of measurement to the point of use. Analog signals for real time temperature, refrigerant pressure, and motor current can be easily derived through the addition of sensors to the equipment. The only sensors likely to require intrusive work to install are those for refrigerant pressure. Routing the measurement signals to the facilities office across the road may prove difficult.

A wired connection between the measurement location to the facilities office will require either an underground or overhead routing of cable, traversing the public road. Permission from state, county, and/or local jurisdictions may be required and present potential barriers to timely completion of the project. The cost to install the cabling will be substantial. The distance may be long enough for signal attenuation to be a concern.

The best solution, in terms of initial cost and time to completion, is to establish a dedicated wireless connection between the walk-in refrigerator and the facilities office.

 A multi-input transmitter is installed at the walk-in refrigerator. The transmitter converts digital (switch) and analog input signals into encrypted digital data and transmits in the 900 MHz band to the receiver installed in the facilities office. The receiver decrypts the received data and mirrors the original analog and digital signals at its output terminals. Wireless overcomes the barriers presented by a wired installation, allowing completion in a timely manner at substantially reduced cost.

If you can operate a walkie-talkie, you can establish industrial wireless connections between remotely located, or mobile, equipment and central monitoring locations. Share your ideas and challenges with industrial wireless experts, leveraging your own knowledge and experience with their application expertise.

Remote Monitoring of Valve Position

Battery powered industrial wireless transmitter
factory installed in hazardous area rated enclosure
with hazardous area rated antenna.

The use of a centralized control or monitoring station is prevalent throughout many industrial applications. Employing a single location as collector and processor of all available information has operational advantages. 

Operations with control valves installed at remote locations face the challenge of determining whether the valve is responding properly to control commands or the demands of the process. Older systems, and some not so old systems as well, may have in-place valves that do not provide a confirmation signal of valve position. Many products are available for retrofitting this capability to a wide range of existing valves, but the challenge of delivering the valve position signal to the control center remains. It is unlikely that spare signal cable conductors were installed at the time of valve installation, so there are two clear options.

• Install signal cable from the valve position indicator to the control center.
• Install a wireless signal transmission system for the valve position indicator.

Unless the valve is located very close to the control center, the wireless option offers a more simple and cost effective method of connecting the valve position transmitter with the control center. Here are the basic tasks.

• Install transmitter at valve location in a suitable enclosure.
• Provide power to radio transmitter. Almost any power source can be accommodated. Power consumption is low enough to allow the use of a small solar panel and battery arrangement, if needed.
• Connect the valve position transmitter output to the radio transmitter input.
• Install a companion radio receiver at the control center, or where a wired signal can be routed easily to the control center. Provide a suitable enclosure.
• Provide power to the receiver, using any of the options available for the radio transmitter described previously.
• Connect the output of the radio receiver to an appropriate input on the central control system.
• Set the communications channels on the radio transmitter and receiver.
• Power up the system.

The transmitter digitizes and encrypts the input signal from the valve position indicator, then transmits the data via 900 MHz or 2.4 GHz to the receiver. The receiver decrypts the data and mirrors the signal that was delivered by the valve position indicator to the radio transmitter. 

This is a simplified illustration, but the implementation of a wireless connection between process components, even with all the details, is not difficult. Transmitters and receivers can be ordered pre-configured, wired, and installed in a specified enclosure, requiring comparatively little field labor. Analynk welcomes customized application challenges and your questions about how to utilize wireless connections in your operation. 

Analynk Wireless manufactures equipment used to establish wireless process connections across the room, across the plant site, across the highway, and around the globe.

Hazardous Area Enclosure for Aurba Access Point... and Others

AP623 Wireless access point enclosure
facilitates installation of commercial equipment
in hazardous industrial locations.

Analynk Wireless continues to expand its extensive line of wireless access point enclosures for hazardous industrial locations. The model AP623 is a recent addition, specially equipped to house the Aruba AP-314 dual band access point.

Analynk access point enclosures accommodate specific wireless access points from a range of manufacturers, facilitating easy installation. Every model includes UL listed explosion proof antennas, a mounting bracket custom tailored for the access point equipment, and RF cables to make the antenna connections. Enclosures have penetrations located to match up with the specified access point. Models are included to house a range of units from Symbol, Cisco, Meru, Aruba, HP, and Motorola, with more models added regularly to accommodate additional wireless access points.

Analynk Wireless specializes in industrial wireless communications. Your wireless communication challenges are welcome at Analynk, so make contact and share your requirements. Combining your process expertise with Analynk's product specialization will produce an effective solution.

The datasheet for the new model is provided below. You can see all the models and their companion access points on the Analynk site.

AP623 Hazardous Area Wireless Access Point Enclosure from Analynk Wireless, LLC

Product Options Match Wireless Equipment to Application

Analynk Wireless offers a broad range of accessories, options, and custom configuration and design services to complement their standard product offering of industrial wireless equipment. The specialty offerings can be used to extend the suitability of a standard product for a specific application, or to create a complete package with all necessary mounting brackets, antennas, cables, and other ancillary equipment needed for a complete installation. Analynk extends the same customizing capability throughout its Telmar line of process measurement and control products, as well.

The company welcomes all inquiries related to custom configuration, accessories, and turnkey system design. Share your wireless connectivity or process measurement and control challenges with the experts and Analynk, combining your process knowledge and experience with their design expertise to develop an effective solution.

Analynk Wireless Enclosure Options and More from Analynk Wireless, LLC

Rotary Vane Actuators for Quarter Turn Valve Automation

The inside of a rotary vane actuator
Image courtesy of Kinetrol

A rotary vane actuator is part of an automated valve assembly. Its role is to change the position of the valve trim, converting the motive force of fluid pressure in the actuator into torque at the valve stem.

Quarter turn valves are widely used in industrial fluid processes. Their application is primarily for operations requiring fully open or fully closed valve trim positions, although some do provide modulating service. A rotation of the valve stem through a 90 degree arc will reposition quarter turn valves between open and closed positions. A rotary vane actuator is well suited for driving this type of valve, with its own 90 degree arc of movement.

A rotary vane actuator is specific for application to quarter turn valves. A pressure tight housing contains a movable vane which is sealed to the sides of the pressure chamber by means of a low friction gasket. Inlets into the chamber on opposing sides of the vane allow a controller to produce a pressure differential across the vane. The vane will move, responding to the pressure differential, in either direction. A shaft is connected to the vane and the vane acts like a lever to rotate the shaft as the vane is moved by fluid pressure. The torque produced by the actuator assembly is primarily dependent upon the applied fluid pressure.

Hydraulic rotary vane actuators have the ability to handle large amounts of fluid and dynamic motions, exhibiting also qualities of durability and compactness. Pneumatic vane actuators use plant air pressure as the motive force. Both types generally have few moving parts and require little regular maintenance. A variety of typical automation accessories and options are available to customize a unit for a particular application.

Analynk Wireless manufactures wireless connectivity solutions for industrial applications and process control. Making cable free connections among process control equipment and instruments, across the room, across the property, across the globe.