Control SystemsAuthor: Matt Ridler
Copyright (c) 2008 Matt Ridler
A HVAC control system is a device or set of devices to manage, command, direct or regulate the behavior of other devices or systems.
There are two common classes of HVAC control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls. There is also fuzzy logic, which attempts to combine some of the design simplicity of logic with the utility of linear control. Some devices or systems are inherently not controllable.
The term "control system" may be applied to the essentially manual controls that allow an operator to, for example, close and open a hydraulic press, where the logic requires that it cannot be moved unless safety guards are in place.
An automatic sequential control system may trigger a series of mechanical actuators in the correct sequence to perform a task. For example various electric and pneumatic transducers may fold and glue a cardboard box, fill it with product and then seal it in an automatic packaging machine.
In the case of linear feedback systems, a control loop, including sensors, control algorithms and actuators, is arranged in such a fashion as to try to regulate a variable at a setpoint or reference value. An example of this may increase the fuel supply to a furnace when a measured temperature drops. PID controllers are common and effective in cases such as this. Control systems that include some sensing of the results they are trying to achieve are making use of feedback and so can, to some extent, adapt to varying circumstances. Open-loop control systems do not directly make use of feedback, but run only in pre-arranged ways.
Pure logic control systems were historically implemented by electricians with networks of relays, and designed with a notation called ladder logic. Today, most such systems are constructed with programmable logic devices.
Logic controllers may respond to switches, light sensors, pressure switches etc and cause the machinery to perform some operation. Logic systems are used to sequence mechanical operations in many applications. Examples include elevators, washing machines and other systems with interrelated stop-go operations.
Logic systems are quite easy to design, and can handle very complex operations. Some aspects of logic system design make use of Boolean logic.
For example, a thermostat is a simple negative-feedback control: when the temperature (the "measured variable" or MV) goes below a set point (SP), the heater is switched on. Another example could be a pressure-switch on an air compressor: when the pressure (MV) drops below the threshold (SP), the pump is powered. Refrigerators and vacuum pumps contain similar mechanisms operating in reverse, but still providing negative feedback to correct errors.
Simple on-off feedback control systems like these are cheap and effective. In some cases, like the simple compressor example, they may represent a good design choice.
In most applications of on-off feedback control, some consideration needs to be given to other costs, such as wear and tear of control valves and maybe other start-up costs when power is reapplied each time the MV drops. Therefore, practical on-off control systems are designed to include hysteresis, usually in the form of a deadband, a region around the setpoint value in which no control action occurs. The width of deadband may be adjustable or programmable.
Linear control systems use linear negative feedback to produce a control signal mathematically based on other variables, with a view to maintaining the controlled process within an acceptable operating range.
The output from a linear control system into the controlled process may be in the form of a directly variable signal, such as a valve that may be 0 or 100% open or anywhere in between. Sometimes this is not feasible and so, after calculating the current required corrective signal, a linear control system may repeatedly switch an actuator, such as a pump, motor or heater, fully on and then fully off again, regulating the duty cycle using pulse-width modulation.
About the Author:
The 5 Key Points to Effective TroubleshootingAuthor: Terry Howsham
You don't realize it, but in the next few minutes you're going to learn to the important skill of troubleshooting an Industrial Control system.
Industrial control equipment can malfunction for a diversity of reasons- that’s life. No matter how well a system is maintained, you cannot prevent all failures. Mechanical contacts, pilot lamps and moving parts such as switches can wear out; on poorly designed systems wires can overheat and burn open or short out. Some parts can even be damaged by the environment. When certain components in a system are damaged equipment may operate in a manner far different than it was designed to, or not at all.
Typically, when process system fails there is a sense of importance to get it fixed and working again as soon as possible. If the defective equipment is part of an assembly line, the whole assembly line could be down causing unforeseen “stoppages” with loss of revenue. If you are at a customer’s site to repair equipment, the customer’s staff may watch you, knowing that they are paying for every minute you spend troubleshooting and repairing their control system. The pressure on you now to solve the problem as quickly as possible! You are now the expert- even though you may have no clue as to what their process does!
So what is troubleshooting?
It is the process of analyzing the behavior of a system to determine what is wrong with it, if anything, and then work out which piece of equipment is not functioning correctly. Now, depending on the type of equipment, troubleshooting can be a very challenging task.
Sometimes problems are easily diagnosed and the problem component is easily visible; such as a blown fuse. Other times the symptoms as well as the faulty component can be difficult to identify. A blown fuse with a visual indicator is easy to spot, whereas an intermittent problem caused by a high resistance connection or loose terminal can be much more difficult to find.
So what makes an expert Troubleshooter?
One quality of expert troubleshooters is that they are able to find virtually any fault in a practical amount of time. By using a basic common sense approach, they find them all. Another quality they have is the knack for finding out exactly what is wrong. No trial and error here. So what is their secret?
Expert troubleshooters have a good understanding of the operation of electrical components, mechanical systems and their components, process controls and control theory. They have an approach that allows them to logically and systematically analyze a system and determine exactly what is wrong. They also understand and effectively use tools such as electrical diagrams, mechanical process diagrams and test instruments to identify defective parts.
Here is a list of skills that you need to troubleshoot a control system.
(1) Work safely! Be aware of your surroundings. This sounds easy, but under pressure to fault find quickly, mistakes can be made. Ask yourself these questions as you work: Are there high voltages in this control panel? Do I need a hard hat or safety glasses to work safe? Are there any dangerous chemicals or processes under high pressure near me?
Arrive on site with an effective amount of tools to help you troubleshoot. Take with you any hand tools, Multimeters, loop calibrators, PC with PLC programming software that you feel will be needed. It is more professional to arrive prepared than to have to keep going back off site for more tools, or even worse, asking the customer to ‘borrow’ his tools.
(2) Listen with an open mind! Ask the operators of the control system what the symptoms are, and also ask any maintenance workers what they think the problem is. How does the system function normally? What has changed? When did it start? You may not be a doctor, but you are diagnosing problem. Only ask pertinent questions.
(3) You need to understand how process controls work. This consists of understanding the operation of components in the system such as PID loops, Industrial ventilation, fans, pumps, valves, PLC systems, Instrumentation such as temperature transmitters, push buttons, contactors, pilot lights, switches, relays, sensors, motors, and much more.
PLC control systems operate mechanical systems such as motors and valves. Could you tell an electrically actuated ball valve from a mechanical check valve? Can you recognize if you are looking at a relay or a contactor in a control panel?
(3) Use a logical, systematic approach to analyze the system’s behavior. This is critical. There are several approaches that troubleshooters use. They may have different steps or processes but they have the following in common: They approach problems systematically and logically thus minimizing the steps and ruling out trial and error.
One such approach used to teach troubleshooting is called the “5 Step Approach”. Here is a summary of the key steps are:
* Observe. A good number of faults provide clues as to their cause. There could be visual clues such as signs of damage, improper operation, lack of control, or no response. Don’t forget to use your other senses; sounds and smells can also provide valuable clues.
* Define Problem Area(s). This is where you apply logic and reasoning to your observations to determine the problem area of the control system.
* Identify Possible Causes. Once you have the problem area(s) defined, you need to identify all the possible causes of the failure.
* Determine The Most Probable Cause. Once the list of possible causes has been made you can prioritize the items as to the possibility of them being the root cause of the system failure.
* Test and Repair. Once you have determined the most probable cause, do some tests to prove it to be the problem or not.
(4) The knowledge of how to use tools. Do you understand how to read prints and diagrams? Can you operate test equipment such as Multimeters, loop calibrators and current probes?
Some of the key things you should be able to determine from electrical prints and process diagrams are:
-How the control system should operate.
-What voltages should you expect at various points on the control system.
-Where components are physically located. Remember, process automation transmitters such as temperature, pressure and flow are located throughout a process control system. They maybe at ground level, up near the roof, or even inside of a large tank!
-Various types of test equipment are available for testing electrical control systems. The ones that you choose depend on the type of system you are working on. A Multimeter is capable of measuring current, voltage and resistance. A loop Calibrator can measure the current signal (4-20Ma) coming from a field device such as a Temperature transmitter or it can simulate the 4-20Ma signal to test analog inputs.
(5) Practice! Troubleshooting, like any other skill, requires practice for you to become proficient at it. Practice can be difficult to get. Until you become reasonably experienced, it is best to practice troubleshooting in a controlled, offline environment.
In summary, troubleshooting a control system takes a high level of knowledge of control systems, patience to handle customers and a keen eye for detail.
About the Author:
Terry Howsham is a Senior Electrical Engineer for Unified Theory Inc, Camarillo, CA. Specializing in Process Control.
For more information please visit our main websiteUnified Theory Inc . We are a full service engineering firm specializing in facility and process design for industrial clients.