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PID Control Theory Made Easy Optimising Plant Performance with Modern Process Controllers 1

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PID Control Theory Made Easy Optimising Plant Performance with Modern Process Controllers 1 Measurement Products PID control theory made easy Optimising plant performance with modern process controllers 1. What are the main types of control applications? Introduction 1. What are the main types of control and rubber processing applications Accurate control is critical to every applications? such as tyre manufacture. In such process. As a means of ensuring that The wide variety of industrial processes processes, the main goal is to ensure tasks such as production, distribution in use today means there are many that specific quantities of products and treatment processes are carried different types of control systems, each are produced in a specific way, under out under the right conditions for the of which are geared towards their own specific conditions and for a specific right amount of time and in the right particular application. period of time. With a single error quantities, control devices form a crucial potentially rendering an entire batch part of virtually every industrial process. It is, however, possible to roughly of products unusable, tight control of In the past, many traditional control segment these processes into three process conditions is critical. processes relied on either mechanical main categories, these being: or solid state electronic technology, – Continuous processes. As their with pneumatics being used to produce – Discrete processes. These types name suggests, these processes are changes in the process, such as in valve of processes are most commonly those which produce a continuous control installations for example. operated in manufacturing and stream of products over a continuous With advances in computerised control packaging applications, where period of time. Examples include technology, there are now a host of new discrete items are being produced large scale production processes in possibilities for the way that processes and handled. This category would the chemicals, refining and plastics can be monitored and controlled. These also include robotic assembly industries. In such applications, the include the use of complex algorithms processes, for example in the car- processes tend to be largely constant, capable not only of reacting to making industry. being geared towards the production changes in process conditions but also of a specific product. increasingly predicting them as well, – Batch processes. Batch processes enabling corrective action to be taken are widely used throughout many Of course, many applications may automatically. industries, obvious examples being actually combine all of these types of food and beverage, pharmaceutical processes, which will have an impact on the control system or systems used. 2 Control theory l White paper 2. Types of control systems: 2. Types of control systems: To obtain a more accurate or more – Improved ability to help track and There are two types of traditional control adaptive control, however, it is trace faults / problems systems: necessary to use a closed-loop system, – Open-loop control systems where the output of the system is fed Some systems use closed-loop and – Closed-loop control systems back to the inputs of the controller. open-loop control simultaneously. In these systems, the open-loop control 2.1 Open loop control systems: 2.2 Closed loop control systems: is termed as ‘feedforward’ and serves Open loop controllers are also known Modern control theory is based on to help improve reference tracking as ‘non-feedback controllers’. These feedback – i.e. signals from a process performance. controllers compute their input into a that can be used to control it more system based on a specific model / effectively. A closed-loop controller uses 2.4 How do closed-loop control known set of conditions. They do not feedback to control states or outputs of systems work? use data from the process to change a system or process. The term ‘closed- As stated above, closed loop controllers their output or vary the process and loop’ comes from the information work by using the sensed value from are unable to compensate for any path in the system – process inputs a process to assess and, if needs be, disturbances in process conditions. to a system have an effect on the alter the process conditions. Instead, any variations in process process outputs, which is measured conditions would need to be achieved with sensors and processed by the A closed-loop or feedback controller by an operator manually adjusting the controller. The result (the control signal) works in the following way: A sensor final control element. is used as an input to the process, of some type measures the output of closing the loop. the system and feeds it back to be As such, open loop control systems compared against a reference value. are best suited to processes where 2.3 Benefits of closed-loop control The controller takes the difference there is a relatively stable set of systems: between the output and the reference operating conditions or where these Closed-loop controllers offer several key value and uses it to change the inputs conditions can be governed by a known advantages over open-loop controllers, to the system to help compensate for relationship. Their inherent simplicity including: the difference. and low cost makes open loop control – Ability to adapt to changing conditions systems ideally suited to simple – e.g. changing loads, process This is called a single-input, single processes where conditions are variations away from the established output (SISO) control system. Those unlikely to change and where feedback norm with multiple inputs and multiple inputs/ is not critical. – Ability to stabilise unstable processes outputs (MIMO) are also common. Control theory l White paper 3 3. PID controllers A. B. PV 100 100% 75 75% ariable Note. Control Set point Proportional 50% 50 Output Band = 50% action set ocess V to reverse Pr Engineering Range 25 25% 0 0% Time Time Fig 1. Proportional action 3. PID controllers The derivative term is used to provide Definitions: The PID controller is probably the most damping or shaping of the response. Proportional – Proportional control widely-used type of feedback controller. The action of derivative is to cater for gives a change in output that is PID stands for Proportional-Integral- disturbances and sudden changes. In proportional to the deviation of the Derivative, referring to the three terms effect, it is used to predict what is going process variable from the set point. operating on the error signal to produce to happen within the process and takes The range over which the output is a control signal. quicker action than the integral term to adjusted from 0 to 100% is called the correct it. proportional band. The proportional The desired parameters/conditions band is expressed as a percentage of for a closed loop system are normally PID controllers are the most well- the engineering range. achieved by tuning the system to the established class of control system. inherent conditions without specific When proportional-only control is knowledge of a plant model. Stability used, manual reset is required. In the can often be ensured using only the diagram above the manual reset is proportional term. Pure proportional set to 50%. This means that when the action will result in control offset. process variable is ‘at set point’ the control output will be 50%. Adjusting The integral term eliminates the offset. the manual reset value has the effect of It does this by repeating the action of shifting the proportional band up and the proportional band every integral time down. This has the effect of changing constant. This enables the system to the control output when the process recover more quickly from a disturbance variable is ‘at set point’. The manual in conditions. reset would normally be adjusted by an operator every time the set point is changed, or when an offset is present for any reason. 4 Control theory l White paper 4. Common control terms A. B. Set point Integral Response = 10% Output ariable Proportional Step = 10% ocess V Pr Note. Control action set to reverse Time Time Fig 2. Integral action (TI) Integral time constant Integral – Due to limitations with the control output in the same direction amount that it has changed due to proportional-only control, which appear as the proportional action, until the proportional action, when a constant as an offset between the process offset or error is removed. deviation is present. Fig 2B shows the variable and the set point, it is often effect that a 20 second integral action necessary to introduce an integral The integral action is expressed as the would have on the control output. action. If an offset is present, the time taken for the output to change integral action will continue to change (due to the integral action) by the same A. B. Time period being evaluated by controller Proportional + Derivative Proportional Only X% ariable Output Note. Control action ocess V X% Y% Pr set to reverse Time Time Derivative Action Time Fig 3. Derivative action Derivative – Derivative action can be is dependant on its setting. Derivative be due to the proportional action after used with proportional control, with action is set as a time constant and the derivative time period. This change or without integral. It has the effect of has the following effect on the control in output shown on the diagram as Y% making changes to the control output, output. Taking only the proportional part is then added to the current control dependant on the rate of change of the of the output as shown by the lower output value as derivative action X%. process variable. The amount of effect line on Fig 3B, the controller calculates that derivative action has on the output forward to find out what the output will Control theory l White paper 5 4.
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