PLC COMMUNICATION

COMMUNICATIONSThere are several methods to communicate between a PLC and a programmer or even between two PLCs.Communications between a PLC and a programmer (PC or Hand held) are provided by the makers and you only have to plug in a cable from your PC to the programming port on the PLC. This communication can be RS232; RS485 or TTY.Communications between two PLCs can be carried out by dedicated links supplied/programmed by the makers (RS232 etc) or via outputs from one PLC to the inputs on another PLC. This direct link method of communication can be as simple as, if an output on the first PLC is on then the corresponding input on the second PLC will be on and then this input is used within the program on the second PLC. If a word of input/outputs (16 bits) are used then numerical data can be transferred from one PLC to the other (refer back to the section on numbering systems).There are many other methods of communication between PLCs and also from PLC to PC. Please refer to the manuals supplied with the PLC that you are using for full details on communications.

integral controller

What Is An Integral Control System?Often control systems are designed using Integral Control. In this control method, the control systems acts in a way that the control effort is proportional to the integral of the error. You should have studied proportional control before tackling this lesson. A proportional control system is shown in the block diagram below.The proportional controller amplifies the error and applies a control effort to the system that is proportional to the error. In integral control, the control effort is proportional to the integral so the controller now needs to be an integrator, and it will have a transfer function of Ki/s - not just a gain, Kp.What Do You Need To Get From This Lesson?This is a short lesson. The goals are simple. Given a closed loop, integral control system,Know that the SSE is zero - exactly! Be able to explain why the SSE can be zero even though there is no input to the integrator.What Is Integral Control? - Some BackgroundIntegral control is what you have when the signal driving the controlled system is derived by integrating the error in the system. The transfer function of the controller is Kp/s, if you think in terms of transfer functions and Laplace transforms. That is what is shown in the diagram below.That's the general outline, but to understand how integral control really works, it helps to understand exactly what an integral is. Let's consider that a while.To use integral control you really need to understand what an integrator is and what an integral is. Let's get back to basics. An integral is really the area under a curve. Let's assume that the independent variable is time, t. Then as time goes on the area accumulates. In math courses when they talk about integration, they picture it as the limit of a process of taking small incremental areas - shown below - and letting the interval, T, shrink to zero. In digital integration, that visualization process is important. Here is an approximation to an integral that is a sum of areas under the curve of the function being integrated.If the integral starts at zero, then the integral is just the area under the curve. Let's look at some implications of that.If the input goes to zero, then the integral stops changing and just has whatever value it had just before the input became zero.The integral can change in either direction as the signal goes positive and negative. Negative area can subtract from positive area, lowering the value of an integral. The first point here is very important because it has implications for the way that the error in the system behaves. The second point has strong implications for overall system behavior, particularly for understanding overshoot in the output of an integral control system.Using Integral ControlLet's look at the structure of a system to control liquid level in a tank. The input is some desired level. The output level is measured and fed back to be compared to the input, generating an error signal. We integrate that error signal to get the voltage to be applied to the pump.Consider this question. What happens when the error signal is zero?The answer to this question is that maybe nothing happens. If the error signal is zero, then the output of the integrator stays constant! That means that the voltage applied to the pump stays constant, and if everything is at steady state, the liquid level in the tank stays constant.If the output level is the desired level, this is a desirable steady state. Let's review this situation.If output level matches the desired level, the error is zero.Because the error is zero, the integrator output does not change.Because the integrator output doesn't change, if the rest of the system is at steady state nothing else changes. All is copasetic! This sounds too good to be true. What could possibly go wrong? Well there could be at least two problemsThe system has to reach steady state. You'll need to learn something about system dynamics to ensure stability. If the system starts to oscillate wildly, then it may not reach a steady state, so the zero state state behavior is never really seen.Although the error goes to zero, no guarantees about speed of response are given. This has been a very cursory look at integral control. You'll need to get into the details to really make it work. The important point you need to take from this lesson is this.If you can design a stable integral control system, the steady state error (SSE) will be zero - exactly!The guarantee of zero steady state error may be important in a system.Questions To Be Answered About Integral ControlYou are a long way from a complete understanding of integral control - but you've made a good start. There are lots of unanswered questions about integral control, and we'll give you some questions and links here and on the next page.Many of the questions revolve around system dynamics. You need to be sure that you can predict how a system behaves.Does the system oscillate a lot before it gets to steady state?How long does the system take to get to steady state? What do we mean by "How long?"? Other questions revolve around how you can implement integral control. There are two basic ways to implement integral control, and both are widely used.Use an analog integrator. Here's the circuit, and you may have seen it already.The analog integrator can be used when the rest of the control system is implemented with analog components.Another option is:Use digital integration. You may want to check the introductory lesson on digital integration and implementation of integral control.If you want to use digital integration, you'll need to learn about digital integration algorithms, and you'll need to be conversant with sampled systems, and particularly Z-transform methods.SummaryThe main thing to take from this lesson is that integral control will produce zero SSE. There's not if's, and's or but's about it. It will be zero. When you think about that, you may wonder why it isn't always used. As with most things in life, there are advantages and disadvantages. The disadvantage is that integral control might produce a closed loop system with significantly slower response times. That's a subject that will take some knowledge about system response times and how they are related to the system you are controlling. You'll need some knowledge about root locus. Using the root locus you can get a handle on response times and how they are related to the parameters of the controlled system, and to the gain you choose for the integral controller.

proportional controller

Proportional controllerA proportional control system is a type of linear feedback control system. Two classic mechanical examples are the toilet bowl float proportioning valve and the fly-ball governor.The proportional control system is more complex than an on-off control system like a thermostat, but simpler than a proportional-integral-derivative (PID) control system used in something like an automobile cruise control.An on-off control is like driving a car by applying either full power or no power and varying the duty cycle, to control speed. The power would be on until the target speed is reached, and then the power would be removed, so the car reduces speed. When the speed falls below the target, with a certain hysteresis, full power would again be applied. It can be seen that this looks like pulse-width modulation, but would result in poor control.Proportional control is how most drivers control the speed of a car. If the car is at target speed and the speed increases slightly, the power is reduced slightly, or in proportion to the error (the actual versus target speed), so that the car reduces speed gradually and reaches the target point with very little, if any, "overshoot", so the result is much smoother control than on-off control.Further refinements like PID control would help compensate for additional variables like hills, where the amount of power needed for a given speed change would vary, which would be accounted for by the integral function of the PID control.

temperature regulator

SAMSON makes its documentation and product information available in different languages. Available language versions can be viewed by clicking the desired two-letter language code given in square brackets. The language codes comply with the international ISO 639 standard. The ISO codes for the relevant languages are as follows:CS = CzechDA = DanishDE = GermanEN = EnglishES = SpanishFI = FinnishFR = FrenchHU = HungarianIT = ItalianJA = JapaneseNL = DutchPL = PolishPT = PortugueseRU = RussianSK = SlovakSL = SlovenianSV = SwedishTR = TurkishZH = ChineseAdditional language codes differing from ISO:BR = special Portuguese version for Brazil CA = special English version for Canada and the USLanguagesDocumentation and product information are available in PDF format. The free Adobe Acrobat Reader is required to view the documents.When viewing PDF documents directly in your web browser, it may happen that only an empty page is displayed. This may be due to the web server having problems handling the browser's Acrobat Reader plug-in. This problem may occur depending on the Acrobat Reader version installed on your system.In such cases, it is recommended to download the file and view it directly in Acrobat Reader.To download a file, right-click the associated link and select "Save Target As" (Internet Explorer) from the context-sensitive menu. Save the file to your system and open it in Acrobat Reader.PDF formatThe spare parts lists contain information in five languages.Western Europe:English, French, German, Italian, and Spanish.Eastern Europe:Russian, further languages in preparation.Spare parts listsAll dimensional drawings are available in vector format as DXF files, compressed to ZIP archives. Unpack the ZIP files to view the drawings. A suitable software is required to view or further process the DXF files.Dimensional drawingsThe × mark displayed next to certain documents indicates that the associated product is no longer available as it has been removed from the SAMSON product range. If you require a replacement device, please contact your local SAMSON subsidiary for an alternative solution.

integral controller

What Is An Integral Control System? Often control systems are designed using Integral Control. In this control method, the control systems acts in a way that the control effort is proportional to the integral of the error. You should have studied proportional control before tackling this lesson. A proportional control system is shown in the block diagram below. The proportional controller amplifies the error and applies a control effort to the system that is proportional to the error. In integral control, the control effort is proportional to the integral so the controller now needs to be an integrator, and it will have a transfer function of Ki/s - not just a gain, Kp. --------------------------------------------------------------------------------What Do You Need To Get From This Lesson? This is a short lesson. The goals are simple. Given a closed loop, integral control system, Know that the SSE is zero - exactly! Be able to explain why the SSE can be zero even though there is no input to the integrator.--------------------------------------------------------------------------------What Is Integral Control? - Some Background Integral control is what you have when the signal driving the controlled system is derived by integrating the error in the system. The transfer function of the controller is Kp/s, if you think in terms of transfer functions and Laplace transforms. That is what is shown in the diagram below. That's the general outline, but to understand how integral control really works, it helps to understand exactly what an integral is. Let's consider that a while. To use integral control you really need to understand what an integrator is and what an integral is. Let's get back to basics. An integral is really the area under a curve. Let's assume that the independent variable is time, t. Then as time goes on the area accumulates. In math courses when they talk about integration, they picture it as the limit of a process of taking small incremental areas - shown below - and letting the interval, T, shrink to zero. In digital integration, that visualization process is important. Here is an approximation to an integral that is a sum of areas under the curve of the function being integrated. If the integral starts at zero, then the integral is just the area under the curve. Let's look at some implications of that. If the input goes to zero, then the integral stops changing and just has whatever value it had just before the input became zero. The integral can change in either direction as the signal goes positive and negative. Negative area can subtract from positive area, lowering the value of an integral. The first point here is very important because it has implications for the way that the error in the system behaves. The second point has strong implications for overall system behavior, particularly for understanding overshoot in the output of an integral control system. --------------------------------------------------------------------------------Using Integral Control Let's look at the structure of a system to control liquid level in a tank. The input is some desired level. The output level is measured and fed back to be compared to the input, generating an error signal. We integrate that error signal to get the voltage to be applied to the pump. Consider this question. What happens when the error signal is zero? The answer to this question is that maybe nothing happens. If the error signal is zero, then the output of the integrator stays constant! That means that the voltage applied to the pump stays constant, and if everything is at steady state, the liquid level in the tank stays constant. If the output level is the desired level, this is a desirable steady state. Let's review this situation. If output level matches the desired level, the error is zero. Because the error is zero, the integrator output does not change. Because the integrator output doesn't change, if the rest of the system is at steady state nothing else changes. All is copasetic! This sounds too good to be true. What could possibly go wrong? Well there could be at least two problems The system has to reach steady state. You'll need to learn something about system dynamics to ensure stability. If the system starts to oscillate wildly, then it may not reach a steady state, so the zero state state behavior is never really seen. Although the error goes to zero, no guarantees about speed of response are given. This has been a very cursory look at integral control. You'll need to get into the details to really make it work. The important point you need to take from this lesson is this. If you can design a stable integral control system, the steady state error (SSE) will be zero - exactly! The guarantee of zero steady state error may be important in a system. --------------------------------------------------------------------------------Questions To Be Answered About Integral Control You are a long way from a complete understanding of integral control - but you've made a good start. There are lots of unanswered questions about integral control, and we'll give you some questions and links here and on the next page. Many of the questions revolve around system dynamics. You need to be sure that you can predict how a system behaves. Does the system oscillate a lot before it gets to steady state? How long does the system take to get to steady state? What do we mean by "How long?"? Other questions revolve around how you can implement integral control. There are two basic ways to implement integral control, and both are widely used. Use an analog integrator. Here's the circuit, and you may have seen it already. The analog integrator can be used when the rest of the control system is implemented with analog components. Another option is: Use digital integration. You may want to check the introductory lesson on digital integration and implementation of integral control. If you want to use digital integration, you'll need to learn about digital integration algorithms, and you'll need to be conversant with sampled systems, and particularly Z-transform methods. --------------------------------------------------------------------------------Summary The main thing to take from this lesson is that integral control will produce zero SSE. There's not if's, and's or but's about it. It will be zero. When you think about that, you may wonder why it isn't always used. As with most things in life, there are advantages and disadvantages. The disadvantage is that integral control might produce a closed loop system with significantly slower response times. That's a subject that will take some knowledge about system response times and how they are related to the system you are controlling. You'll need some knowledge about root locus. Using the root locus you can get a handle on response times and how they are related to the parameters of the controlled system, and to the gain you choose for the integral controller. --------------------------------------------------------------------------------Problems --------------------------------------------------------------------------------Links To Related Lessons Other Introductory Lessons General Introduction Introduction To Proportional Control Introduction To Integral Control Introduction To Block Diagram Representations More Advanced Material On Integral Control Integral Control Implementing Integral Control --------------------------------------------------------------------------------Send us your comments on these lessons.

Derivative Control

Derivative ControlWith derivative action, the controller output is proportional to the rate of change of the measurement or error. Some manufacturers use the term rate or pre-act instead of derivative. Derivative, rate and pre-act are the same thing. The controller output is calculated by the rate of change of the error with time.controller output = Td*d(error)/dt = Td*d(SP - PV)/dtwhere the parameter Td is called derivative time. Derivative control is mathematically the opposite of integral action, but while we might have an integral-only controller, we would never have a derivative-only controller. The reason for this is that derivative control only knows the error is changing. It does not know what the setpoint actually is.Derivative action has the potential to improve performance when sudden changes in measured variable occur, but is should be used with care. It is mostly a matter of using enough, not too much.Derivative Gain LimitationIn most commercial processes sudden changes in process output may appear. In most casesa sudden change in the slope of such a process output cannot be avoided at all times. Using such a process output in controllers with pure derivative action, would lead to unwanted steps in the controller output. Moreover, high frequency noise in the measured signals may lead to unwanted large outputs of the controller.To prevent this unwanted effect, the derivative action can be filtered by a first-order system with time constant Td/N.This approximation acts as a derivative for low-frequency signal components. The gain, however, is limited to K*N. This means that high-frequency measurement noise is amplified at most by a factor KN. This is why the parameter N is called the derivative gain limitation. Typical values of N are 8 to 20. Sometimes the reciprocal value of N is used, mostly with the name beta (beta = 1/N).

smart transmitter

major difference between electronic and pneumatic transmission systems is the time required for signal transmission. In an electronic system there are no moving parts, only the state of the signal changes. This change occurs with virtually no time lost. Signal Transmission for Electronic and Pneumatic Signals As we stated previously, mechanical movement takes place whenever any pneumatic process signal changes. When devices move mechanically, time is lost. In addition, pneumatic systems, because they contain moving parts, are higher maintenance and subject to vibration, as well as rotational or gravitational mounting problems. However, pneumatic systems are still in place in many plants because they are safer than electrical systems in certain environments containing potentially explosive atmospheres. 4.1 Transmission lag Pneumatic Transmission Signal Lag The figure above shows the time lost with a pneumatic system. This figure represents a system using 3/16 ID tubing for the transmission line. As shown at the bottom of the graphic, in short distances, the effect of time is small. Under 200 feet, a signal can change 15 psi to 3 psi (the span of a pneumatic device) in roughly 0.4 seconds or less. This lost time represents the time needed to make up the air volume difference in the line (either replacing or releasing the air volume). A lag of 0.4 seconds is not critical, but as the distance of the signal line increases, so does the lag. At 400 feet, the lag time rises to about 1.3 seconds. At 1000 feet, the time is nearly 7 seconds -- in some processes, a critical period of time. Note: This time measurement represents the time required for the signal to travel from the sensing device to the controlling device. If there is a change and the controller responds to it immediately, the amount must be doubled for the signal transmission to reach a final control device. Pneumatic devices are best used for safety applications, simplicity, and for valve actuators, always in applications where the line length is kept under 100 feet; otherwise electronic signals should be used. 4.2 Transmitter gain A transmitter's gain, that is the ratio of the output of the transmitter to the input signal, is constant regardless of its output. In other words, an electronic transmitter's gain will remain constant whether it's output is 0% of span (4 mA) or 100% of span (20 mA) or any other point between those extremes. Transmitter Gain for an Electronic Transmitter 4.3 Smart Transmitters So far, the discussion has centered around electronic and pneumatic transmitters. The input and output of both of these types of transmitters is an analog signal -- either a mA current or air pressure, both of which are continuously variable. There is another kind of transmitter -- the "smart" transmitter. Smart Transmitter Components and Function The figure above illustrates functions of a smart transmitter. They can convert analog signals to digital signals (A/D), making communication swift and easy and can even send both analog and digital signals at the same time as denoted by D/A. A smart transmitter has a number of other capabilities as well. For instance, inputs can be varied, as denoted by A/D. If a temperature transmitter is a smart transmitter, it will accept millivolt signals from thermocouples and resistance signals from resistance temperature devices (RTDs), and thermistors. Components of the smart transmitter are illustrated in the lower figure. The transmitter is built into a housing about the size of a softball as seen on the lower left. The controller takes the output signal from the transmitter and sends it back to the final control element. The communicator is shown on the right. The communicator is a hand-held interface device that allows digital "instructions" to be delivered to the smart transmitters. Testing, configuring , and supply or acquiring data are all accomplished through the communicator. The communicator has a display that lets the technician see the input or output information. The communicator can be connected directly to the smart transmitter, or in parallel any where on the loop. 4.4 Smart transmitter microprocessor-based features Smart transmitters also have the following features: Configuration Re-ranging Characteristics Signal conditioning Self-diagnosis 4.4.1 Configuration Smart transmitters can be configured to meet the demands of the process in which they are used. For example, the same transmitter can be set up to read almost any range or type of thermocouple, RTD, or thermistor. Because of this, they reduce the need for a large number of specific replacement devices. 4.4.2 Re-ranging The range that the smart transmitter functions under can be easily changed from a remote location, for example by the technician in a control room. The technician or the operator has access to any smart device in the loop and does not even have to be at the transmitter to perform the change. The operator does need to use a communicator, however. A communicator allows the operator to interface with the smart transmitter. The communicator could be a PC, a programmable logic controller (PLC), or a hand-held device. The type of communicator depends on the manufacturer. Re-ranging is simple with the smart transmitter. For instance, using a communicator, the operator can change from a 100 ohm RTD to a type-J thermocouple just by reprogramming the transmitter. The transmitter responds immediately and changes from measuring resistance to measuring millivoltage. There is a wide range of inputs that a smart transmitter will accept. For instance, with pressure units, the operator can determine ahead of time whether to use inches of water, inches of mercury, psi, bars, millibars, pascals, or kilopascals. 4.4.3 Characteristics Another characteristic of a smart transmitter is its ability to act as a stand-alone transmitter. In such a capacity, it sends the output signal to a distributed control system (DCS) or a PLC. 4.4.4 Signal conditioning Smart transmitters can also perform signal conditioning, scanning the average signal and eliminating any "noise" spikes. Signals can also be delayed (dampened) so that the response does not fluctuate. This is especially useful with a rapidly changing process. 4.4.5 Self-diagnosis Finally, a smart transmitter can diagnose itself and report on any problems in the process. For example, it can report on a circuit board which is not working properly. 4.4.6 Summary of smart transmitter benefits There are distinct advantages in using a smart transmitter. The most important include ease of installation and communication, self-diagnosis, improved and digital reliability. Smart transmitters are also less subject to effects of temperature and humidity than analog devices. And although vibration can still affect them, the effects are far less than with analog devices. Smart transmitters also provide increased accuracy. And because can replace several different types of devices, using them allows for inventory reduction