to download the Eurotherm piccolo⢠Manual in PDF format
to download the Eurotherm piccolo⢠Manual in PDF format to download the Eurotherm piccolo⢠Manual in PDF format
User Manual Piccolo Range 7. Control Parameters in this section allow the control loop to be set up for optimum control conditions. An example of a temperature control loop is shown below:- The actual temperature measured at the process (PV) is connected to the input of the controller. This is compared with a setpoint (or required) temperature (SP). If there is an error between the set and measured temperature the controller calculates an output value to call for heating or cooling. The calculation depends on the process being controlled but normally uses a PID algorithm. The output(s) from the controller are connected to devices on the plant which cause the heating (or cooling) demand to be adjusted which in turn is detected by the temperature sensor. This is referred to as the control loop or closed loop control. Control Output Control Method Power Regulator Setpoint Error PV Control Loop Measured temperature Heater 7.1 Types of Control Two types of control loop may be configured. These are On/Off control, PID control. 7.1.1 On/Off Control On/Off control is the simplest means of control and simply turns heating power on when the PV is below setpoint and off when it is above setpoint. As a consequence, On/Off control leads to oscillation of the process variable. This oscillation can affect the quality of the final product but may be used on non-critical processes. A degree of hysteresis must be set in On/Off control if the operation of the switching device is to be reduced and relay chatter is to be avoided. If cooling is used, cooling power is turned on when the PV is above setpoint and off when it is below. It is suitable for controlling switching devices such as relays, contactors, triacs or digital (logic) devices. 7.1.2 PID Control PID, also referred to as ‘Three Term Control’, is an algorithm which continuously adjusts the output, according to a set of rules, to compensate for changes in the process variable. It provides more stable control but the parameters need to be set up to match the characteristics of the process under control. The three terms are: Proportional band PB Integral time Derivative time ti TD The output from the controller is the sum of the contributions from these three terms. The combined output is a function of the magnitude and duration of the error signal, and the rate of change of the process value. In Operator Level 2 it is possible to turn off integral and derivative terms and control on proportional only (P), proportional plus integral (PI) or proportional plus derivative (PD). PI control might be used, for example, when the sensor measuring an oven temperature is susceptible to noise or other electrical interference where derivative action could cause the heater power to fluctuate wildly. PD control may be used, for example, on servo mechanisms. In addition to the three terms described above, there are other parameters which determine how well the control loop performs. These include Cutback terms, Relative Cool Gain, Manual Reset and are described in detail in following sections. 64 Part No HA031260 Issue 1 May -12
Piccolo Range User Manual 7.1.3 Proportional Band ‘PB’ The proportional band, or gain, delivers an output which is proportional to the size of the error signal. It is the range over which the output power is continuously adjustable in a linear fashion from 0 to 100 (for a heat only controller). Below the proportional band the output is full on (100), above the proportional band the output is full off (0) as shown in the diagram below. The proportional band is measured in engineering units (e.g O C). The width of the proportional band determines the magnitude of the response to the error. If it too narrow (high gain) the system oscillates by being over responsive. If it is too wide (low gain) the control is sluggish. The ideal situation is when the proportional band is as narrow as possible without causing oscillation. Output 100% Proportional band wide narrow Temperature Setpoint 50% Increasingly narrower proportional band 0% Setpoint Temperature Time The diagram also shows the effect of narrowing proportional band to the point of oscillation. A very wide proportional band results in straight line control but with an appreciable initial error between setpoint and actual temperature. As the band is narrowed the temperature gets closer to setpoint. If the proportional band is very narrow the loop becomes unstable resulting in an oscillatory response. The proportional band is set as a percentage of the controller range. 7.1.4 Integral Term ‘Ti’ In a proportional only controller, an error between setpoint and PV must exist for the controller to deliver power. Integral is used to achieve zero steady state control error. The integral term slowly shifts the output level as a result of an error between setpoint and measured value. If the measured value is below setpoint the integral action gradually increases the output in an attempt to correct the error. If it is above setpoint integral action gradually decreases the output or increases the cooling power to correct the error. The diagram below shows the result of introducing integral action. Temperature Setpoint Proportional only control Proportional + Integral control The units for the integral term are measured in time (1 to 9999 seconds). The longer the integral time constant, the more slowly the output is shifted and results in a sluggish response. Too small an integral time will cause the process to overshoot and even oscillate. The integral action may be disabled by setting its value to OFF. Time Part No HA031260 Issue 1 May-12 65
- Page 15 and 16: Piccolo Range User Manual 2.6 Outpu
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- Page 19 and 20: Piccolo Range 2.15 Wiring Examples
- Page 21 and 22: Piccolo Range 3.1 Installation Safe
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Piccolo Range<br />
User <strong>Manual</strong><br />
7.1.3 Proportional Band ‘PB’<br />
The proportional band, or ga<strong>in</strong>, delivers an output which is proportional <strong>to</strong> <strong>the</strong> size of <strong>the</strong> error signal. It is <strong>the</strong><br />
range over which <strong>the</strong> output power is cont<strong>in</strong>uously adjustable <strong>in</strong> a l<strong>in</strong>ear fashion from 0 <strong>to</strong> 100 (for a heat only<br />
controller). Below <strong>the</strong> proportional band <strong>the</strong> output is full on (100), above <strong>the</strong> proportional band <strong>the</strong> output is full<br />
off (0) as shown <strong>in</strong> <strong>the</strong> diagram below. The proportional band is measured <strong>in</strong> eng<strong>in</strong>eer<strong>in</strong>g units (e.g O C).<br />
The width of <strong>the</strong> proportional band determ<strong>in</strong>es <strong>the</strong> magnitude of <strong>the</strong> response <strong>to</strong> <strong>the</strong> error. If it <strong>to</strong>o narrow (high<br />
ga<strong>in</strong>) <strong>the</strong> system oscillates by be<strong>in</strong>g over responsive. If it is <strong>to</strong>o wide (low ga<strong>in</strong>) <strong>the</strong> control is sluggish. The ideal<br />
situation is when <strong>the</strong> proportional band is as narrow as possible without caus<strong>in</strong>g oscillation.<br />
Output<br />
100%<br />
Proportional band<br />
wide<br />
narrow<br />
Temperature<br />
Setpo<strong>in</strong>t<br />
50%<br />
Increas<strong>in</strong>gly narrower<br />
proportional band<br />
0%<br />
Setpo<strong>in</strong>t<br />
Temperature<br />
Time<br />
The diagram also shows <strong>the</strong> effect of narrow<strong>in</strong>g proportional band <strong>to</strong> <strong>the</strong> po<strong>in</strong>t of oscillation. A very wide<br />
proportional band results <strong>in</strong> straight l<strong>in</strong>e control but with an appreciable <strong>in</strong>itial error between setpo<strong>in</strong>t and actual<br />
temperature. As <strong>the</strong> band is narrowed <strong>the</strong> temperature gets closer <strong>to</strong> setpo<strong>in</strong>t. If <strong>the</strong> proportional band is very<br />
narrow <strong>the</strong> loop becomes unstable result<strong>in</strong>g <strong>in</strong> an oscilla<strong>to</strong>ry response.<br />
The proportional band is set as a percentage of <strong>the</strong> controller range.<br />
7.1.4 Integral Term ‘Ti’<br />
In a proportional only controller, an error between setpo<strong>in</strong>t and PV must exist for <strong>the</strong> controller <strong>to</strong> deliver power.<br />
Integral is used <strong>to</strong> achieve zero steady state control error.<br />
The <strong>in</strong>tegral term slowly shifts <strong>the</strong> output level as a result of an error between setpo<strong>in</strong>t and measured value. If <strong>the</strong><br />
measured value is below setpo<strong>in</strong>t <strong>the</strong> <strong>in</strong>tegral action gradually <strong>in</strong>creases <strong>the</strong> output <strong>in</strong> an attempt <strong>to</strong> correct <strong>the</strong><br />
error. If it is above setpo<strong>in</strong>t <strong>in</strong>tegral action gradually decreases <strong>the</strong> output or <strong>in</strong>creases <strong>the</strong> cool<strong>in</strong>g power <strong>to</strong> correct<br />
<strong>the</strong> error.<br />
The diagram below shows <strong>the</strong> result of <strong>in</strong>troduc<strong>in</strong>g <strong>in</strong>tegral action.<br />
Temperature<br />
Setpo<strong>in</strong>t<br />
Proportional<br />
only control<br />
Proportional +<br />
Integral control<br />
The units for <strong>the</strong> <strong>in</strong>tegral term are measured <strong>in</strong> time (1 <strong>to</strong> 9999 seconds). The longer <strong>the</strong> <strong>in</strong>tegral time constant, <strong>the</strong><br />
more slowly <strong>the</strong> output is shifted and results <strong>in</strong> a sluggish response. Too small an <strong>in</strong>tegral time will cause <strong>the</strong><br />
process <strong>to</strong> overshoot and even oscillate. The <strong>in</strong>tegral action may be disabled by sett<strong>in</strong>g its value <strong>to</strong> OFF.<br />
Time<br />
Part No HA031260 Issue 1 May-12 65
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