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RTD PT100 calibration

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how to calibrate RTD PT100 to work with plc and what is the best type for sensitive range from 0 to 100 C

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Resistance Temterature Detector (RTD) – Pt100 An RTD is a device used from sensing the temperature of a process. These devices work on the principle that the electrical resistance of metals or semiconductors changes with temperature. The most commonly used metals are platinum, nickel, or nickel alloy. The resistance changes in a linear manner with temperature, although the actual change in resistance per degree is small. For example, a Pt100 has a resistance of 100? at 0°C and 138.4 ? at 100°C . This implies a 1°C increase in process temp will cause the resistance of the RTD to increase by 0.384?. There are also PT1000 sensors that have a resistance of 1000 ? at 0 °C. The RTD is then connected to the 4-20 transmitter where is converted to a 4-20mA current signal. 4-20mA Transmitter Signals from plant sensors are many and varied, ranging from a few millivolts to a hundred volts. Signals can be DC volts, AC volts or a resistance, such as that from a Pt100. The 4-20mA transmitter is a standard way of transmitting information between sensors and the measurement and control equipment. Figure 1: 4-20mA current loop diagram. Source: http://www.iqinstruments.com/iqshop/technical/signals.html As can be seen from fig.1, the transmitter is a current sinking circuit, which means that it will draw a current from an external power supply. This is usually a 4-20mA signal powered from 24V DC supply which is often an incorporated into the measuring instrument to which the transmitter is connected. The amount of current flowing through the loop is determined by the resistance of the Pt100. At 0°C, the resistance of the Pt100 will be 100?, this will allow 4mA to flow in the loop. Similarly, at 100°C, the resistance of the Pt100 will be 138.4?, this will allow 20mA to flow in the loop. The 4-20mA signal is fed to a the Analogue to Digital converter where it is converted into it’s digital equivalent, which can be processed by the PLC program. The 4mA ‘zero’ signal has a dual purpose. The 4mA offset provides the current that the transmitter/transducer needs to operate. It also protects against transducer or cable damage. If the transducer fails, or the signal cable is open or short circuited, the 4mA current will be lost and can be used to give a ‘transducer fail’ alarm. A current signal is used as this is less affected by noise than a signal represented by a voltage and a current signal can travel over long distances. Mitsubishi (FX2N-4AD) Analogue to Digital Converter (ADC) The ADC converts a continuously varying analog signal to a digital form that can be used inside the PLC program. The 4-20mA signal must first be calibrated so that 4mA gives a O in a register D0 and 20mA gives 100 in the same register. This is done by using a current simulator and writing a program (see figure 2) into the PLC. Figure 2: 4-20mA current loop calibration program. The 4-20mA signal is then fed into the FX2N-4AD module. The ADC will give a series of on/off signals on it’s 12 output channels which is the digital equivalent of the inputted analog signal. The digital signal is then fed into the PLC to be processed by it’s microprocessor. This module has four input channels, so, 4 analog signals can be fed into the ADC. The i/p resolution of this ADC is 20µA - a 20µA change in the analog i/p signal will give rise to a change of one digital bit on it’s o/p. The o/p resolution of this block is 12. A resolution of 12 means that the 4-20mA analog signal will be represented by 2^12 [+2047-(-2048)] bits at the output of the PLC. For example, 4mA will be represented by binary number 0000 0000 0000 and 20mA will be represented by 0111 1111 1111. The higher the o/p resolution of an ADC, the more accurate it will be. From the above information and to illustrate this idea, consider the following:- ? Temperature Pt100 Resistance (?) 4-20mA Current Digital O/P of ADC 0°C 100 ? 4mA 0000 0000 0000 40°C 115.36? 4.600mA 0000 0010 1000 50°C 119.20? 4.768mA 0000 0011 0010 60°C 123.04? 4.936mA 0000 0011 1100 100°C 138.40 ? 20mA 0111 1111 1111 Figure 3: Temperature – ADC output conversion table Mitsubishi PLC In this application the o/p of the ADC is connected to the i/p of the PLC. A PLC is an industrial microprocessor used for controlling processes and machines in an industrial environment. The PLC is the ‘brains’ of the system and provides an o/p according to the status of it’s inputs and user defined program. PLC’s have the following characteristics: Rugged and designed to withstand vibrations, temperature, humidity and noise. Have interfacing for inputs (sensors) and outputs (relays, motors) Are easily programmed and have an easily understood programming language that can be altered in position by a maintenance/electrician person. The 12 bit digital signal (which represents the temp of the liquid) from the ADC is inputted into the PLC. The PLC will have the setpoint (desired liquid temp) programmed in by the user, this will also be a 12 bit digital word which is stored in the PLC’s Random Access Memory. The two binary words are compared and if the temp of the liquid falls outside the acceptable limits, corrective action will be taken by the PLC. The PLC will then give a digital o/p signal which is converted to a 0-5V DC signal. This signal is fed into the phase controller which will attempt to bring the PV closer to the SP. Mitsubishi (FX2N-4DA) Digital to Analogue Converter (DAC) A DAC is a device that converts a digital (usually binary) code to a continuous analogue voltage or current signal. The FX2N-4DA can accept the 12 bit binary word (11 bits and sign bit) from the PLC and converts it into a 0-5V DC analog signal. When the digital input changes, the analog o/p changes in a stepped manor. In this 12 bit converter the o/p is made up of 2^12 [+2047-(-2048)] bits. This means, for an o/p range of 0-5V, one bit of a change will be 5/2047 = 2.4426mV. So, if the DAC o/p is 0000 0000 0000 (O Decimal), then this will give a 0V DC signal at it’s o/p and a DAC output of 0111 1111 1111 (2047 Decimal) will give an o/p of 5V DC. Figure 4 gives an example of the DAC’s operation. A B DAC I/P Decimal DAC O/P in Volts ( Column B x 2.4426mV) 0000 0000 00010 0 0V 0011 1110 1000 1000 2.4426V 0111 1001 1001 1945 4.75V 0111 1111 0110 2038 4.97V Figure 4: DAC input and output characteristics The 0-5V DC signal is then fed into the Phase Controller to control the temperature of the heating element. Phase Controller The Phase Controller is used to control the current supplied to the heating element, which in turn will control the temp of the liquid. The Phase Controller also called phase cutting, is a method of power limiting, applied to AC voltages. It works by modulating a thyristor, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform. The 0 – 5V analogue signal from the DAC is fed into the Phase Controller. The Phase Angle Controllers operate by varying the firing point of a silicon controlled rectifier's (SCR) input. A SCR is a solid state switching device which can provide fast, proportional control of electric power to the heating element. The power delivered to the load is proportional to the command input signal. In this application the SCR is zero fired. That is, if 0V is applied to the controller from the DAC, the SCR will be conducting, and full current will be delivered to the heating element. If 5V is applied to the controller, the SCR will be non-conducting and no current will be delivered to the heating element. And if 2.5V is applied to the controller, half the 220V A.C. power will be delivered to the heating element. it will SCR Controllers allow you to apply more precise control to any heater application, regardless of the type of heater elements you choose Not only does this give maximum control of your heat process, but it can extend heater life many times over other control methods. Electronic References http://www.picotech.com/applications/pt100.html http://www.iqinstruments.com/iqshop/technical/signals.html Bibliography Programmable Logic Controllers, W. Bolton, Newnes 2009 Programmable Controllers – An Engineer’s Guide, Second Edition, E.A. Parr, Newnes

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impressive , but where are the figures u pointed to. and will I find that conversion table for any kind of plc

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I took that info out of an assignment I did. Alot of it was useless to you but it helps a small bit. I have information on how to calibrate a PT100 to a Mitsubishi PLC - I will dig it out for you. I will have to scan it in at work for you as I don't have it electronically.

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thank you for your cooperation but if i use fatek plc where will i find it's information . IS it obligatory for each company to attach those files(RTD and analog module) with her plc ? or it's different from company to another.

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ger1, you didn't complete your answer. I am waiting if you are busy

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