Programmable Logic Controllersprv Contents Analog Inputs Data Representation Data Handling Analog Outputs 2 Data Representation Data Handling prv/PLC and its Applications 11-Oct-14 What is Analog PLC can understand only discrete language Examples inputs: 3 prv/PLC and its Applications of analog Flow transducers Humidity transducers Load cell transducers Potentiometers Pressure transducers Vibration transducers Temperature transducers 11-Oct-14 . 4 prv/PLC and its Applications 11-Oct-14 .Analog Input Module Analog input modules digitize analog input signals It uses an A/D converter for the purpose. PLC reads the information stored in either BCD or Binary format. Steps in conversion 5 prv/PLC and its Applications 11-Oct-14 . Note that the value stored in register 1000 is in binary. the preceding bits are filled with 0s. for computational purposes. is 2767. 11-Oct-14 . Since the value is represented in 12 bits. 6 prv/PLC and its Applications After the instruction is executed. Its decimal equivalent.An example What will the contents of register 1000 be after the multi bit instruction shown in Figure is executed? Note that the digitized value corresponding to the analog transformation shown in the figure is represented by 12 bits in binary format. the contents of register 1000 will be: 0000 1010 1100 1111 This number corresponds to the digitized value generated by the module. Analog input data representation 7 prv/PLC and its Applications 11-Oct-14 . 8 prv/PLC and its Applications 11-Oct-14 . Thus. the binary value that the PLC receives is the digital equivalent of the incoming analog signal. The transmitter sends this information to the input module as an amplified current or voltage proportional to the signal being measured. the analog input interface digitizes the current or voltage by converting it into an equivalent binary number. Next. The interface then sends the digitized signal to the controller.Analog conversion The field sensing device sends a very low-level current or voltage analog input to the transmitter. the input signal into many digital counts. the most common formats are binary and BCD.A/D conversion The A/D converter divides. which in turn. the module or processor must perform an extra linearity computation to provide a valid BCD number. or digitizes. however. 9 prv/PLC and its Applications 11-Oct-14 . This division of the input signal is called resolution. In BCD format. makes them available for use in register or word locations. The format of these values varies according to the format used by the PLC. An A/D converter transfers its digital- equivalent values to the processor. it provides. and (b) find the same values for a module with a 10-bit resolution. 10 prv/PLC and its Applications (a) Find the equivalent voltage change for each count change (the voltage change per degree Celsius change) and the equivalent number of counts per degree Celsius. a +1 to +5 VDC signal compatible with the analog input module. assuming that the input module transforms the data into a linear 0 to 4095 counts. When the temperature transducer receives a valid signal from the process (100 to 600°C). which is connected to a temperature transducer. has an A/D with a 12-bit resolution. 11-Oct-14 .Try this! An input module. via a transmitter. Solution for (a) An input module. a +1 to +5 VDC signal compatible with the analog input module. it provides. When the temperature transducer receives a valid signal from the process (100 to 600°C). 11 prv/PLC and its Applications 11-Oct-14 . via a transmitter. which is connected to a temperature transducer. has an A/D with a 12-bit resolution. has an A/D with a 12-bit resolution. When the temperature transducer receives a valid signal from the process (100 to 600°C). 12 prv/PLC and its Applications 11-Oct-14 . via a transmitter. which is connected to a temperature transducer. a +1 to +5 VDC signal compatible with the analog input module.Solution for (b) An input module. it provides. 2 13 prv/PLC and its Applications 11-Oct-14 .Question No. the processor reads the value from the module and transfers the information to a location specified by the user.Analog data handling During the input reading section of the scan. This location is usually a word or register storage area or an input register. 14 prv/PLC and its Applications 11-Oct-14 . g.. which is necessary to properly interface with their compatible field sensing devices.Analog Input Connections Analog input modules usually provide a high input impedance (in the mega ohm range) for voltage-type input signals. T his allows the module to interface with high source- resistance outputs from input- sensing devices (e. Current-type input modules provide low input impedance (between 250 and 500 ohms). transmitters or transducers). 15 prv/PLC and its Applications 11-Oct-14 . This type of valve. 16 prv/PLC and its Applications 11-Oct-14 . thereby changing the speed of the ram or platen. which is used in hydraulic-based punch presses. An example of this type of field device is a volume adjust valve (see Figure). requires a 0–10 VDC signal to vary the volume of oil being pumped to the press cylinders.Analog Output Channels Analog output interfaces are used in applications requiring the control of field devices that respond to continuous voltage or current levels. Analog Output Module The controller transfers the contents of a register. from digital to analog and passes it to the field control device 17 prv/PLC and its Applications 11-Oct-14 . generally specified by 12bits. to the output module upon execution of the instruction The module then transforms this value. whether BCD or binary. Question and Answer Figure illustrates the binary transfer of information to an analog output module via a multi bit instruction. the valve is open approximately 1/65th. 18 prv/PLC and its Applications 11-Oct-14 .53%). or 1. while it converts a value of 1111 1111 1111 (4095 decimal) to an analog value that makes the valve be fully open. not the number of 1s and 0s—a binary number with half 1s and half 0s does not indicate that the valve is half open. which is equivalent to decimal 63. Note that the position of the valve is determined by the decimal equivalent of the binary value. of its fully open position (63 ¸ 4095 = 1.53%. the output module would have converted the value to decimal to determine the valve position. closed. Assume that the module converts a digital signal equal to the binary value 0000 0000 0000 (0 decimal) to an analog value that makes the control valve be completely closed. What will the state of the valve be according to the contents of register 2000? The value stored in register 2000 is 0000 0011 1111. Thus. If the value in the register had been in BCD. except that the data direction is reversed.g.Analog Output Data representation An analog output interface operates much like an analog input module. This analog output value is proportional to the digital numerical value received by the module.. 0 to 10 VDC) 19 prv/PLC and its Applications 11-Oct-14 . The magnitude proportional to the minimum and maximum capable analog voltages or currents of the field device (e. A D/A transforms the numerical data (BCD or binary) sent from the processor into an analog signal. During the execution of the output update. Since the program calculates the register/word value. when sent to the module. The module transforms the register/word’s binary or BCD value into an analog output voltage or current.Analog Output Data Handling The storage or I/O table section of a PLC’s data table area holds the data to be sent to an analog output module (see Figure ). This data comes from program computations that. 20 prv/PLC and its Applications 11-Oct-14 . the processor sends the register/word contents to the analog module specified by the address in the instruction. will control an analog output device. the user should take precautions during programming to avoid computing or sending nonvalid ranges to the module. Output data handling 21 prv/PLC and its Applications 11-Oct-14 . Principles and Applications.1997. Bryan. An Industrial Text Company Publication. Newnes. ISBN 0-944107-32-X John R.References W. Ronald A. Programmable Controllers – Theory and Implementation. Fifth Edition. 2006. Hackworth. Hackworth Jr.A. E. Fredrick D. Bolton. Bryan. 2003. (ISBN-13: 978-0-7506-8112-4) John W. Programmable Logic Controllers: Programming Methods and Applications 22 prv/PLC and its Applications 11-Oct-14 . Fourth Edition. Programmable Logic Controllers. Webb. Reis. (ISBN-81-203-2308-4) I.A. Programmable Logic Controllers. Second Edition. Prentice Hall of India. Programmable Logic Controllers and Industrial Automation. Thomson Delmar Learning.References Madhuchandra Mitra. ISBN: 81-87972-17-3 Gary Duning. ISBN 0 – 7668 – 1768 -7 23 prv/PLC and its Applications 11-Oct-14 . Penram International Publishing (India)P Limited. 2005. Samarjit Sen Gupta. 2005. an Introduction. Introduction to Programmable Logic Controllers. 2nd Edition. Have any? Thanks for listening… 24 prv/PLC and its Applications 11-Oct-14 . A passing thought… Greatness knows itself 25 prv/PLC and its Applications 11-Oct-14 .