7 -6 O V E RV I E W OF A NALOG O UTPUT S IGNALS
7 -6 O V E RV I E W OF A NALOG O UTPUT S IGNALS
Analog output interfaces are used in applications requiring the control of field devices that respond to continuous voltage or current levels. An example of this type of field device is a volume adjust valve (see Figure 7-16). This type of valve, which is used in hydraulic-based punch presses, requires a 0–10 VDC signal to vary the volume of oil being pumped to the press cylinders, thereby changing the speed of the ram or platen. Table 7-4 lists some other common analog output devices.
1 1C 2
2C Voltage or Current
Analog
Output Signal
Signal Common
3C – (Voltage Pressure)
To hydraulic
from oil reservoir
From
platen or ram
press cylinder system
oil resevoir
Figure 7-16. Representation of a volume adjust valve.
A n a l Analog Outputs o g O u t p u t s
Table 7-4. Typical analog output field devices.
7 -7 I N ST R U C T I ON S F OR A N A L OG O U T PU T M OD U L ES
Multibit output instructions, which are similar to those used with multibit Multibit analog output instructions, which are similar to those used with discrete outputs, are used to send analog information to field devices. The multibit discrete outputs, are used to send analog information to field devices. controller transfers the contents of a register, generally specified by 12 bits, The controller transfers the contents of a register, generally specified by 12 to the output module upon the execution of the instruction (see Figure 7-17). bits, to the output module upon execution of the instruction (see Figure 7-17). The module then transforms this value, whether BCD or binary, from digital
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S ECTION Components The Analog C HAPTER 2 and Systems
Input/Output System 7
The module then transforms this value, whether BCD or binary, from digital to analog and passes it to the field control device. Figure 7-18 illustrates a multibit instruction transferring 12 bits of data from register 2000 to an analog output module that is connected to a control valve. These 12 bits of informa- tion, which are transferred to the field device for control, may be the result of other computations in the PLC program. Chapter 9 explains PLC instructions in more detail.
PLC
Analog
Register/Word
Output
Memory Location
Module
Transforms
Continuous Signal
data from PLC
(voltage or current)
to analog signal
for analog control actuator
Figure 7-17. Conversion of register data to an analog signal.
Block transfer out
instruction
Analog Output Module
1110 9 8 7 6543210 12 Output
D/A
Transducer/Actuator (e.g., control valve)
Word/ 17 16 15 14 13 12 11 10 7 6 5 4 3 2 1 0 Register
Storage Area
Figure 7-18. Steps in converting a binary value into an analog signal.
to analog and passes it to the field control device. Figure 7-18 illustrates a E X AM PLE 7 -4
multibit instruction transferring 12 bits of data from register 2000 to an analog output module that is connected to a control valve. These 12 bits of informa- Figure 7-19 illustrates the binary transfer of information to an analog tion, which are transferred to the field device for control, may be the result of output module via a multibit instruction. Assume that the module other computations in the PLC program. Chapter 9 explains these instructions converts a digital signal equal to the binary value 0000 0000 0000 (0 in more detail. decimal) to an analog value that makes the control valve be completely
closed, while it converts a value of 1111 1111 1111 (4095 decimal) to an analog value that makes the valve be fully open. What will the state
of the valve be according to the contents of register 2000? E X AM PLE 7 -4
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S ECTION Components The Analog C HAPTER 2 and Systems
Input/Output System 7
Figure 7-19 illustrates the binary transfer of information to an analog Word/ Storage Area
output module via a multibit instruction. Assume that the output Register 17 16 15 14 13 12 11 10 7 6 5 4 3 2 1 0 module converts a digital signal equaling the binary value 0000 0000 2000 0000000000111111 0000 (0 decimal) to an analog value that makes the control valve be completely closed, while it converts a value of 1111 1111 1111 (4095
decimal) to an analog value that makes the valve be fully open. What Block transfer in instruction Analog Output Module
will the state of the valve be according to the contents of register 2000? 1110 9 8 7 6543210 12
Control Valve The value stored in register 2000 is 0000 01 1111, equivalent to Decimal Binary
decimal 63. The valve is open approximately 1/64th (or 1.56%) of its 0 0000 0000 0000 Valve Closed fully open position. Note that the position of the valve is determined by the decimal equivalent of the binary value, not the number of 1s—a 4095 1111 1111 1111 Full Open binary number with half 1s and half 0s does not mean that the valve is
Figure 7-19. half open. If the value in the register had been in BCD, the module Block transfer of register contents to an analog output module.
would have converted that number to determine the valve position. S OLU T I ON
The value stored in register 2000 is 0000 01 1111, which is equivalent to decimal 63. Thus, the valve is open approximately 1/65th, or 1.53%, of its fully open position (63 ÷ 4095 = 1.53%). Note that the position of the valve is determined by the decimal equivalent of the binary value, 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. If the value in the register had been in BCD, the output module would have converted the value to decimal to determine the valve position.