7 -8 A NALOG O UTPUT D ATA R E P R E S E N TAT I O N
7 -8 A NALOG O UTPUT D ATA R E P R E S E N TAT I O N
Like analog inputs, analog output interfaces are usually connected to controlling devices through transducers (see Figure 7-20). These transducers amplify, reduce, or change the discrete voltage signal into an analog signal, which in turn, controls the output device. Since there are many types of controlling devices, transducers are available in several standard voltage and current ratings. Table 7-5 lists some of the standard ratings used in program- mable controllers with analog output capabilities.
Binary data
Analog
to module
from processor
Module Transforms
Takes voltage or
current and affects or voltage or current controls the process
digital value to
Example:
Effect:
Voltage to pressure
Increase or decrease psi in process
Figure 7-20. Analog output device connected to a transducer.
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Output Interfaces
4–20 mA 10–50 mA
0 to +5 volts DC 0 to +10 volts DC
2.5 volts DC
5 volts DC
10 volts DC
Table 7-5. Analog ouput ratings.
An analog output interface operates much like an analog input module, except that the data direction is reversed. As mentioned earlier, a PLC processor can
only interpret digital binary numbers, so it assumes that all other devices operate in the same manner. An analog output module’s responsibility, then, is to change the PLC’s data from a binary value to an analog real-world signal that can be understood by field devices.
The data transformation that occurs in an output interface is exactly opposite of the transformation in an analog input interface (see Figure 7-21). A digital- to-analog converter (D/A or DAC) transforms the numerical data (BCD or binary) sent from the processor into an analog signal. This analog output value is proportional to the digital numerical value received by the module. Thus, the D/A converter creates a continuous analog signal with a magnitude proportional to the minimum and maximum capable analog voltages or currents of the field device (e.g., 0 to 10 VDC).
DC Voltage
Bit 5
D/A or Current
Digital-to -Analog
Not Used 17 16 15 14 13 12 11 10 7 6 5 4 3 2 1 0
16-Bit PLC Register
Figure 7-21. Digital-to-analog conversion of numerical data in a PLC register.
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The resolution of a digital-to-analog converter is defined by the number of bits that it uses for the analog conversion. For example, a D/A with a 12-bit resolution creates an analog signal ranging from 0 to 4095 counts (4096 total
values), which is proportional to a 12-bit digital signal (2 12 = 4096). Therefore, the analog value 2047 in a 12-bit resolution is equal to half of the full range. For an analog field device with a range of 0 VDC (closed) to 10 VDC (fully open), a 2047 analog value would be equal to a 5 VDC signal. Table 7-6 shows the current, voltage, and psi output values from a D/A with a 12-bit resolution.
PLC Register
0–10 VDC 4–20 mA (psi)
12 mA 1000 psi
20 mA 2000 psi
Table 7-6. Output values for a 12-bit analog output module.
An analog output module ensures that the value provided by the processor is proportional to the signal or variable that is being controlled by the field device. For instance, if an output device provides pressure control ranging from 100 to 800 psi, the values from the processor, in counts, will be proportional to this range. Output modules can have both unipolar and bipolar configurations, which provide control voltages with either all positive values or negative and positive values, respectively.
E X AM PLE 7 -5
A transducer connects an analog output module with a flow control valve capable of opening from 0 to 100% of total flow. The percentage of opening is proportional to a –10 to +10 VDC signal at the transducer’s input. Tabulate the relationship between percentage opening, output voltage, and counts for the output module in incre- ments of 10% (i.e., 10%, 20%, etc.). The bipolar output module has a 12-bit D/A (binary) with an additional sign bit that provides polarity to the output swing.
S OLU T I ON
Since the analog output module has a sign bit, it receives counts ranging from –4095 to +4095, which are proportional to the –10 to +10 VDC signal required by the transducer. Figure 7-22 graphically illustrates the relationship between the module’s counts, the output voltage, and the percentage opening.
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100% +10 VDC
Control Voltage -10 VDC to
Figure 7-22. Relationship between counts, voltage, and percentage. To formulate the desired table, first determine the equivalent values for
each variable. Since the solution should be expressed in increments as a function of percentage, the percentage changes are calculated as follows:
∆ Percentage ∆ Voltage ( − 10 to + 10) ∆ Counts ( − 4095 to + 4095)
1% change as function of voltage =
1% change as function of counts =
= 81.90 counts
Note that these computations are magnitude changes. To implement the table, the offset values for the voltage and counts must be added,
taking into consideration the bipolar effect of the module and the negative-to-positive changes in counts. Therefore, to obtain the volt- age and count equivalents per percentage change, add the offset voltage and count values when the percentage is at 0%. Thus:
Percentage as function of voltage = (0.2 × P) − 10 VDC Percentage as function of counts = (81.9 × P) − 4095 counts
where P is the percentage to be used in the table. Therefore, to calculate the required table, multiply each voltage and count relation- ship by the desired percentage of opening (see Table 7-7).
The PLC’s software program calculates output counts according to a predetermined algorithm. Sometimes, the output computations are expressed in engineering units that indicate a 0000 to 9999 (binary value or BCD) change in output value. These values must be ultimately converted to counts—in this case, –4095 to +4095 counts.
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Table 7-7. Equivalent counts, voltages, and percentages.