Up to this point we continue to treat the transducer as a black box having an input and an output. We have already discussed the various kinds of inputs a transducer can have; we now want to look at the kinds of outputs we can have.
Transducers must be designed to interface with certain devices, such as displays and other circuits, and because they must also be able to transmit signals over a variety of communications paths (wires, space, fibre optics), not to mention the fact that these devices must also be able to cope with various types of signal noises (i.e., interference) encountered during data transmissions, it is important to understand the characteristics of various signal transmission types that are available. There are certainly advantages and disadvantages associated with each. In Section 1-3 we looked at an overview of the various types of transducer outputs available. In Chapter 2 we want to go into much more detail to increase our understanding of these transmission methods. We want to answer the question most often asked about transducer devices: "Why can't I simply just hook up the output of the transducer to some sort of data receiver regardless of the circuits involved and transmission type being used, and be done with it?"
2-1.1 - Analog Current and Voltage Signals
In the early days of transducers, analog data transmissions were the only practical and fairly well understood means of sending signals. (One notable exception was Samuel Morse's invention in 1859 of the telegraph sender and receiver utilizing a digital code for communications.) Immediately following the industrial revolution in the United States around the beginning of this century, individuals began experimenting with methods of sending voltages and currents through wires from strategically placed sensors to remotely placed recording stations. The magnitude of the current flow or the magnitude of voltages being transmitted were directly correlated with the magnitude of the quantity (i.e., the measurand) being detected and measured. Many of the same methods are still being applied in industry and are very effective. We now discuss these systems in detail to obtain a better understanding.
4- to 20-mA current loop system (and others)
The current transmission method is one of the oldest of all existing methods used for sending transducer output information to a remote location. Because of the magnitude of currents being transmitted and because of electrical circuit noise limitations, a
transmitting distance of approximately 2500 ft is considered the maximum reliable distance before requiring additional amplification for this type of system.
Figure 2-1 shows a general representation of a 4- to 20-mA current loop system. We see a power supply whose current output is varied in the range 4 to 20 mA, depending on the varying impedance created by the transducer. Often, the power supply is built right into the transducer housing itself to form a single integrated package, as shown in Figure 2-1. The transducer also contains, of course, a sensor whose output response is varied by the measurand. It is this fluctuating output response that is converted to the varying impedance by the transducer's onboard circuitry. The current flow can be converted very simply to an output voltage by inserting a resistor into the loop as shown in Figure 2-1. The 4-mA current represents the minimum measurand amount (often a zero value), whereas the 20-mA value represents the maximum measurand value. The reason it is desirable to have a current flow, in this case 4 mA, represent a zero measurand measurement is so that the receiving or measuring circuit may continue receiving operating power despite no signal being present. Although this is not obvious in Figure 2-1, the resistor is just part of an indicating circuit whose operation is dependent on the current and voltage supplied by the power supply.
Transducer
Figure 2-1 4-to-2O-mA current loop system.
Actually, there are other popular analog systems that are used in addition to the 4-to-20-mA system. There are also 10-to 50-mA systems and 1-to 5-mA systems in use.
They function just like the 4-to 20-mA systems; however, the 4-to 20-mA system is probably the most popular one in use at present. However, regardless of the current range being used, there must be a circuit provision onboard the transducer to allow for adjustment of the upper and lower current limits so that the proper span and range are suitable for the measurand span and range. In addition, it is highly desirable to have a transducer that has a very linear output (Figure 2-2). Then it becomes very easy to develop an algorithm using linear components (such as resistors, capacitors, or inductors) such that a simple multiplier circuit can be used to convert the developed current values into actual measurand values. Let's look at an example here using the information given in Figure 2-2.
Example 2-1
Let's say that we will want 1V (dc) of output from our converter to represent 1°C.
Therefore, our converted output (Figure 2-3) will be voltage, whereas our input signal to
the converter corning from the transducer's output will be the 4-to 20-mA current signal.
The converted output would then be calculated as follows. We would first divide the transducer's output change by its input change as if we were determining the sensitivity of the transducer [see Section 1-9.1, specifically eq. (1-29)]:
( )
output change 60 V dc ( 80 C - 20 C)
= = 3.75 V dc/mA
input change 16 mA 20 mA - 4 mA
← ° °
←
Since V dc/mA = kilohms (from Ohm's law), we can, instead, give the answer as 3750Ω. What this means is this: All we have to do is to install a 3750 Ω resistance (this would have to be a low-tolerance resistor, probably ±1%, since this is not a "standard value" resistor) into our current loop and then measure the occurring voltage drops.
The value of each recorded drop would then have the same numerical value as the temperature value being measured. The newly installed resistor is now our
"multiplication" device since I x R = V.
Figure 2-2 Typical output response of a linear transducer.
Figure 2-3 Block diagram showing converter for converting a 4-20-ma signal from a temperature transducer into an output voltage.
Admittedly, Example 2-1 is the simplest of examples one might encounter in transducer design, but it does demonstrate a vital principle of unit conversion.
To receive a current analog signal it is only necessary to use a milliammeter that is calibrated within the current range of the transducer. Then you must use a calibration chart to convert the milliammeter readings to the measurand units of measure. A much better method, however, is to use a milliammeter whose scale has been directly marked out in the measurand units of measurement, thereby eliminating the need for conversion charts. In either case, however, the transducer must be designed so that the user can make span and range adjustments of its current output for calibration purposes.