regarding the status or
perfor-mance of a component or
system. Transducers also are
used in a control environment
wherein the data collected
from the transducer is used to
control a machine or process
that the transducer is
monitor-ing. Although many problems
and challenges are similar in
both categories, some issues
are unique to the industrial
control environment.
turnkey operational packages should they be needed.
Measurement link 5: Receiver sig-nal processing — Sigsig-nal processing on the receiving end may or may not be needed. The most common possibility is that the sending device transmits a 4-to 20- mA current-loop signal but the receiver requires a voltage — that is the receiver has a high input impedance.
The typical industrial servo amplifier is an example of a voltage-processing re-ceiving device. If a servo amplifier is the receiving device, a resistor, usually in the range of 100 to 500 Ω, will con-vert the current signal into a propor-tional voltage that the receiver then can process. Should the transmission me-dium be a radio signal, the receiving-end signal processor must be a radio re-ceiver and a suitable radio-frequency (RF) demodulator.
Measurement link 6: The receiv-ing device — The form this device can take has many possibilities but two common ones are a proportional valve amplifier or a servovalve ampli-fier, most often called simply a servo amplifier. Given the popularity of pro-grammable logic controllers in indus-try and the growing use of motion con-trollers, the receiving device of the future increasingly will be a special-purpose digital computer. If this is the case, interfacing decisions take on
new dimensions. If the transducer generates an analog signal, and ana-log-to-digital converter must be some-where in the measurement chain or in the receiving computer. If the physical sensing device is, say, an absolute-po-sition encoder with parallel digital output, the encoder can be coupled di-rectly to the parallel input port of the receiving computer.
Control link 7: Controller — A controller is a component that receives the conditioned data that represents the measured variable, compares it to the command, and issues any necessary corrective signal to the power ampli-fier. The controller could be an analog proportional or servo amplifier or it could be a special- purpose digital computer. The controller could even be a general- purpose desktop or laptop computer if it is configured with the proper hardware and software.
Control link 8: Power amplifier — For electrohydraulic control systems, the power amplifier is the driver stage of the servo/proportional amplifier.
Motion controllers may have the power amplifier as an integral part of the con-troller. Amplifier output drives the valve coil.
Control link 9: Power output — The power output component is the hydraulic actuator: a cylinder or ro-tary actuator.
The loop formed in Figure 1 is the measurement-control closed-loop chain of which the measurement links form a vital part. It can be shown that the measurement device and its sup-porting equipment alone dictate the performance of a high-gain closed-loop system. That is, deficiencies in the measurement network chain propagate one-for-one into deficiencies in the control of final system output. Trans-ducer performance is crucial and its se-lection is critical.
Three elements
Usually, three elements of Figure 1 are chosen early in the design process or are mandated by application require-ments:
● the control problem at hand usually dictates the measurand. If one wants to control the speed of a motor, for exam-ple, it is unlikely that a temperature-measuring transducer would be called upon to do this task; a speed transducer is the more likely choice. Once the measurand is selected, the choice of transducer most often narrows consid-erably: there are no commercially available speed transducers in which strain gages perform the basic act of transduction; frequency-generation methods are more popular and can be cost effective. Suppose that a fre-quency-generation transducer has been selected; the signal conditioning equip-ment must then be able to process a fre-quency input as opposed to an ampli-fier for a strain gage bridge or thermocouple analog voltage
● application circumstances many times control the data transmission method. The transmission medium in most industrial hydraulic systems will be a multiplicity of wires, but if neces-sary to remotely control a farm tractor, for example, radio waves may be a real possibility. If the distance between sending and receiving devices is great, a 4- to 20- mA current loop may be the choice. If there is a distance problem as well as a ground-loop noise problem, then opto-isolation and fiber-optic ca-bling may be called for. Whatever con-straints and requirements apply, the data- transmission medium will signifi-cantly affect the nature and extent of the signal processing/conditioning equipment on the sending and
receiv-C O N T R O L N E T W O R K S
Commands Machine or process to be controlled
To load
Fig. 1. A transducer and its support equipment for several links in the chain of the mea-surement-control, closed-loop network. All measurement links may not be present or necessary in all systems.
1998/1999 Fluid Power Handbook & Directory
A/41
ing ends of the measurement network chain, and
● the receiving device will affect inter-facing decisions. If the receiving de-vice can only receive digital signals, the receiving signal processor must produce digital outputs. If the receiver is an analog device such as a servo am-plifier on the other hand, the receiving signal processor must output analog signals. Carrying this point further: if the sending-end signal conditioner gen-erates a high-level DC output (more than ±1 V) and the physical distance between the transducer and servo am-plifier is just a few feet, there is no need for receiving-end signal processing; the servo amplifier can do it all. Make note of this — the servo amplifier is at once the receiving device and the controller.
What we find then is an extensive jargon and terminology of transduction and control along with a block diagram that shows function more than it does physical hardware. This is not unusual in the technology of control systems.
That is, the block labeled Receiving de-vice, Figure 1, may in one instance be a piece of real and separate hardware such as an analog-to-digital converter.
But is also may be, for example, noth-ing more than the 500 Ωresistor in a servo amplifier that converts a 4- to 20-mA current signal into the proportional voltage that the servo amplifier needs.
All of this recognizes that a great deal of the problem for the fluid power practitioner is one of terminology, definition, and common usage. Be aware that the diagrams used by con-trol-systems artisans probably will contain blocks that define function as opposed to depicting separate, identi-fiable boxes that one would buy in a store. The hardware needed to imple-ment a function may come all in one box or in several boxes. It is impossi-ble to predict without dealing with specific applications and choice of specific hardware.
Selection questions
When selecting actual hardware, al-ways ask this question: of all the functions needed to perform the desired control action, how many are contained in this specific piece of hardware and how many must be obtained with other pieces of hardware?
The answer to this question will be enhanced if the fluid power practitioner can remember, appreciate, and under-stand the:
● overall picture suggested in Figure 1, and distinguish between function and hardware boxes. Eventually, the hard-ware boxes must be identified, but at planning and design time, it helps to consider only function
● basic physics of the transducer in question because its operation will help select the transducer
● variety of formats in which data can exist. For example, when dealing with analog or digital signals, will the infor-mation be the frequency or the ampli-tude? and etc.
● various data transmission media and what factors assist in the decision to se-lect one over the other. If application circumstances dictate a particular trans-mission medium, understand the signal conditioning/processing implications attendant with that decision, and
● nature of the controller and whether the other elements in the measurement network chain require a separate re-ceiving device or can the rere-ceiving-de- receiving-de-vice function be adequately performed by the controller.
Adding to this multitude of possibili-ties is the fact that there are so many different transducers that perform the same function. For example, there are at least 20 different ways to measure liquid flow. Each method has its own particular technology, generates its own output data form, and requires its own special signal- processing
equip-ment. Add to that the possibility that there are several different controllers and data transmission media, and the number of combinations becomes truly astronomical. What follows reduces that astronomy into manageable pro-portions by looking at things the practi-tioner of the electrohydraulic art may encounter in the work-a-day world.
Forms and methods for data transmission
In the electrohydraulic control situa-tion, components are connected in a chain-like fashion to form the feed-back-control loop. The purpose of the several connections is to transmit infor-mation or data from one point to an-other in the loop to achieve the desired degree of control. Obviously, the infor-mation exists in a variety of forms at various points within the loop; for ex-ample, the transducer output may be a voltage; at the valve, input usually is a current; within the actuator, informa-tion is in the form of a pressure and a flow, and so on.
When expanding the purely elec-tronic elements within the loop, there are a variety of forms within that me-dium alone. The transducer probably outputs an analog voltage but within the controller (depending upon the type), the data may exist as a digit in computer memory. In between, there may be pulse-position modulation, a frequency-modulation element, or even a double- sideband, suppressed-carrier, amplitude-modulated signal.
The possibilities may be endless.
C O N T R O L N E T W O R K S
No standards for industrial controls
Fig. 2. The analog signal family tree of data indicates the different forms that data may take in an electrohy-draulic system. Several forms may exist in different parts of a single system.
The general subject of the forms and methods of data transmission is covered in detail in an electrical engi-neering course called Information Theory or Information Transmission.
That course deals with the many pos-sible forms in which information can be processed and transmitted. Fortu-nately, for the electrohydraulic engi-neer, there are only a few forms used for data transmission.
There are two broad data-form cate-gories that must be understood to cover most electrohydraulic applications:
digital and analog. In the following, electronic counters will be thought of as components or black boxes in the control/measurement network chain mostly to show methods of converting data from analog to digital form when the data is not a simple analog DC volt-age. Because all these data forms are common in electrohydraulic control systems, they should be understood at least in a conceptual sense.
Analog data forms
A family tree of the analog signal data forms likely to be encountered in electrohydraulic systems, Figure 2, shows the two major branches of the tree, namely DC and AC. Strictly
speaking, the DC branch is really a sub-set of the AC branch, but the fluid power technology does not make this distinction. The fact of the matter is that DC, by definition, is absolutely constant and therefore cannot transmit information because it never changes.
But information is transmitted in the fact that the DC level does change. For example, when the output voltage of a pressure transducer changes, one knows that the pressure must have changed commensurately. That change is the data; it is the information.
But, the argument goes, if the volt-age changes, then by definition it must be an AC system. The concept seems almost philosophical and it is treated that way here. But it is more than that for the electronic circuit de-signer because indeed, AC circuit the-ories must be used for designing this thing that is categorized as a DC data transmission system. In spite of these arguments, the tree of Figure 2 has a DC and an AC branch.
DC data transmission forms
While no standards govern the volt-age level in the generation or trans-mission of DC analog signals, actual practice is slightly different. There
are a large number of systems that use a 0- to 10-V signal or a ±10-V signal.
Most commercial transducers come with options that often include those va l u e s , or t h e t r an sd u ce r c a n b e equipped with signal conditioning modules that provide that output. Fur-thermore, receiving devices that ac-cept and process 10-V signals are easy to design, economical, and com-monly available. This also is true of 5-V signals; 5- and 10-5-V signals are consistent with the normal 5- to 24-V range of DC power-supply voltages and each gives good noise immunity because the noise levels usually are well below peak signal levels.
Some of the options that should be considered in a DC analog data trans-mission system are indicated in Figure 3. When the system uses voltage as the data-carrying variable with the so-called high-level signals between 100 mV and 10 V, the interconnection is very straightforward. A signal wire and ground connection are all that are needed when the interconnection dis-tances are, say, below 20 ft. In any event, it is always good practice to use a shielded, twisted pair as the intercon-necting method, understanding that shielding becomes more important as the separation distance increases. This shielded, twisted pair refers to the type of cabling that has two signal wires and a common conducting sheath usually made of braided wire called the shield.
When transmission distances exceed 20 ft, line 2, Figure 3, it may be desir-able to include a buffer amplifier at the sending end of the data link. This am-plifier provides a high input impedance for the transducer to look into, and at the same time provides a low output impedance to drive the transmission line. Both impedances will work to en-hance the noise immunity of the data link. When the voltage level at the sending end is less than 100 mV, am-plification at the sending end is almost mandatory. This statement is made as-suming that the receiving-end device, such as a proportional servo amplifier, will best be served by a 5- or 10-V maximum signal level. That may not always be the situation at hand, so the suggestion must be interpreted as a rule-of-thumb; each application has its own specific considerations.
C O N T R O L N E T W O R K S
Transmission medium
V<100mV
Receiving device options
Analog meter Oscilloscope
Servo or proportional valve amplifier
Digital multimeter A/D converter
Computer Zero
offset
Current-to-voltage resistor Optional attenuator 100mV to 10V
V>10V 1
2
3
4
A
A A
R1 R2
Sending end
Source of analog DC voltage signal
Voltage-to-current converter
Source of analog 2 - 20 mA data signal
Fig. 3. Either a voltage or a current source, located at the sending end (left) of shielded and twisted-pair analog DC data- transmission lines 1, 2, 3, and 4, sends low-amplitude voltage (±100 mV) in line 1; mid-amplitude voltage (100 mV to 10 V) through line 2, and high-level amplitude (±10 V) through line 3 to the receiving device.
The fluid power industry uses 250- or 500-Ωinput resistors to convert the current sig-nal to a proportiosig-nal voltage. Any asig-nalog receiving device (right) can be used depend-ing on needs of the application.
1998/1999 Fluid Power Handbook & Directory
A/43
Note that the correct position of the amplifier is at the sending end, espe-cially if the distances are more than two or three ft. This will raise the voltage, probably to the 5- or 10-V range, with the amplification taking place in the noise-protected environment of the sending device. With this increased voltage level, noise induced in the more-vulnerable interconnecting cable will have less effect that if the voltage were transmitted at the millivolt level.
That is, the signal-to-noise ratio of the system will be higher when using a sending-end amplifier.
For signals that exceed 10 V (the ex-ception rather than the rule), it proba-bly will be necessary to attenuate the signal so that it is in the 5- or 10-V range. If the attenuator is placed at the sending end of the transmission line, it can pose a problem. Voltage divider R1
and R2, line 3, Figure 3, will undoubt-edly increase the effective output im-pedance of the sending device and will reduce the noise immunity of the line.
Another reason it may be necessary to include a buffer amplifier at the send-ing end is to reduce the effective out-put impedance of the sending end. It may be possible to get by without the buffer amplifier if the attenuator is placed at the receiving end.
The 4-to-20 mA current loop, line 4, Figure 3, grew up in the process-con-trol industries where transmission dis-tances between senders and receivers often are measured in terms of miles in-stead of feet or inches. The 4-to-20 mA current loop, or simply the current loop, was invented to solve the noise problems attendant with such long lines. The current loop has found a niche in electrohydraulic control sys-tems when noise immunity is desired and/or long distances are involved.
A current loop uses the concept of duality compared to the voltage method of data transmission. Using duality, the sending- device output stage contains a current amplifier rather than a voltage amplifier. Current amplifiers have high output impedances, usually in the kΩrange.
In contrast, the ideal receiving device has zero input impedance. The reader is encouraged to compare this scenario with that of the ideal sender and ideal receiver for voltage transmission.
In practical terms, zero impedance is an unachievable goal, so designers must settle for low impedance. In the case of servo and proportional valve amplifiers, the practice is to output the input stage of the receiving amplifier with a 250- or 500-Ωresistor which converts the incoming current signal into a proportional voltage — what the amplifier was designed for. Note then, that Ohm’s Law tells us that with a maximum incoming signal current of 20 mA, the effective input voltage at the receiving amplifier is 5 V with the 250-Ωresistor and 10 V with a 500-Ω resistor, just about ideal. On the other hand, the minimum input current by standardized practice is 4 mA, which converts to 1 and 2 V, respectively, for 250- and 500-Ωinput resistance.
One of the disadvantages of the cur-rent loop is that the incoming signal does not pass through zero although it may be necessary to have a condition that corre-sponds to zero. With the current loop, system zero must correlate to a non-zero input current. The usual practice is to split the current spread down the middle,
One of the disadvantages of the cur-rent loop is that the incoming signal does not pass through zero although it may be necessary to have a condition that corre-sponds to zero. With the current loop, system zero must correlate to a non-zero input current. The usual practice is to split the current spread down the middle,