Topic Finder for Chapter 1
Introduction
Electromechanical Relays
Motor Control
Process Control
Automatic vs Human Process
Control
Digital Control
Simplified 7408
Control Application Example
Logic Devices
Logic Thinkers
Chapter Overview
Review Questions
List of Figures
Figure 1.1 Simple
Relay Motor ON/OFF Control Circuit
Figure 1.2 Simplified
Solid State Motor ON/OFF Control Circuit.
Figure 1.3 Pinout
for Common TTL Logic Device Packages
Figure 1.4 Example
Function Diagram for Unknown Application
Figure 1.5 Logical Thinkers
List of Tables
Table 1 Summary of Important
Areas where Digital Technology has made a Significant Impact
Table 2 Example of Human
Vs. Automatic Control Situations
Table 3 Summary of Important
TTL Logic Devices
Digital devices including microprocessors have revolutionized the way Chemical, Mechanical and Industrial engineers look at control situations. Table 1 summarizes important areas where digital technology has made a substantial impact. Although each of the topics in this table will be explored, for now, it is convenient to quickly consider an example that touches on the first two items.
Figure 1.1 illustrates the idea and component parts of an electromechanical relay circuit employed in a simple motor control application. In this situation the motor will operate a large industrial saw used to cut shapes out of sheets of plywood. The figure shows a dotted vertical line to separate the two electrical circuits involved. The motor to be controlled, ( i.e. in this case turned ON and OFF), together with a 120 volt alternating current power source in series with a contact switch are shown to the right of this line. To its left, a coil and an ON/OFF switch are shown in series with a 24 volt direct current power source. If a human places the switch in the ON position, current flows through the coil in the 24V circuit. The resulting magnetic field pulls the moveable contact toward the stationary contact. Once these two contacts touch, the 120V power supply provides the motor with the current it requires to rotate the saw.
The electromechanical relay system has been the classical way to provide the interface between humans in the factory and the motors and other electrical equipment they wish to operate. Figure 1.2 is a simplified illustration of a newer alternative motor control interface technology. Again the vertical dotted line distinguishes the high voltage "working" circuit on the right from the low voltage control circuit on the left. The left side of the figure indicates that a Motorola triac triggering device, MOC 3011, is connected in series between a current limited 5 volt power source and the output of a 7408 AND device. (Electron current flow from pin 1 of the MOC 3011 into the positive terminal of the 5 volt power supply is controlled, limited, by the value of the resistor, R2, in that part of the circuit. The larger the value of the resistor the less current flow through the 5 volt power supply from pin 1 of the MOC 3001.) The right side of the diagram indicates that a triac, 2N 6071B, is now in series with the saw motor and the 120v power supply.
The circuit details in Figure
1.2 will be made clear later. For now, its important to recognize
a few key points.
First, the coil and contact interface have been replace by the triac
triggering device, MOC 3011, and a triac, 2N 6071B.
Second, the new circuit has no moving parts. Such electronic
circuit components are know as solid state devices. They function
because of electron and charge transfers within the devices. From
the prospective of this course there is another very significant addition
to Figure 1.2.
The new circuit has an useful additional circuit component, i.e. the 7408
AND device. This logic device and others like it are extremely important
to the controls engineer because it permits low power DC circuits to place
conditions on the operation of higher voltage equipment.
The example illustrated in Figure 1.1 and Figure 1.2 outline the activities involved in process control. Since a course goal is to learn how to implement process control with digital devices, it is useful to establish a rudimentary idea of what process control means. Table 2 provides a few examples of process control. Although the table makes a distinction between human and automatic process control it does not actually define a process.
For now, a process is defined as a set or collection of sequential actions that when replicated produce the same results. For example, the collection of actions involved with you getting through the hotel door is a process.
You approach the door.
The doorman notes your approach.
The doorman springs into action and opens the door. You enter the hotel.
The doorman closes the door.
These collective events summarized below in automatic as well as human forms represent the process of you entering the hotel.
Automatic Vs Human Process Control
The role of process control is to assure that the process works correctly every time. The collection of circuits and equipment that execute all of the elements necessary for you to successfully get through the door without human assistance constitute an automatic process control system. As the engineer adds more considerations to the process, i.e. a conveyer belt that starts when you step on it and then stops after you have stepped off on the other side of the door, the process control sequence becomes more complicated and the need for an automatic process control scheme increases. As the process becomes more complicated and is conducted repeatedly over shorter intervals of time, the ability of humans to successfully control the process deteriorates rapidly.
Automatic process control can be simplified if it is broken into two large liberal classifications; digital control and analog control. The quality and degree of control improves as digital and analog control are integrated into the process. As this integration becomes more sophisticated, it becomes harder and harder to distinguish the digital from the analog control actions and with one notable attribute exception, the controller behaves more and more like a human. It is important to remember that the automatic controller has an attribute humans do not possess. The controller will never get tired. Therefore, it never makes a mistake because it is tired. An automatic controller does, however, make mistakes! It only has the skills defined for it by the humans that created it in the first place. Perhaps the best thing that can be said about those creatures is that; they, after all, "are only human".
Figure 1.2 illustrates the general role that solid state devices play in the elimination of electromechanical relays in industrial control circuits. Solid state devices can be digital, analog or hybrid in nature. In this example, the 2N 6071B triac falls in the hybrid class while the MOC 3011 and the 7408 are classified as digital devices. The triac will permit an analog alternating load current to flow through the saw provided it senses enough trigger current from pin 4 of the MOC 3011. The MOC 3011 trigger current flows in a loop from pin 4 into the triac through R1 and back to pin 4 by way of pin 3 of the solid state switch. The trigger current value is set by resistor R1. The MOC 3011 trigger current will flow once 5 volts is applied to pin 2 when pin 1 is at 5 volts. Once the triac is ON, the 120V power supply provides the AC voltage and the appropriate current to operate the saw. The amount of load current required by the saw will vary as the saw performs its work. As long as this current demand does not exceed the rating for the triac or the power supply the saw will run. If the input signal conditions at pin 2 change such that the MC 3011 lowers its trigger current so that the current flow from pin 4 through the triac gate pin and back to the MOC 3001 is below the current value necessary for the triac to remain at the ON level, the triac will essentially break the series connection between the motor and the power supply and the saw will not run.
(No one is expected to know off hand exactly how the MOC 3011 solid state switch operates. The manufacturer of such devices will always provide that information. Most engineers do, however, understand the general operation of a solid state switch and only refer to the manufacturers literature to find out the exact voltage and current values required to turn the switch ON or OFF. In general solid state switches function like relays. Solid state switches have two inputs that control the current that flows on two output pins. In this case, the MOC 3011 has two low voltage control signal input pins, pin 1 and pin 2, and two output pins for high voltage circuit control, pin 4 and pin 3.)
The 7408 AND device diagrammed in Figure 1.2 provides the control signal to the MOC 3011 that determines whether the trigger current is supplied to the triac or not. The AND's output signal on pin 11 is used as the ON/OFF control signal for pin 2 of the MOC 3011. If pin 11 on the 7408 is at 5 volts, the MOC 3011 will allow the trigger current to flow through R1, pin 3 and pin 4 of the MOC 3011 and then back to the triac gate pin. Since R1 has been selected to assure that the correct trigger current is supplied the 2N 6071B gate pin, the saw will operate. If pin 11 on the 7408 is at zero volts, the signal conditions at pin 2 are not satisfied and the MOC 3011 will not permit the triac trigger current to flow and the saw will no run.
The output voltage signal on pin 11 of the 7408 in Figure 1.2 determine the condition of the signal of pin 2 of the MOC 3011. The voltage on pin 11 depends on the input voltage signals at pins 13 and 12. The 7408 is designed such that pin 11 is at 5V only when pins 12 and pins 13 are both at 5 volts. If this is not the case then the voltage at output pin 11 is at zero volts. Pin 11 never produces any intermediate voltage values between zero and five volts. The output is either 5V or 0V. This "one or the other" characteristic of the AND device output pin is what classifies it as a digital device.
Simplified 7408 Control Application Example
The addition of the 7408 AND device to the control circuit on the left side of Figure 1.2 allows the engineer to add conditions, constraints, or requirements to the situation before the saw is permitted to operate. In this particular case, the saw can not run unless the safety shield switch and the ON/OFF switch are both in the ON position. Of course, it is up to the engineer to make sure that the only time the shield safety switch is in the ON position is when the safety shield has been lowered between the human and the saw blade.
Although a pair of electromechanical relay circuits could have been used in Figure 1.2, it turns out that the 7408 solid state AND device is much easier to install. In fact, the 7408 AND, the 7400 NAND, the 7404 INVERTER, and the 7432 OR devices are very popular because they are such convenient replacements for electromechanical relays. This is especially true if the logic idea to be implemented is important as well as complicated. The characteristics of each of these devices is discussed below.
The most amusing aspect of developing skills that allows an engineer to create process control systems is the fact that a great fraction of what is to be learned is already understood by the person in the first place. The problem is the person does not recognize that knowledge as a valuable tool nor does the person know how to use that tool to create a controllable situation. The use of logic devices is a classic example of this predicament.
Most new engineers think logically but have great difficulty recognizing the elements of logical thought they use and how those elements were arranged to solve the problem at hand. If an engineer is going to transfer the control of a process from a human to an automatic process control system, then each of the process's logical operational steps and its correct operational sequence must be identified. (The step by step breakdown of the hotel door situation provided earlier is one example.) If any of these logic steps are omitted, improperly identified, or incorrectly sequenced then the automated control system will fail. Therefore an engineer will create a useful automatic process control system only if intense attention is spend establishing the exact details as to how the process works as well as what logic hardware to used to implement the control system.
Fortunately, humans are not the only logic devices, i.e devices that "think" logically, in the world. (A comforting fact since humans often make mistakes when they try to think logically and indeed spend a reasonable faction of the day not thinking at all.) Non human logic devices have been made for centuries. Table 3 provides a summary of the now common but important transistor, transistor logic, TTL, solid state logic devices. Although this 7400 Series of logic devices is fabricated from a reasonably complicated arrangement of transistors, the user need be concerned with just two practical issues. The first is the location of the package power and ground pins as well as the location of the input and output pins for the devices in the package. The second concern is that the user's own logic thought process is sound so that the selection of logic devices for the task is correct.
Figure 1.3 does address the first user concern. It illustrates the pin assignments, i.e. the pinout, for the 7400, 7402, 7404, 7408, and the 7432 logic packages. Pinouts such as these are published in a TTL Data Book. This reference manual is available from several TTL device manufacturers such as Texas Instruments, Motorola and Signetics. Note that there are common pin assignments for the power, 5V, and ground, GND, functions on all of the packages. These power/ground assignments are popular with the 7400 series logic devices but are by no means the rule. Always check the data sheet for the correct power and ground connections. Never assume that they will be pins 14 and 7 respectively. Also note that the 7400, 7404, 7408 and 7432 packages have their devices arranged from left to right, inputs on the left and outputs on the right. Although this a convenient memory tool, as the arrangement of the 7402 suggests it is not a hard and fast rule.
At this point, it is worth a few lines of text to provide a "put it into perspective" caution. Certainly, from a perspective that can see as far as the first few weeks of this course, knowledge of the pinouts for common TTL devices as shown in Figure 1.3 is important. However, a view further to the horizon, will switch the emphasis to the information found in Figure 1.4. Figure 1.4 is a function diagram that conveys a piece of a process control idea. At this point, there is no reason to have any knowledge of the process control idea or the piece of that idea that the figure relates to. However, it is expected that a good controls engineer can read this diagram and understand its part in the a control scheme even if the whole process control story is not known at the time.
To acquire the skill to read function diagrams and swallow the entire concept represented by that diagram, an engineer has to learn to "read" the symbols of the function diagram much like we learned to read a paragraph in a book. The role of a paragraph is to string the words, phrases and sentences together to convey a complete subset of ideas or images that are associated with the theme of the book or a chapter in that book. The role of the string or collection of symbols in process control function diagrams is to clearly give the control's engineer the exact idea associated with that portion of the control scheme. Therefore, as you learn to read these symbols please keep a global view for the symbols as they are used in function diagrams. Figure 1.4 provides your first opportunity to try this global way to view function diagram symbols. However, the significance or even meaning of the box labeled "Timer NE 555" is not expected to be known at this time. Thus your review of Figure 1.4 is constrained to the 7400 and 7404 devices depicted in the graphic. By contrast, Figure 1.5 is a completely different story. This figure provides an excellent opportunity for the reader to step back from a parochial view of the logic symbols and consider a much more global but useful perspective of the logic symbol set.
For some of you, the symbol for the AND, OR, NAND
and NOR are not new. For some of you, the exact symbols used in this
course for these logic concepts may be new but the concept as expressed
in "truth tables" is known and understood. For some of you, this
is all new knowledge with no estimate of its value in the overall scheme
of things. For all of you, Figure 1.5
represents totally different way to think about logic operations.
It is presented at this time, not to focus your thoughts on the AND, OR,
NAND and NOR logic concepts, but to bring you to a broader perspective
of the role of logic thinkers as elements of a process control scheme.
Please proceed to the next section of this introductory material with a
blank slate so that you will not be trapped by any preconceived notions.
This leap of faith will be rewarded with a global way to view logic operations
that is quick and efficient to use but will never violate any correct concepts
you may already know about the AND, NAND, OR and NOR logic operations.
Figure 1.5 offers a non conventional way to view the basic logic operations. It is not provided as an academic exercise to test logical thinking skills but as a way to teach an engineer a method of transferring a set of logical constraints that must be imposed on a process to a set of diagram symbols that clearly and unambiguously reflect those control constraints. The first point to gleam from the diagram is the fact that it represents a visual tool or trick to accomplish the above stated task. The reader is encouraged to take a grand view of the symbols to assure that the bigger picture of the symbols and the ideas associated with the symbols will eventually be approached. The reader is also encouraged to return to this section every once in a while as more of the images associated with logic control constraints are developed through out this course.
Figure 1.5 is sectioned into three general parts. The top of the graphic provides a symbolic and certainly liberal definition of logic thinkers. In fact, it does not provide a definition at all but rather two partially complete symbols, one in blue one in red. The blue symbol represents a generalized AND thought process while the red symbol portrays the generalized OR thought process. ( the colors are of no significance except to facilitate the initial identification of the symbols in question.). The significance and difference in the two symbols can be stated in the following rules. These rules should be memorized until they are comfortably resident in your background knowledge base. (At this point, these rules may make little sense since they use the undefined term "active".)
An AND thinker is a human or non human thinker that will make its output
active only when all of its inputs are active.
An OR thinker is a human or non human thinker that will make its output
active when at least one of its inputs is active.
The second two sections for Figure 1.5 provide function diagram symbol examples of AND and OR thinkers respectively. Although all of the diagrams seem to be a disjointed collection of lines with or without circles connected to one of the two symbols in question, there are some immediate visual conclusions to be gleamed from the diagrams. First, the logical thought process described in figure (A), (B), (C), (D), (E), and (F) all include the AND thinker symbol. Thus these are all different examples of control concepts that require the AND thought process. Similarly, the function diagram symbols shown as (G), (H), (I), (J), (K) and (L) all represent process control constraints that require the OR thought process. But what of the differences in the collection of AND or OR thinkers.
Function diagram element (A) in Figure 1.5 has two inputs and one output while element (E) has four inputs and one output. In addition, element (C) has two inputs and one output but the output has a circle attached to it. These distinctions are significant but have nothing to do with the AND thinking nature of the diagrams as shown. ( Some of you, because of your previous knowledge of logic devices, are now tempted to note that diagram (A) is the symbol of an AND device while diagram (C) is the symbol of a NAND device. To facilitate your acceptance of this alternate and perhaps radical way to view logic operations, please enjoy the feeling of having this special subset knowledge about the symbols for an AND and a NAND devices and promptly forget it so that you can move on to the bigger picture.)
Function diagram element (G) in Figure 1.5 has two inputs and one output while element (H) has four inputs, two with circles attached, and one output. In addition, element (L) has two inputs and one output but the output has a circle attached to it. These distinctions are significant but have nothing to do with the OR thinking nature of the diagrams as shown. ( Some of you, because of your previous knowledge of logic devices, are now tempted to note that diagram (G) is the symbol of an OR device while diagram (L) is the symbol of a NOR device. To facilitate your acceptance of this alternate and perhaps radical way to view logic operations, please enjoy the feeling of having this special subset knowledge about the symbols for an OR and a NOR devices and promptly forget it so that you can move on to the bigger picture.)
Obviously, there is meaning to the presence or absence of a circle on an input or output of any of the symbols in Figure 1.5. However, there presence or absence does not alter the fact that the way an AND thinks is as stated above. When all of the inputs to an AND thinker are active the output of the AND thinker is active. Likewise the presence or absence of a circle on the input or output of an OR thinker does not alter the fact that the way an OR thinks is as state above. When at least one of the inputs to an OR thinker is active the output of the OR thinker is active. Hopefully, an equally obvious statement about Figure 1.5 is the fact that there is only one output to an AND or OR thinker and that it makes no difference how may inputs the AND or OR thinker possesses. The rules of operation are always the same. The output of an AND thinker is active when all of the inputs to the AND thinker are active. The output of an OR thinker is active when at least one of the inputs to the OR thinker is active.
At this point, the complete picture of the significance
of the presence or absence of the circle on an input or output of an AND
or OR thinker needs to be introduced. Chapter
5, "Alternative Graphic Symbolism", deals with these issues.
In addition, the chapter takes the reader through the concepts associated
with the terms Active and Passive. Some of the material in Chapter
5 may need to be revisited for better comprehension as the lectures
during the first few weeks of this course progress.
This introduction provided initial concepts associated with process control. A fundamental final control element, the relay, was introduced as an electromechanical device and then later as a solid state device. The common concept in both versions was the ability for a low power and low voltage signal from a human or a process variable to control the run state of a high power high voltage motor. Introductory information was also provided about an important class of TTL devices, the 7400 series logic devices. Information about the AND, OR, NAND and NOR with respect to their input signal requirement and output signal expectations were provided. The questions below should assist in a review of the material discussed in this section.
1. 1 Figure 1.1 illustrates a Motor ON/OFF Control Circuit. As the diagram stands now,
a) what is the voltage across the fixed and movable contact if the ON/OFF switch is not in the ON position?
b) What is the voltage across the fixed and movable contact if the ON/OFF switch is moved to the ON position?
c) what is the voltage across the coil if the ON/OFF switch is not in the ON position?
d) What is the voltage across the coil if the ON/OFF switch is moved to the ON position?
1. 2 Figure 1.2 illustrates a Triac modification of the Motor ON/OFF Control Circuit in Figure 1.1.
a) What is the Triac device symbol in the Figure?
b) What is the symbol for the logic device in the Figure?
1. 3 What device in Figure 1.2 has replaced the coil in Figure 1.1?
1. 4 Study the function of the components in Figure 1.2 and determine
a) what the voltage across the Triac is if the ON/OFF switch is not in the ON position?
b) What the voltage across Triac is if the Safety Shield is down and the ON/OFF switch is moved to the ON position ?
c) what the voltage at pin 11 is if the ON/OFF switch is not in the ON position?
d) What the voltage at pin 11 is if the ON/OFF switch is moved to the ON position?
1. 5 What is the operating voltage to the left of the vertical dotted line in Figure 1.2?
1. 6 What is the operating voltage to the right of the vertical dotted line in Figure 1.2?
1. 7 What is the identification number for the logic device in Figure 1.2?
1. 8 What is the biggest flaw in any automatic control system?
1. 9 Figure 1.3 illustrates several common TTL devices.
a) What is the TTL number for the AND device?
b) What is the TTL number for the OR device?
c) What is the TTL number for the NAND device?
d) What is the TTL number for the NOR device?
1.10 What is the
a) power pin connection for the NAND package?
b) power pin connection for the AND package?
c) power pin connection for the OR package?
d) power pin connection for the NOR package?
1.11 What is the
a) ground pin connection for the NAND package?
b) ground pin connection for the AND package?
c) ground pin connection for the OR package?
d) ground pin connection for the NOR package?
1.12 What is the symbol for:
a) the AND device? b) the OR device? c) the NOR device?
d) the NAND device?
1.13 Draw a relay circuit that has an AND device output attached to one of the coil inputs of a relay with the other end of the coil connected to ground (O volts). Show the load side of the relay connected to a 220V saw motor. ((THIS PROBLEM WILL BE ON THE FIRST OR SECOND TEST))
1.14 Figure 1.4 has a function diagram with several logic devices in it. Draw the wiring diagram for this function diagram.