The best way to approach the subject of remote and automatic control systems is to start with a simple, easy to understand, arrangement and then add features to make it more complex. All control systems have much in common and this simple example will make it easier to understand the more elaborate systems that follow.
One of the simplest control systems I can think of is an outside light which is controlled by a switch inside the house. The usual arrangement is shown in ill. 1-1A. This is more properly called a remote switching system, rather than a remote control system, because the power line that operates the controlled device is carried right to the control point.
A true remote control system for an outside light is shown in ill. 1-1B. Here the light is actually controlled by a relay which in turn is controlled by a low-voltage dc circuit. As simple as it's , this arrangement has many advantages over the remote switching circuit of ill. 1-1A. e.g., in as much as low voltage is used, light wiring can be employed without any risk of fire or electric shock. Furthermore many different switches can be added so that the outside light can be turned on or off from many different points inside the home.
The system of ill. 1-1B can be made even more flexible by replacing the conventional relay with a latching relay that will latch whenever it's actuated. With this setup there is no need to send a continuous control signal to keep the light on. All we have to do is to transmit a brief pulse that will latch the relay. To turn the light off we would transmit another brief pulse that would release the latch. We will discuss systems of this type in detail in a later section.
One limitation of this simple system is obvious. Unless the switch is located where the outside light can be seen through a window there’s no way of telling whether or not the light is on. What we need is some sort of indicator inside the house near the control switch that will tell us when the outside light is on. You will find that this is true of all but the most simple control systems. it's almost always necessary to have an indicator that will tell us the state of whatever it's that we’re trying to control. In ill. 1-1C we have added an indicator to our simple outside light control system. Here additional contacts on the relay drive a low-voltage circuit that actuates a small indicator light inside the house. When the indicator light is on, the outside light is also on.
The arrangement of ill. 1-1B or 1-1C is fully adequate for controlling an outside light but it does have limitations. e.g., there is no way of knowing if the outside light happens to be burned out. If the object being controlled was something much more important than a simple light, this could be a serious limitation. In such a case, we may wish to use a more elaborate arrangement. A sensor such as a light detector might be used so we could be really sure that the outside light was on.
Without going into anything more complicated than a simple light that is controlled remotely, we can see that it's possible to add many features to improve the system. e.g., we might add a timer to the circuit and fix it so that the outside light would turn on for two or three minutes every time anyone rang the doorbell. This of course would mean that whenever the doorbell rang the light would go on-h—night or day. If this were objectionable, we could use a light sensor or another timer to be sure that the outside light only turned on after dark.
This example is admittedly trivial but it does serve to illustrate some of the functions that are common to all control systems. These are summarized in the block diagram of ill. 1-2. In any control system we must have some device that actually controls whatever it's we want controlled. In our example, the control device is the relay that supplies power to a light. In most cases the control devices will be more complex. Connected to the control device we have a signal path which carries the control signal. The type of path used depends upon the nature of the control signal. To simplify the requirements for the signal path it's advisable to use low-voltage control signals. These may be either simple on-off dc signals or ac tones.
The control initiators are merely push-button switches at the control point. We will see later that we can also use the push-button arrangement of a Touch-Tone telephone system to generate control signals.
Unless the device that we are controlling is in plain sight we will need some sort of sensor and indicator which will tell us the state of whatever it's that we are controlling.
The functional elements shown in ill. 1-2 are typical of all remote control systems.
Note that this system is not fully automatic. A human operator is still required to operate the switches. To see what would be needed to make a system like this fully automatic, let’s again consider the simple case of an outside light being operated from inside the house. Assume that we don’t want to have to push the switch to turn the light on. Also assume that we want the light to turn on at dark and stay lit until say 11:00 P.M. An arrangement that will do this is very similar to the remote control system that we discussed earlier with a couple of added functions. Figure 1-3 shows the arrangement. A light sensor is located outside. This sensor will tell the system when it's dark outside. Instead of driving an indicator the output of the sensor is now used to generate a control signal that will turn on the outside light.
We can’t use the sensor alone to turn on the outside light because we have also stipulated that for the light to turn on automatically it not only must be dark outside but must be before 11:00 P.M. Therefore in addition to the signal from the light sensor we need another signal from a timer. The switches in the sensor and timer are converted in series as shown. When both devices are active, that is when it's dark outside and it's before 11:00P.M, both of the switches will be closed and the light will be turned on.
CLOSED-LOOP and OPEN-LOOP SYSTEMS
Fully automatic systems, that is systems that can operate without the intervention of a human operator can be classified into two general categories. These are closed-loop and open-loop systems. Figure 1-4 shows a block diagram of a closed-loop system. The example we have chosen is the familiar household thermostat that is used with a furnace to control the temperature in a home.
The sensing element of the system is a thermostat. This device senses the temperature in the house and compares it with the desired temperature that is set by means of a dial. The thermostat generates a signal that corresponds to the difference between the actual temperature in the house and the desired temperature set on the dial. This signal is usually known as an error signal.
One thing that the error signal must do is tell the furnace the direction of the error; that is, it must tell whether the temperature in the house is higher or lower than the desired temperature. Otherwise the furnace would not know whether it should turn on or off. The error signal in some systems may also tell how great the error is. That is, the signal may be proportional to the amount that the actual temperature differs from the desired temperature.
The next element of the system is the transmission channel by which the error or control signal is transmitted to the device that actually controls the furnace. In the case of the ordinary household thermostat, the error signal is usually a low-voltage ac signal. The channel is a pair of small wires.
The final element of the control system is the relay that actually turns the furnace on or off. This is the output or control device of the system.
Figure 1-4 shows a very elementary temperature control sys tem. In most household temperature control systems, many other features are included such as an arrangement that will allow the forced air fan on the furnace to run after the burner has been turned off. We need not concern ourselves with these other features at this time; our simple system will serve to demonstrate the principle of the closed-loop system.
This type of system is called a closed-loop system because there is a continuous path around the entire control system. The input to the system is the temperature in the house which is converted into an electrical signal that controls the furnace. The furnace generates heat which, in turn, controls the temperature in the room. and as the temperature is the input to the system there is a complete closed loop around the whole system.
Probably the most important characteristic of a closed-loop system as far as the designer is concerned is that it can be unstable; that is, it can oscillate. Suppose that the thermostat is located a considerable distance away from the nearest furnace register and that there isn’t much air circulation in the area. As the temperature in the home drops, the thermostat develops an error signal that asks the furnace for more heat. The furnace then turns on and begins heating the house. Because of the large distance between the thermostat and the nearest furnace register, it will be a long time before the temperature at the thermostat becomes high enough to tell the furnace to shut off. As a result, the temperature in the rest of the area, particularly near the registers, will become much higher than the desired temperature. Now the house begins to cool. Let’s say that any heat leaks such as doors or windows are closer to the registers than they are to the thermostat. The result is that by the time the temperature at the thermostat becomes low enough for it to ask the furnace for more heat, the temperature in the rest of the house will be well below the desired temperature. The result is that instead of the automatic control system providing a nice even temperature, it will continually oscillate between the two conditions—too hot and too cold.
In the temperature regulating system that I have described, instability is annoying but not particularly dangerous. If the system were controlling the position of a door and the instability meant that the door continually swung open and closed, the system would not only be worthless but also hazardous.
Almost all closed-loop systems have the potential of instability. In the design of large control systems for industrial equipment, one of the most important considerations is making sure that the system will be stable under all conditions. This usually isn’t a major problem in most home or office control systems because there are usually comparatively simple ways of assuring that the system will be stable. Often this consists of opening the control loop giving us an open-loop system.
Figure 1-5 shows an example of an open-loop control system. The system is automatic in that it will operate automatically without any human intervention. The purpose of the system is to turn on a coffee maker at 6:00A.M. every morning. The thing that is sensed is not the condition of the coffeepot, but simply the time of day. This is sensed by a timer which is actually an electric clock. Control is initiated by a pair of contacts that can be set to close at any desired time. At 6:00A.M. every morning, the contacts close turning on the coffee maker. If we wish, we may have another pair of contacts to turn it off at some later hour. This is an open-loop system because there is no closed path around the system. The input to the system is the time of day. The output is turning on a coffee maker. The condition of the coffee maker can in no way control the time of day so there is no closed path around the system.
The open-loop system is always stable. It can’t break into undesirable oscillations. Its biggest drawback is that in many cases it has no way of knowing the present condition of whatever it's that we wish to control. This isn’t significant in the case of the coffee maker because someone must fill the coffee maker the night before and be sure that the switches are in the proper positions and it's ready to go. It could be significant in a system that closed a door at the same time everyday, because it might try to close a door that is already closed causing some damage.
In many applications, an open-loop system is entirely adequate. In other cases, a closed-loop system is much better suited to the application. The decision of which type of system best suits a particular control problem will become clearer in later sections.
PLANNING A CONTROL SYSTEM
The best way to tackle the design of a remote or automatic control system is to start with the output or control device. This is the device that we use to operate whatever it's that we want to control. This is where most electronics technicians or hobbyists get into trouble. Very few of them have had much practical experience outside of electronic circuits. Usually once the control device has been selected, the rest of the design is comparatively simple.
The easiest case to handle is where the device that we wish to control is electrically operated. Here we can use an electrical device such as a relay to accomplish the control function. The design of such a system is usually quite straightforward. The trouble usually arises when we are trying to control the physical position of something ranging from a door or window to draperies. Here we need a control device that will not only provide a mechanical motion, but also the right kind of motion in the right direction. This field is usually foreign territory to the average electronics technician. For this reason we have devoted a great deal of attention to this phase of the problem in the following sections.
The way to start solving the problem is to select a mechanical arrangement that will produce the desired motion. The device must be able to be powered with a readily available source of mechanical power such as an electric motor or solenoid.
Once the control device is selected, a method of sending control signals to it must be provided. This is usually simply a matter of finding a way to use a low-voltage ac or dc signal to control a larger voltage. The different ways that this can be done are described in detail later on. The signaling system should always use low voltage and current so that the wiring will not be critical and will not constitute a fire or electric shock hazard.
In most applications, the thing that is being controlled will not be visible from the control point so it will be necessary to provide a sensing and indicating system. This is how the operator of a remote control system will know what he is doing and it's the feedback portion of a fully automatic control system.
The next decision that must be made is just how elaborate the system will be. Will it be a remote control system where the various devices are controlled from a single control point? Will there be several control points? Will some of the devices be controlled automatically to operate at a certain time of day? Or will it be an elaborate fully automatic system where everything that is con trolled is related to something else? it's these considerations that dictate the complexity of the system.
DECIDING WHAT TO CONTROL
One of the most confusing aspects of designing a control sys tem is deciding just what things should be controlled by the system. After all, you have probably been living for many years without any control systems around the home. A home control system is one of those things like an automatic transmission in an automobile that you never miss if you have never had it. it's only after living with a control system for a while that you begin to wonder how you ever managed to get along without it.
Once most technicians become interested in a home control system, there is a tendency to want to control or automate almost everything in sight. The result is a plan for a system that is so elaborate that its chances of ever being completed to the point where it's operational are negligible. it's better to start with a simple system that can be expanded later.
One of the best ways to decide what should be controlled is to carefully define your reasons for wanting a control system in the first place. If the reason is a physical handicap, the decisions are usually obvious. There are many things around a home that a handicapped person simply can't cope with without some aid or assistance. If a handicapped person can't move about without considerable difficulty, the system should control as many things as possible from one or more central locations. These things can range from turning lights on and off and automatically unlocking or opening doors to controlling the volume of a stereo or a TV set.
When the purpose of the system is personal convenience, the selection of just what functions should be controlled is much more difficult. it's really after you have the convenience of controlling many things from an easy chair that you begin to realize the value of such a system. The coffee lover who has once had a system that has the coffee ready before the alarm clock sounds will have great difficulty in going back to the old way of life where coffee had to be made after arising. One way of tackling the problem is to make a list of inconveniences. These can range from preparing the coffee in the morning and answering the door at inconvenient times to getting up at night to let the cat in. A little conscious attention to the inconveniences of everyday life will soon result in a list of things that might well be handled by a control system.
One of the strongest motives behind all sorts of human behavior, but one that is rarely acknowledged openly, is the desire to favorably impress other people. The motive is rarely acknowledged because it seems childish to go to a lot of trouble to build a system to simply favorably impress others with your knowledge and expertise. Although the motive is rarely acknowledged, much of our behavior ranging from dress style to table manners is geared primarily to group acceptance.
If impressing others with your expertise is one of your motives, you don’t have to acknowledge it openly. Neither is there any reason to be ashamed of it. After all, you have spent a great deal of time and effort to reach your state of electronic competence. If your accomplishments are confined to knowledge, you will have great difficulty sharing them with nontechnical people. If, on the other hand, your knowledge and ability result in a system that will make life more convenient, you can indeed show it to nontechnical people with justifiable pride.
One electronics engineer, with a great deal of ingenuity, won his wife with the aid of an automatic control system. The system was initiated by a timer that started the functions on the hour— when TV programs change. The system turned on the stereo and gradually increased its volume, decreased the volume of the TV audio, and dimmed the lights to a more cozy level. After awhile, the TV turned off. All of this was accomplished without our hero leaving his cozy position on the sofa.
An automatic control system is a natural for the computer hobbyist. The average computer buff will talk for hours about all the wonderful things that his computer can do for him around the house. Only too often, all it will really do is play games and display fancy graphics on a screen. These are not particularly impressive to other computer hobbyists who have similar systems and all too often they fail completely to impress anyone who has little technical know ledge. A nontechnical visitor is more impressed when a computer does something like turning on a light, than he is by some data transformation that is completely incomprehensible to him.
Table 1-1 lists many of the things around the home that lend themselves to automatic or remote control. From this list you can probably recognize those things that are worth the trouble of con trolling electronically.
If a system is properly designed, it will be flexible so that functions can be added as time passes. There is no reason why the whole thing has to be operational at the start. Perhaps the system will only handle a single light when it's first installed. If enough control positions are provided on the control box, and if a sufficient number of wires are run when it's installed, it can be expanded as time permits to control more and more things.
Probably the best advice that can be given to the technician or engineer planning a system for the home is contained in the so- called KISS formula. KISS stands for “Keep It Simple, Stupid.” The biggest reason for many proposed systems never reaching completion is that in the planning stage they become so elaborate that they would require many man years of effort to build and get working. The simpler a system is, the better chance it has of being completed.
Table 1-1. Functions Suitable for Control In a Home.
Simplicity has many other advantages. The simpler a system, the easier it will be to troubleshoot it in the event of failure. Furthermore, a simple system with few components will be much more reliable.
Updated: Tuesday, January 11, 2011 2:31 PST