| The Olympic Torch relay, soon to culminate in London, is the grand daddy of all relays, starting in one country, traversing many others, then ending its journey at the site of the Olympic Games. It passes through many athletes’ hands while on its journey, its final purpose being to light the Olympic Flame. Less glamorous, though still useful, is the relay race that often takes place within digital controls.
Last time we looked at my design solution for the control of a microprocessor controlled medical x-ray film developing machine, where a field effect transistor (FET) acted as a digital control interface between a 5 volt direct current (VDC) microprocessor and a 12 VDC buzzer. Well, controlling the buzzer wasn’t the only function of the microprocessor. It also had to control a 120 volt alternating current (VAC) drive motor, the purpose of which was to move x-ray film through a series of processes within the machine. Yet another requirement was that the machine’s drive motor run 40 minutes upon activation by a start button, then shut off.
One of the challenges presented by this specification was that an FET standing alone is only suited to control direct current devices like the buzzer, but not alternating current devices like electric motors. Direct current flows in one direction only when traveling through wire, and since an FET can only pass current in one direction it is the perfect match for those applications.
Now, as the name would imply, alternating current flow alternates, that is, it reverses direction and varies in intensity many times each second. This is a scenario that FETs are not equipped to handle because they can’t deal with reverse flow. This meant that, for the purpose of my developing machine, I could not use an FET to directly control the 120 VAC motor. Now let’s take a look at Figure 1 to get a basic look at how I solved the problem.
Figure 1 shows not one, but two green FET’s, each branching off from the microprocessor chip. We’ll call them FET 1 and FET 2. If you recall from last time, the buzzer works on 12 VDC, so FET 1 works well as a direct interface between it and the microprocessor chip. But in the case of FET 2 we need an intermediary device to handle the alternating current motor. That device is a 12 VDC electric relay.
In an earlier blog series on industrial controls I discussed how electric relays use electromagnets to power light bulbs and motors on and off in response to someone pressing a button on a control panel. We have very much the same mechanics at play in our developing machine. The relay will power the motor on and off in response to the computer program running within the 5 VDC microprocessor, a programming sequence that is initiated by someone pressing a button.
To get the motor control to work in the machine, the gate (G) of FET 2 is connected to another output lead on the microprocessor. We’ll call that Output Lead 2. Then, the source (S) of FET 2 is connected to the wire coil in the relay. To tap into the power source for the relay, the drain (D) of FET 2 is connected to the 12 VDC supply. Finally, the other end of the relay coil is connected to electrical ground.
Next time we’ll activate the pushbutton and see how the control initiative passes along a path in a manner reminiscent of the flame in an Olympic Torch relay, but here it passes between the microprocessor, the FET and electrical relay, culminating in power to the drive motor.
Posts Tagged ‘start button’
Tags: 12 VDC, 120 VAC, alternating current, digital control, digital control interface, digital input, digital output, direct current, drain, drive motor, electric current, electric motor control, electric motors, electric relay, electrical ground, electronic device, engineering expert witness, FET, field effect transistor, forensic engineer, gate, machine control, microprocessor, microprocessor chip, output lead, pushbutton, relay coil, source, start button, transistor, x-ray film machine
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| You’ve probably heard the saying, “asleep at the switch.” It’s usually associated with some sort of disaster, found later to have been caused by human error. Someone wasn’t paying attention, and something very bad happened. The meltdown of the Soviet nuclear power plant Chernobyl in 1986 comes to mind. You may be surprised to learn that the saying has its origins in the world of industrial controls, or more specifically, manual controls, as we’ll see in this article.
Last week when we opened our discussion on manual controls, we talked about how they work just as their name implies, that is, someone must manually press a button or throw a switch in order to initiate a factory operation. In other words, a manual control requires human intervention to initiate an action, such as pushing the start button. The machine will then continue to run until a person hits the stop button.
Let’s go now on a virtual field trip into a telephone factory to see how a basic manual control system works. It has a conveyor belt operated by an electric motor, and this motor is connected by wires and a power switch to a 120 volt power source of alternating current. Figure 1 illustrates what we mean. It shows that when the power switch is in the open position, a physical air gap exists within the electrical circuit. This prevents electricity from flowing through the wire because electricity can’t jump over gaps.
Figure 1 – Open Power Switch
Enter a human into the scenario, someone who grabs the power switch handle and manually closes it, eliminating the air gap. See Figure 2.
Figure 2 – Closed Power Switch
When the power switch is closed, a metal conductor bridges the gap, causing electricity to flow through the metal conductor to the electric motor in the circuit. This brings life to the conveyor belt. As long as the power switch remains closed, the conveyor belt will continue to operate.
That’s it, that’s a basic manual control system. It’s simple to operate, but it does have one major flaw. It requires constant monitoring by a human. Aside from opening and closing a power switch, humans are required to monitor operations, in case something goes wrong. The operator watching over an industrial machine performs the same function as the pilot on a plane, that is, to start-stop operations, and to intervene in case of an emergency. Computers fly modern jets. Pilots serve as trouble shooters when the unanticipated disaster situation occurs, because computers can’t yet creatively problem solve.
Next time we’ll introduce the element of an automatic control system, which will virtually eliminate the need for human intervention and with it human error.
Tags: air gap, alternating current, asleep at the switch, assembly line, control system, conveyor belt, electricity, electricity flow, engineering expert witness, factory, forensic engineer, industrial controls, machine, manual control, metal conductor, motor, operator, power plant, power source, power switch, production line, start button, stop button, telephone, wire
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