| When I was in engineering school in the mid 1970s microprocessor chips were still a fairly new concept. Scientific calculators were the size of a brick back then, and they weighed almost as much, and there were no personal computers.
I remember doing homework on the UNIVAC 1108 mainframe computer at school. To program it I had to sit at a monster of a keypunching machine for which I punched an endless array of holes into paper cards. These holes acted as the programming logic to instruct the computer what functions to perform. The 1108 computer’s mainframe was so huge it was housed in an adjoining room the size of a house. Since the 1980s advances in microprocessor technology have increased computing power and dramatically reduced the size of components, making things like laptops, smart phones, and sophisticated electronic products possible.
Last time we began looking at my design solution for the control of a machine which developed medical x-ray film and made use of a microprocessor chip to automate its operation. A field effect transistor (FET) acts as a digital control interface between its 5 volt direct current (VDC) microprocessor and a 12 VDC buzzer. Figure 1 shows what happens when someone presses the button to put everything into action and the microprocessor starts timing.
With the button depressed the chip senses 5 VDC from the power supply on its input lead. This in turn signals the computer program to turn the product on. The program then begins counting down the minutes, all the while maintaining a 0 voltage output from the chip’s output lead. With no voltage present on its G lead, the FET does not permit electrical current to flow from the 12 VDC supply, through the buzzer, through D and S, and down to electrical ground. The buzzer remains silent.
Figure 2 shows what happens when the program begins its 40-minute warming sequence. The chip raises the output lead voltage to 5 VDC and applies it to G, then the FET permits electric current to flow through it to ground from the 12 VDC supply and the buzzer. Now supplied with power, the buzzer sounds. Then, per programming instructions, after 2 seconds the program shuts off the voltage in the chip’s output lead, current is cut off, and the buzzer goes silent.
Next time we’ll see how an FET can be used as an interface between a microprocessor and another higher powered device, that of a 120 VAC motor that’s used to move x-ray film through a series of processes within the developer.
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Tags: 12 VDC, 5 VDC, alarm, automate, buzzer, current, D, design solution, digital control, electric relay, electrical design, electronics, engineering expert witness, FET, field effect transistor, forensic engineer, G, hardware design, machine control, medical product, microprocessor chip, MOSFET, motor, power supply, programming logic, pushbutton, S, voltage
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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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