| If you’ve been following along with our blog discussion on the systems engineering approach to medical device design, you should by now be convinced that instructions are important. In fact, the meticulous instructions produced during the manufacturing, operating, and maintenance phases of the Development stage are also crucial to later stages, that of Production and Utilization. Let’s finish up our discussion on the Development stage by taking a look at its final aspect, Preproduction.
The Preproduction aspect is instrumental to nipping potential problems in the bud before the medical devices go into actual production. In the initial Preproduction stages, systems engineers coordinate with the manufacturing and purchasing departments within the company as well as outside suppliers. The goal is to acquire all parts and equipment necessary to build a limited number of medical devices on the assembly line. Subjects such as preference in molded plastic components, motors, gears, pumps, springs, electronic components, circuit boards, wire, and tubing are discussed and agreed upon. Vendors are assessed with regard to their ability to produce parts when they are needed and that meet design specifications, satisfy quality requirements, and have costs that fall within budgetary constraints. The assembly of Preproduction devices provides an opportunity for systems engineers to validate manufacturing and quality control instructions and assess the device design with regard to manufacturability, meaning, the extent to which devices can be manufactured with relative ease, at minimal cost, while maintaining maximum reliability. Devices manufactured during this aspect of the Development stage serve as a test. Are instructions clearly written? Do the device parts fit together as they should? Are parts strong enough to withstand the assembly process? Can the devices be assembled as quickly and easily as expected? If the answer is “no” to any of these questions, then the device design and instructions must be returned to the design engineers and technical writers. Heads come together to rehash things and work out the bugs. Next time we’ll continue with the Preproduction aspect of the Development stage to see how laboratory and field testing enables systems engineers to shake out any more bugs from the medical device design, operating instructions, and maintenance instructions. ___________________________________________
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Posts Tagged ‘wire’
Systems Engineering In Medical Device Design – Preproduction, Part I
Monday, February 4th, 2013
Industrial Control Basics – Unlatching the Latching Circuit
Sunday, February 5th, 2012| When I had the misfortune of getting stuck in my Uncle Jake’s outhouse as a kid, I would allow my hysteria to get the best of me and forget my uncle’s instructions on how to get out. It was a series of raps and a single kick that would prove to be the magic formula, and once I had calmed myself down enough to employ them I would succeed in working the door’s rusty latch open. Our relay circuit below has a much less challenging system to effectively unlatch the pattern of electric current.
Figure 1 shows our latched circuit, where red lines denote the flow of current.
Figure 1
If you recall, the relay in this circuit was latched by pressing Pushbutton 1. When in the latched state, the magnetic attraction maintained by the wire coil and steel core won’t allow the relay armatures to release from their N.O. contacts. The relay’s wire coil stays energized via Button 2, the red bulb goes dark while the green bulb remains lit, even though Button 1 is no longer actively depressed. Now let’s take a look at Figure 2 to see how to get the circuit back to its unlatched state.
Figure 2
With Button 2 depressed the flow of current is interrupted and the relay’s wire coil becomes de-energized. In this state the coil and steel core are no longer magnetized, causing them to release their grip on the steel armatures. The spring will now pull them back until one of them makes contact with the N.C. contact. The red bulb lights again, although Button 2 is not being actively depressed. At this point the electric relay has become unlatched. It can be re-latched by depressing Button 1 again. Let’s see how we can simplify Figure 2’s representation with a ladder diagram, as shown in Figure 3.
Figure 3
We’ve seen how this latching circuit activates and deactivates bulbs. Next time we’ll see how it controls an electric motor and conveyor belt inside a factory. ____________________________________________ |
Industrial Control Basics – Latching Circuit
Sunday, January 29th, 2012| When I think of latches the first thing that comes to mind is my Uncle Jake’s outhouse and how I got stuck in it as a kid. Its door was outfitted with a rusty old latch that had a nasty habit of locking up when someone entered, and it would take a tricky set of raps and bangs to loosen. One day it was being particularly unresponsive to my repeated attempts to open it, and the scene became like something out of a horror movie. There was a lot of screaming.
When latches operate well, they’re indispensable. Let’s take our example circuit from last time a bit further by adding more components and wires. We’ll see how a latch can be applied to take the place of pressure exerted by an index finger. See Figure 1.
Figure 1
Our relay now contains an additional pivoting steel armature connected by a mechanical link to the original steel armature and spring. The relay still has one N.C. contact, but it now has two N.O. contacts. When the relay is in its normal state the spring holds both armatures away from the N.O. contacts so that no electric current will flow through them. One armature touches the N.C. contact, and this is the point at which current will flow between hot and neutral sides, lighting the red bulb. The parts of the circuit diagram with electric current flowing through them are denoted by red lines. Figure 1 reveals that there are now two pushbuttons instead of one. Now let’s go to Figure 2 to see what happens when someone presses on Button 1.
Figure 2
Again, the parts of the circuit diagram with current flowing through them are denoted by red lines. From this diagram you can see that when Button 1 is depressed, current flows through the wire coil, making it and its steel core magnetic. This electromagnet in turn attracts both steel armatures in our relay, causing them to pivot and touch their respective N.O. contacts. Electric current now flows between hot and neutral sides, lighting up the green bulb. Current no longer flows through the N.C. contact and the red bulb, making it go dark. If you look closely at Figure 2, you’ll notice that current can flow to the wire coil along two paths, either that of Button 1 or Button 2. It will also flow through both N.O. contact points, as well as the additional armature. So how is this scenario different from last week’s blog discussion? That becomes evident in Figure 3, when Button 1 is no longer depressed.
Figure 3
In Figure 3 Button 1 is not depressed, and electric current does not flow through it. The red bulb remains dark, and the green bulb lit. How can this state exist without the human intervention of a finger depressing the button? Because although one path for current flow was broken by releasing Button 1, the other path through Button 2 remains intact, allowing current to continue to flow through the wire coil. This situation exists because Button 2’s path is “latched.” Latching results in the relay’s wire coil keeping itself energized by maintaining armature contact at the N.O. contact points, even after Button 1 is released. When in the latched state, the magnetic attraction maintained by the wire coil and steel core won’t allow the armature to release from the N.O. contacts. This keeps current flowing through the wire coil and on to the green bulb. Under these conditions the relay will remain latched. But, just like my Uncle’s outhouse door, the relay can be unlatched if you know the trick to it. Relays may be latched or unlatched, and next week we’ll see how Button 2 comes into play to create an unlatched condition in which the green bulb is dark and the red bulb lit. We’ll also see how it is all represented in a ladder diagram. ____________________________________________ |
Industrial Control Basics – Introduction to Electric Relays
Tuesday, January 3rd, 2012| I’ve always considered science to be cool. Back in the 5th grade I remember fondly leafing through my science textbook, eagerly anticipating our class performing the experiments, but we never did. For some reason my teacher never took the time to demonstrate any. Undeterred, I proceeded on my own.
I remember one experiment particularly well where I took a big steel nail and coiled wire around it. When I hooked a battery up to the wires, as shown in Figure 1 below, electric current flowed from the battery through the wire coil. This set up a magnetic field in the steel nail, thereby creating an electromagnet. My electromagnet was strong enough to pick up paper clips, and I took great pleasure in repeatedly picking them up, then watching them unattach and fall quickly away when the wires were disconnected from the battery. Figure 1
Little did I know then that the electromagnet I had created was similar to an important part found within electrical relays used in many industrial control systems. An example of one of these relays is shown in Figure 2. Figure 2
So, what’s in the little plastic cube? Well, a relay is basically an electric switch, similar to the ones we’ve discussed in the past few weeks, the major difference being that it is not operated directly by human hands. Rather, it’s operated by an electromagnet. Let’s see how this works by examining a basic electrical relay, as shown in Figure 3.
Figure 3
The diagram in Figure 3 shows a basic electric relay constructed of a steel core with a wire coil wrapped around it, similar to the electromagnet I constructed in my 5th grade experiment. If the coil’s wires are not hooked up to a power source, a battery for example, no electric current will flow through it. When there is no current the coil and steel core are not magnetic. For purposes of our illustration and in accordance with industrial control parlance, this is said to be this relay’s “normal state.” Next to the steel core there is a movable steel armature, a kind of lever, which is attached to a spring. On one end of the armature is a pivot point, on the other end is a set of electrical switch contacts. When the relay is in its normal state, the spring’s tension holds the armature against the “normally closed,” or N.C., contact. If electric current is applied to the wire leading to the pivot point on the armature while in this state, it will be caused to flow on a continuous path through the armature and the N.C. contact, then out through the wire leading from the N.C. contact. In our illustration, since the armature does not touch the N.O. contact, an air gap is created that prevents electric current from traveling through the contact from the armature. Next week we’ll see how these parts come into play within a relay when electric current flows through the coil, turning it into an electromagnet. ____________________________________________ |
Industrial Control Basics – Manual Control
Monday, December 12th, 2011| 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. ____________________________________________ |
Ground Fault Circuit Interrupters
Sunday, June 26th, 2011| I’ve been talking about how I was asked to be a subject matter expert for an upcoming series on The Discovery Channel titled Curious and Unusual Deaths. Most of the accidents discussed involved electrocutions, and in each case the electrocution occurred because the victim’s body, usually their hand, inadvertently contacted a source of current. When that happened their bodies essentially became like a wire, providing an unintended path for current to travel on its way to the ground. Why does it travel to the ground, you ask? Because electric current, by its very nature, always wants to flow along a conductor of electricity from a higher voltage to a lower voltage. The ground is the lowest voltage area on our planet. When electricity flows to ground along an unintended path it’s referred to as a “ground fault,” because that’s where the electricity is headed, to the ground, or Earth. By “fault” I mean that something in an electrical circuit is broken or not right, allowing the electrical current to leak out of the circuit along an unintended path, like through a person’s body.
For example, in one of the Curious and Unusual Deaths segments I was asked to explain how a fault in wiring caused electrical current to flow through a woman’s body to the ground that she was standing on. This happened when she unintentionally came in contact with a metal door that was, unbeknownst to her, electrically charged from an unanticipated source. The current was strong enough to cause her death. Where did the electric current originate from? Watch the program to find out, but I’m sure you’d never guess. To say that it was an unlikely source is an understatement. When ground faults pass through a person’s body, bad things often happen, ranging from a stinging shock to stopping your heart muscle to burning you from the inside out. The severity depends on a number of factors, including the strength of the current to the amount of time your body is exposed to it. It might surprise you to know that if your skin is wet at the time of contacting a current, you risk a greater chance of injury. Water, from most sources, contains dissolved minerals, making it a great conductor of electricity. But what exactly is electrical current? Scientifically speaking it’s the rate of flow of electrons through a conductor of electricity. Let’s take a closer look at a subject close to home, a power cord leading from a wall’s outlet to the electric motor in your kitchen hand mixer. That power cord contains two wires. In the electrical world one wire is said to be “hot” while the other is “neutral.” The mixer whirrs away while you whip up a batch of chocolate frosting because electrons flow into its motor from the outlet through the hot wire, causing the beaters to spin. The electrons then safely flow back out of the motor to the wall outlet through the neutral wire. Now normally the number of electrons flowing into the motor through the hot wire will basically equal the number flowing out through the neutral wire, and this is a good thing. When current flow going in equals current flow going out, we end up enjoying a delicious chocolate cake. Since the human body can conduct electricity, serious consequences may result if there is an electrical defect in our hand mixer that creates a ground fault through the operator’s body while they are using it. In that situation the flow of electrons coming into the mixer from the hot wire will begin to flow through the operator’s body rather than flowing through the neutral wire. The result is that the number of electrons flowing through the hot wire does not equal the flow of electrons flowing through the neutral wire. Electrons are leaking out of what should be a closed system, entering the operator’s body instead while on its way to find the ground. Next time we’ll look at a handy device called a Ground Fault Circuit Interrupter (GFCI) and how it keeps an eye on the flow of electrons, which in turn keeps us safe from being electrocuted. _____________________________________________ |
