Archive for the ‘Systems and Control’ Category

S15 Health and safety

Posted: October 10, 2010 in Systems and Control

I need to look closely at health and safety within systems and control.  Extreme vigilance must be taken, BS4163 and CLEAPSS need to be examined (see health and safety resistant materials) and due care and attention taken at all times.

SOLDERING IRON

I have used this tool to solder the components onto my PCB.  I have used this tool before safely and was able to conduct a safe practise again, although my soldering left a lot to be desired in the first few attempts.

BS4163 Main points:
Hazards: Electric shocks.  Leads that can be tripped over.  The  hot soldering iron tip is hot enough to burn.  Fumes from rosin based fluxes can cause respiratory sensitisation.  Melted solder or flux could burn if in contact with the user.

Suitable materials and processes:
To help create a joint between 2 faces of sheet metal or to create a connection between wires and its terminals using molten solder with a flux

PPE
Protective glasses, use in a well ventilated room

CLEAPSS Main points:
Hazards:  There is a possible hazard of an electric shock via the bit to the metal being soldered ( this is because it is hard to provide insulation between the bit and the element without reducing heat conduction).  The mains cable could also be vulnerable to damage from the hot tip of the iron.  Although I did not use one, the gas heated types could be a fire hazard because of the butane fuel.  There is a hazard of burning as the tip and the stem of the iron get hot enough to burn skin.

Risk assessment:  Burns from soldering are usually minor but can be easily avoided.  Circuit components are more likely to be damaged than the person using the iron.  Heat damage to a mains lead is a high risk.

Common problems:
Stray bits of solder or splashed solder can burn.  The tip of the soldering iron must be kept clean to provide a more accurate result.  Soldering must take place in a well ventilated room.  The iron must be kept in its rest when not in use, not on the table.  Soldering does not provide a very strong mechanical joint.  Care must be taken to avoid any short circuits.

PCB ETCHING

I have observed this machine when making my pcb for the ambiance light.

BS4163 main points: Ferric chloride is an irritant and can be harmful.  Sodium persulfate is an irritant and an oxidizing agent.

Risk assessment: It is essential that protective goggles and gloves are worn when preparing ferric chloride solution and when emptying tanks.  Any contact with skin should be avoided and if contact does occur should be washed off immediately.

CLEAPSS main points: Ferric chloride and sodium persulfate are harmful if swallowed, although not classed as this it can cause dizziness and headaches if consumed.  Toxic, if the two etchants are mixed a toxic chlorine gas is produced.  Irritant, solid etchings or solutions can be an irritant to the skin or the respiratory system.  Sodium peroxdisulphate is highly flammable, it releases oxygen when wet which can enhance fire.

PPE: Protective gloves and goggles

Risk assessment: Solid sodium peroxdisulphate is harmful so measures must be taken to avoid indigestion.  If applied with a brush these concentration will not be approached, even locally in a school electronics area.  If the two etchants are used in the same area there is a possibility of them being mixed deliberately or by accident.  Sodium peroxdisulphate can create a mist which makes it less suitable for a bubble etch tank, if used in a small manual developing tray though minimal mist is produced.  The presence of sodium peroxdisulphate does not enhance the fire risk.

To dispose of solid waste it should be put in appropriate containers and consigned to an authorized contracted waste disposal.  Used sodium hydroxide should be neautralised with 1 M ethanoic acid before pouring away.  A siphon pump is recommended to empty tanks and great care must be taken when doing so.  To store the substances they should be placed in a secure, dry, well ventilated area and ferric chloride should be kept away from metals.

The tests I have done with a multimeter have all been on low voltage circuits and they cannot hurt you too much but it is worth noting that if it was necessary to test a high voltage then careless use of the multimeter would cause serious harm.  Even if not actively testing a high voltage circuit dangerous current can be exposed and it is important to remember to keep your fingers away from the metal tips of the meters test leads.


		
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Initial knowledge – 0

Current working knowledge – 2

Although structures are evident throughout my life I have never really taken the time to analyse them.  When I think of structures the first thing that comes to mind are buildings or bridges,  large, seemingly solid structures.  Using these as examples it indicates that the purpose and function of a structure is to primarily support itself, but to also support the load that it has been designed to take.

So what are the different classifications of structures?

There are mass structures, these types of structures rely on their own weight to resist loads.  So in theory a single brick could be a mass structure.  As could a Dam made of numerous bricks as it too is a solid mass structures.  I resisted the opportunity to use a house as an example as nowadays these can often use frame structures as well.

the pyramids are a mass structure

Framed structures are supported by a skeleton that can be made of materials such as metal, wood and re-enforced concrete.  The rigid frames have fixed joints that enable the frames to resist lateral forces.  Some other frames require diagonal bracing such as the Eiffel Tower or shear walls and diaphragms for lateral stability.

framed structure

There are also shell structures.  If I think of one of the largest recent shell structures the Millennium Dome it has a large shell that has been bent to give it stability.  A more common example would be a car.

the dome, a shell structure

Of course all structures are put under a certain amount of strain or force, and this too can come in different forms.  Compression would be squashing something, whereas a tension force would be stretching.  Torsion is a twisting force and a shear force would be a fracturing.  Finally there is bending which is pretty self-explanatory!

The amount of force is measured in Newtons (N) with 1N being the amount of force to hold up a weight of 100g

These forces can all be applied in different ways too.  If I was to stand still and hold some weights without moving, the force that my body would be put under is called a static force.  If I was standing still and the wind was blowing against me this moving or live force is known as a dynamic force.

A concentrated force is applied at a single point on a beam or structure.   Beams are commonly used for structural support in homes, commercial buildings, and bridges  and so the beam must be designed to withstand forces and stress, while minimizing weight, space requirements, and material cost. Incorrectly designed beams can fail prematurely and this would obviously have catastrophic effects.  The two most important characteristics of a concentrated load in the designing of a beam are the magnitude of the force and the location where it is applied.  How a beam or structure is supported plays an important role in its ability to support this type of load.   A concentrated load applied at the center of a long beam, which is supported at both ends, will behave very differently than the same load applied to the end of a cantilevered beam.

A concentrated load can cause a beam to deflect, or bend, when the force is applied so the design and construction of a beam will influence its ability to resist bending when exposed to a heavy weight. The deflection of a beam is a function of its cross-section, how it is supported, the material it is made of, and where the forces are applied.

Steel beams are most commonly used in commercial buildings due to their strength and resistance to bending, but beams are also manufactured using other materials, such as wood and aluminium.

If the weight is spread evenly across a beam this is described as UDL or uniformly distributed load and so each unit of length has the same amount of load as the others.

Of course a lot of structures must be reinforced for additional strength.  A rectangle is not a particularly strong shape and would easily turn into a rhombus with applied pressure .  It is often common for triangular trussing to be used within a square or rectangular shape to add additional stability.  When a force is applied to a triangular frame, two of its members stretch the third one, making it tense. This in turn will pull the other two members towards it, making the structure rigid and spreading the force between all three members of the triangle.  Triangulation is used in most construction and in the building of bridges and other large structures.

steel structure with triangulation

Forces acting on the outside of a structure such as gravity pulling it down are known as external forces, these in turn cause internal forces or stresses  to the materials that the structure is made of.  These stresses can cause movement and a change in shape or the size of a structure.  This is known as deformation and can lead to repair or the permanent damage of a structure.

In a structure, the various parts can either be in compression (being squashed) or in tension, ( being stretched).  A member in compression is called a strut and a  member in tension is called a tie.  A simple way to look at them would be a triangular house roof:

On a more complex level if I look at a bridge I can see a whole range of struts ties and the triangulation that offers it stability.

Initial knowledge – 1

Current working level – 2

When I think of basic mechanical systems I automatically go back to my school days and a trip to the Museum of Automata  in York and constructing a simple mechanical toy in DT with the use of a single cam.  Of course mechanics and mechanisms play a massive part in our everyday life whether it be travelling in various automobiles or using an everyday household object such as a pair of scissors.  Mechanisms is an extremely broad subject and its uses are almost endless, so to understand the basics of designing and analyzing basic mechanisms it is probably best to start by looking at the types of movement that mechanisms can make and their types of motion.

There is linear motion which is a straight motion such as a rack and pinion and guillotine.  Rotary motion is a rotating movement such as a set of gears or a carousel.  Reciprocating is a continuous motion such as a saw going back and forward or the needle of a sewing machine continuously going up and down.  Finally there is oscillating motion which again goes back and forward but will move in a circular arc such as a pendulum.

There are three different classes of lever.  A class 1 lever has its fulcrum in the middle and operates like a see-saw.  The fulcrum is simply the levers pivoting point. A class 2 lever has its fulcrum at one end and the effort is applied at the opposing end like a wheelbarrow.  A class 3 lever again has its fulcrum at one end but the effort is applied in the middle such as a pair of tweezers.

There are 5 different types of linkage that can feature in different mechanisms including parallel linkage.  A parallel-motion linkage creates an identical parallel motion.   By pulling (or pushing) a linkage in one direction, it creates an identical parallel motion at the other end of the linkage.  There is also reverse motion linkage A reverse-motion linkage changes the direction of motion.   By pulling (or pushing) the linkage in one direction, it creates an exact opposite motion in the other direction. If the fixed pivot was not central, it would create a larger or smaller motion in the opposite direction.  Another form of linkage is a treadle linkage,  this linkage shows how linkages can be used to change one type of motion into another. In the case of treadle, the rotary motion of the cam moves a parallel-motion linkage. The parallel-motion linkage controls the identical side-to-side, or oscillating motion in windscreen wipers for example.  The final form of linkage is a bell-crank linkage, this linkage changes the direction of movement through 90°. A bell-crank linkage tends to look a little like an L shape.  For example, a bell-crank linkage could be used to turn a vertical movement into horizontal movement which is the sort of motion used in bike brakes.

To calculate a moment of force between two points the equation is M= FxD .  The moment is the force multiplied by the distance.

Other calculations I may need to make when analyzing mechanisms is its mechanical advantage.   Mechanical Advantage is the ratio of the existing weight or load to the acting force; or, the ratio of the distance through which the force is exerted to the distance the weight is raised. For example, a machine has a mechanical advantage of 5 if an applied force of 1 kg can counterbalance a weight of 5 kg.  Or in simpler terms the load is divided by the effort.  So the load ( or weight)  5kg is divided by the force (or effort) of 1 kg to equal a mechanical advantage of 5.

To work out a velocity ratio the equation is distance moved by effort divided by the distance moved by load.  I can calculate a velocity ratio by looking at pulley systems.  Pulleys are used to change the speed, direction of rotation or the turning force or torque.  A pulley system typically consists of two pulley wheels each on a shaft that will be connected by a belt.  This will transmit rotary motion and force from the input, or driver shaft, to the output or driven shaft.

If the pulley wheels are different sizes, the smaller one will spin faster than the larger one. The difference in speed is called the velocity ratio. This is calculated using the formula:

Velocity ratio = diameter of the driven pulley ÷ diameter of the driver pulley

So Velocity ratio = 120mm ÷ 40mm = 3

If the pulley system is a two pulley one or a four pulley one the distance moved by effort is multiplied by 2 an4 respectively

If you know the velocity ratio and the input speed of a pulley system, you can calculate the output speed using the formula:

Output speed = input speed ÷ velocity ratio

So the output speed = 100rpm ÷ 3 = 33.3 rpm

The velocity ratio of a pulley system also determines the amount of turning force or torque transmitted from the driver pulley to the driven pulley. The formula is:

output torque = input torque × velocity ratio.

To work out efficiency I have to use the equation: mechanical advantage divided by velocity ratio x 100%

Moving back to the days of the old cam driven toys at school  I can look at the different types of cams available.  A cam is used to change a motion ( commonly a rotary one) to a reciprocating or linear motion.    There are different shaped cams and these are the most common shapes:

Although there are of course many variations of these standard shapes.There are also different types of followers available:

a)  knife edge follower: In theory there is not a limit on the shape of cam that can be used with this follower

b)  Roller Follower: The roller follower has the advantage that the sliding motion between cam and follower is largely replaced by a rolling motion. Note that sliding is not entirely eliminated since the inertia of the roller prevents it from responding instantaneously to the change of angular velocity required by the varying peripheral speed of the cam. This type of follower also produces a considerable side thrust.  The roller follower demands that any concave portion of the working surface must have a radius at least equal to the radius of the roller.

c)  Flat of Mushroom Follower : These have the advantage that the only side thrust is that due to friction between the contact surfaces of can and follower. The relative motion is one of sliding but it may be possible to reduce this by off setting the axis of the follower as shown in the diagram. This results in the the follower revolving under the influence of the cam.

Flat faced Follower These are really an example of the mushroom follower and are used where space is limited. The most obvious example being car engines.

A spring can be attached to some followers to keep it in permanent contact with the cam and the cams motion caused by its shape can be tracked by a displacement graph or diagram.

Another mechanism that uses rotary motion is gears they can not only transmit motion but force also.  When gears are ‘meshed’ like this they can act in a similar way to levers.  Each tooth of the gear could be regarded as an individual lever with the fulcrum being at the centre of the gear.

To work out a gears mechanical advantage you divide the number of teeth on the driven gear by the number of teeth on the driver gear so in this instance it would be 18 / 8 = 2.25.

To work out the gears velocity ratio or gear ratio we divide the number of teeth on the driver gear by the number of teeth on the driven gear so again it is 18/8.

There are also numerous types of gears as well:

Initial knowledge 1

Current working knowledge 2.5

Having designed and made circuits on a pcb and on prototype boards I certainly have had to do my fair share of fault finding.  The problems have usually come from either poor circuit design or poor construction.  One thing I did not utilise in ED217 were prototype boards.  The first circuit I made worked and as far as I was concerned at the time that was that, of course when it came to rearranging my circuit I started having all sorts of problems and would have benefited from laying it out on a prototype board before I jumped in at the deep end again.  It should be the first point of call when fault finding really checking if the original circuit actually works!

After that its time to move onto the visual checks (after the [power has been turned off).  Although I have improved, in the early stage of the year my soldering left a lot to be desired so that would be the first thing to inspect, checking for dry joints and look for any solder bridging across the tracks.  One thing I didn’t realise was that the track that runs around the PCB can actually lead to a faulty circuit if any solder has managed to work its way onto it.

Whilst checking the tracks it is wise to inspect for any breaks that may have occurred,  this could be quite common if you end up trying to scratch off a large ball of solder that shouldn’t be there!   But it could also be due to over-etching when making the board itself.   Its probably best to use the multimeter at that point to check and see.

Whilst the multimeter’s out its wise to check that the power supply is actually supplying the circuit with the voltage required.  There is probably nothing more frustrating than running a vigorous fault finding procedure only to find out that the batteries are dead.  In fact its probably best to check this first.

Then all the components need checking.  is the chip heating up? can you smell burning? If they are polarised are they in the right way?  Are the values the correct?

When using the prototype boards I have encountered times when I have simply turned on the power and nothing has happened.  This is when fault finding comes into play again.  As there is no soldering involved the visual checks made are different to if I was checking a PCB.  On one of my first attempts I’m not quite sure what I did wrong but the 555 timer got extremely hot so the first check I make now is to see if any part of the circuit is over heating.  I need to be careful here because when I say hot I mean it could actually be enough to burn and there is the possibility that you might actually be able to smell burning.  Fortunately that has been a one off but due to the nature of the fact that it can become a safety hazard it is my first point of call.  Following this it is time to do the visual checks.  First and foremost double check the circuit does actually match the diagram, if not make the alterations and try again.  Next I will make sure that everything is pushed in properly.  I don’t have the smallest of hands and some of the smaller wires can be tricky to get in and when working in a close knit space it is not uncommon to accidently knock out a wire while trying to insert another one.  Whilst doing this it is possible to see if any of the connections have snapped and of course if they have then they need to be replaced.  I am not adverse to putting components in the wrong way round occasionally, so the next thing to do is check that all polarised components such as leds ( long leg positive, smooth side negative) and capacitors are in the right way round, if not turn them round and try again otherwise the circuit will simply not work.  Whilst checking the components it is also important to check the values.  I purchased some transistors from Maplins, I had asked for BC547’S and what I actually received were BC557’S, sometimes components can be exchanged for a similar type but in this case when I checked the data sheet and although numerically similar one is a NPN and the other a PNP so the circuit was never going to work.  Finally one of my personal favorites was putting the IC in the wrong way round or even better just putting the wrong one in, it might sound daft but when concentrating on all the other things it is sometimes the most obvious that stump you.

By using the multimeter it is easier to actually see where the circuit may be faulty.  First check that the power supply is reading at the required voltage before testing components, and then I tend to follow the circuit round testing each individual component to see if they are receiving the correct amount of power.  By doing this you can see if a resistor is offering too much resistance and therefore swap it for a lower value etc

When testing the LED’S on my ambience light I used PIC AXE. By downloadingg a small program specifically for just one of the outputs say to flash on and off it is easy to quickly see if that individual component is working or indeed if that is where the fault lies.

I have never worked with programmable systems before, it was certainly a first and was actually quite a  daunting prospect.  Although having looked at the complexity of Steve’s alarm clock I realized that the PIC 08 chip was merely a drop in the ocean.

I used this chip for my ambiance light and having printed the PIC 08 template circuit from the picaxe website and manufactured the PCB it was time to try and attempt to program the chip.

The PIC is a Programmable Integrated Circuit micro controller and is basically a computer on a small chip.  It has a processor and a memory that will run a program responding to inputs and controlling outputs.  It is able to achieve complex functions which would normally require several integrated circuits.

Although I was a little apprehensive before using the PIC chip I actually found the PICAXE system fairly user friendly.  It uses a standard computer (my laptop) to program and re-program the PICs, all I needed was a download lead.  There was extensive documentation to download off  www.picaxe.co.uk and all I needed to do was work out how to use the PICAXE programming editor to transfer the information I wanted to control my ambiance light from the program to the chip.

The programming editor was like a more complex version of flowel.  There were inputs and outputs but they had to be assigned to the corresponding number on the 8 pin chip.  At first I found the program a little frustrating but once I got used to it and started to think about the way the current was travelling through the circuit and how the inputs were effecting it I found it a fairly simple package.  Of course that could be something to do with the fact that I purposely chose an easy program to start off with, but I got to grips with the basics of it and would not feel as intimidated if I wanted to produce something a little more complex.

Here is the program that I downloaded onto the chip

picaxe for ambience light.cad

It is worth noting that the PIC is static sensitive and can be damaged when touched  because your body may have become charged with static from your clothes for example.  So be careful when handling as it can lead to stressful and lengthy fault finding!

My Experience of interpreting electronic flow charts was at best extremely limited.  When designing my ambience light for technologies ED217 I had to learn how to use the programme flowell 3.0.  My light features 2 switches and 3 LED’s amounting to 1 light.  One of the switches will simply turn the light on and off.  The second switch can be pressed to turn the light off gradually in 10 minute intervals, this enables the light to be used as a child’s night light or as a timed reading light.

In the flowell program the elongated circle is used to represent a processes start or finish.

The diamond contains the input of a process, in the case of my ambience light these are 2 switches,  1 to turn the light on and the second to activate the ‘sleep’ mode.

The rectangle depicts an action taken during the process,  in this case it is the 10 minute interval that occurs in between the LED’s turning off 1 by 1.

My flowell chart was as follows:

flowell 3.0 flow chart

Initial knowledge – 0

Current working knowledge – 2

I did not use a commercial module for ED217, instead I chose to use the capabilities of the PIC chip.  I did however purchase a ‘crawling micriobug’ kit from Maplins which used LDRs.  The robot worked first time and helped me to improve and practice my circuit assembly skills.