Mechatronics Assignment

These notes shouldn't be taken too much as direct instructions about what to include in your assignment and what not to. Engineers as a group tend to want to be told precisely what to do, how to do it and by when. Undergraduate study is about encouraging you to think for yourselves. You may come up with some better ideas than mine and I don't want to over prescribe how you should think. If you have a different approach that meets the spec that's not just fine it's to be encouraged.

Notes:

I've had questions about the word count indications. I'm not actually much of a fan of word counts but have found that if I leave them off then good students are inclined to write a novel for each assignment. The key motivation here is to encourage you to write well and write as little as you can to make your point. Like an engineering or business report, so long as all the information is there, is well explained and well set out then shorter is better. I won't be counting words or enforcing a word limit, just be brief without missing out anything that matters.
If this assessment seems a bit random in its content that's because it is. In their glorious wisdom BTEC have mixed a discussion of sensors and actuators, their strengths and weaknesses and fitnesses for certain purposes, with a discussion of mathematical models. To cap that, you're not expected to be able to use the models generated to solve real problems, just to describe systems mathematically. The mixture of ideas makes no sense to me either but c'est la vie. Your best approach is to tackle the tasks as written, they make as much sense as I can make them.

Task 1 Scenario A

You'll find it easier to tackle part b) first for reasons that will become clear below. There's loads of stuff on the net about this topic and there's a limit to what I can tell you without doing the assignment for you. However, I can tell you that the difference between this scenario and ones you have seen before is that the force in the spring depends its extension which is the difference between the change in position of the building and the ground and the force in the damper depends on how fast this difference changes.

By way of an analogy, let's consider the suspension of a car modelled (approximately) as a lump mass supported by a single spring and damper (shock absorber) over a bumpy road.

If we take the tyres to be rigid, the mass of the wheels & suspension to be small compared to that of the car (live with these simplifications or the maths gets horrible), the height of the car (measured from its original position, not the road) to be y and the height of the road (again measured from its original position) to be x then:

The force in the spring is k(y - x)

The force in the damper is b( y ?- x ?)

These have to add up to mass times acceleration for the car in the vertical plane so:

my ¨+b(y ?- x ? )+k(y - x)=0
my ¨+by ?+ ky=bx ?+kx

Take some Laplace transforms:

ms^2 y+bsy+ ky=bsx+kx
y(ms^2+bs+ k)=x(bs+k)
y/x=(bs+k)/((ms^2+bs+ k))

If I was being ultra-pedantic I ought to have noted that x and y now represent their Laplace transforms so should be written as x(s) and y(s), and the equation for this final transfer function should formally be written as:

ms^2 y(s)+bsy(s)+ ky(s)=bsx(s)+kx(s)
y(s)(ms^2+bs+ k)=x(s)(bs+k)
(y(s))/(x(s))=(bs+k)/((ms^2+bs+ k))

But then mathematical rigour is akin to rigour mortis for engineers and the simpler equations are easier to read.

Now play spot the similarity to the scenario set.

Part a) is the same as part b but there's no spring so k = 0. Go figure.

Task 1 scenario B

Study the argument above and you'll see that it applies to this task. Your equations are in Q (charge) and you need a Qc for the charge in the capacitor (y?) and Qi for the injected charge (x?). Mass is replaced by inductance damping constant by Resistance and spring stiffness by Capacitance. With those substitutions you'll be able to see whether or not this analysis gives an exactly analogous transfer function or not. Bear in mind that I was required to give you electrical and mechanical systems and ask yourself how different I was likely to make them.

Task 2

Answer this for yourself based on your answers from task 1. You should be able to explain your conclusion in a few sentences, not paragraphs.

Task 3

The key issue here is to realise that we need to measure each of the quantities asked for in the example. Suggest some appropriate sensors for each value and some actuators for the ground. Keep this to a small number each and don't write an essay about how they work. A couple of sentences for each will suffice. Your answers will be deemed correct so long as they are credible. For example, don't suggest measuring the temperature with a mercury thermometer, they are too slow to respond, hard to read when moving and very tricky to get an electronic output from. Suggest something like a thermistor, thermocouple or remote IR device.

For the M1 criterion you'll have to make some guesses about the characteristics needed in each case. My advice would be to read the stuff I provided, have a dig around t'internet and have a think about what manufacturers bang on about in their blurb. Combine this with your own thoughts and slim this down to the stuff that matters (we're unlikely to be interested in the colour of devices) and then make a list. This bit is designed to find out whether you can think for yourself so I'm not going to say any more about it other than to suggest you keep the lists short - 3 or 4 criteria per device.

For M2, use your answer to M1 to assess the suitability of the devices suggested at the beginning of this task for this purpose. Use published materials to help you make your choices and list your references. This requirement is there to justify the award of the M2 criterion and is a bit artificial. Spec sheets are available online as are discussions about the strengths and weaknesses of devices. Please don't right reams on this. A tight, well-reasoned discussion is needed. Try to keep it to a page if you can.

For D1, choose the most appropriate type (there isn't a definitive right answer in all cases, just make sure you make reasonable selections) dig out some technical blurb and explain your reasoning. This is a distinction criterion so I will be picky about whether your choices are sensible and appropriate in size, range, cost, accuracy, etc.

Attachment:- Mechatronics-Assignment-Brief.rar