# Block Diagrams - Control Engineering. Basic Question

1. Nov 22, 2011

### Femme_physics

(Go on easy on me, this is my first attempt and first course at control engineering)

1. The problem statement, all variables and given/known data

In systems who has a huge consumption of hot water, such as food factories, chemical industries or hospitals, it's customary to heat the water through fuel type "mazut". the hot water supply is modifiable, according to consumers use. The changes in water supply affect the hot water temperature. The control system in the drawing is based on regulating the fuel supply to the blowtorch.

http://img408.imageshack.us/img408/4324/schemeeng.jpg [Broken]

Build a block diagram of the system, when the entrance and exit signal are the desire temperature (Tr) and the existing temperature (Tc) of the hot water.

3. The attempt at a solution

http://img842.imageshack.us/img842/320/61723306.jpg [Broken]

Last edited by a moderator: May 5, 2017
2. Nov 22, 2011

### MisterX

Your diagram doesn't seem right to me. The desired temperature is an input to the system. Nothing should go into this input (no arrows should point to Tr block). You don't have any arrows in your diagram, which makes it hard to determine what is an input to a block, and what is an output. This kind of block diagram should have arrows.

The desired temperature is specified with an adjustable voltage. The desired temperature signal would not come from a transmitter, and I'm not sure sensor is the right word either. The diagram for this input indicates the desired temperature signal (Ur) comes from a potentiometer. A person would adjust the potentiometer to set the desired temperature.

The summing junction essentially should compare the desired temperature with the actual temperature from the sensor. This would produce an error signal (Ue). The error signal would represent the amount the temperature needed to be changed. Your diagram has only one signal going into the summing junction, which is wrong.

Now there are a number of things which would happen to the error signal (Ue). First it is amplified resuling in Um. Then the signal Um is applied to an electromagnet. The electromaget signal determines the fuel flow. The fuel flow can be thought of as the input to another system, which detemines the temperature of the water. Obviously this system would have memory because the temperature would depend not only on the fuel flow but also the starting temperature. The temperature would be measured by a sensor producing the "Meatured Temp" signal.

Your block diagram should have blocks that correspond to this chain of things starting with the error signal and ending with "Meatured Temp" signal.

Last edited: Nov 22, 2011
3. Nov 22, 2011

### I like Serena

Hi Fp!

I like that you got a new field of learning and posted on PF!
Just out of curiosity, which courses do you have this semester?

As for you diagram, I believe there is usually an input.
Which would the input be?

Energy is usually not modelled.
It is assumed to be universally present whenever you need it.

Here's a somewhat similar diagram, showing what I mean:

It also shows the basic elements (F=feedback sensor, C=controller, P=plant or system).

4. Nov 22, 2011

### Femme_physics

Last edited by a moderator: May 5, 2017
5. Nov 22, 2011

### I like Serena

Yes, this makes more sense.

Now let's see if we can match the components and signals you have in your drawings.
I'm going to limit myself to the components that have an electrical input or output.

Can you fill in the empty cells, or mark them in your block diagram?

$$\begin{array}{| l | l |} \hline \\ \textbf{Original electrical component} & \textbf{Block diagram} \\ \hline \\ Potentiometer & \textit{Potentiometer (introductory)} \\ \hline \\ ? & \textit{Junction node} \\ \hline \\ ? & Controller \\ \hline \\ Amplifier & Amplifier \\ \hline \\ \textit{Electro-magnet} & \textit{Electro-magnet + Fuel valve} \\ \hline \\ & Blowtorch \\ \hline \\ ? & \textit{Feedback potentiometer} \\ \hline \\ Reductor & ? \\ \hline \\ \textit{Temp. sensor} & ? \\ \hline \\ \ & \ \\ \hline \\ \textbf{Electrical Signals} & \\ \hline \\ U_r & ? \\ \hline \\ U_e & ? \\ \hline \\ U_m & ? \\ \hline \\ U_b & ? \\ \hline \\ \textit{Measured temperature} & ? \\ \\ \end{array}$$

Oh, and you didn't say which courses you are having this semester.
Or don't you want to?

Last edited by a moderator: May 5, 2017
6. Nov 22, 2011

### Femme_physics

Sorry, I was distracted!

Control Engineering
Electronics (many classes)
Hydraulics
Robotics CIM (introductory robotics class)
Materials Strength
Materials Strength lab

At any rate, I will try solving it tomorrow. thanks.

7. Nov 23, 2011

### Femme_physics

Last edited by a moderator: May 5, 2017
8. Nov 23, 2011

### I like Serena

I know you're new to it!
But you want to impress everyone don't you?

So if you try to explain to anyone, you should be able to say what in the original diagram goes where in your block diagram.
And the main thing is that you think about it.

I don't expect you to map the electrical signals to blocks in your block diagram.
But I'd like you to mark in your block diagram the lines with the electrical signals.
An electrical signal goes over a wire you know, so they're between blocks.

Now let's see.
I'll pick out a couple of your choices.

You have added "Blowtorch" as "Original electrical component" and mapped it to the "Controller" in your block diagram.
But the Blowtorch doesn't control anything does it?
And it isn't electrical.

You have mapped the Reductor to the Fuel Valve.
But if you look to your original figure, you should see that the Reductor is nowhere near the Fuel Valve.
Furthermore the Reductor has 3 electrical connections, but your Fuel Valve is not electrical at all.

And you mapped the "Temperature Sensor" to the "Container".
But "Container" is not one of the blocks in your block diagram.

Last edited by a moderator: May 5, 2017
9. Nov 24, 2011

### Femme_physics

Hmm, as I think it over I think I need a good tutorial about building a block diagram.
I really did try google but it's not helping me. I am truly lost, and feel like I have a too deep hole and no idea of how to build a block diagram, how to do proper analogy from original electrical component to block diagram.

Can I be helped to some resources that are relevant?

10. Nov 24, 2011

### I like Serena

Well, a block diagram is nothing other than that you draw each component in your original diagram as a block.
And then you connect the blocks with lines, just like the components are connected in your original diagram.

The best way to learn it, is by simply doing it, and thinking about it.
Perhaps it would help to see a few examples.

You did fine, except that it was not obvious to me which component went where in your block diagram.
And I was also not clear what the connections between the blocks represented

So any component that does not have an electrical input or output might be left out (or not as you please).

So the first question is: which electrical components do you have exactly?
And the second question is: which inputs and outputs do they have?

11. Nov 25, 2011

### Femme_physics

Done :P

And how do I mark those connections? With Ur, Ue, Um and Ub?

Reductor, Amplifier, Electro-Magnet, Blowtorch (if activated by an electrical device), Temp' Sensor

Fuel that begins an electro-mechanical chain of reaction.

12. Nov 25, 2011

### I like Serena

Yes.

Good!

But the blowtorch does not have an electrical input, but it is indirectly controlled by the Electro-Magnet.

Huh?
Fuel is not electrical. It is a fluid that burns.

13. Nov 25, 2011

### I like Serena

Btw, you can draw 2 types of block diagrams.
One for only the electrical components to model the electrical signal.
And one for all the components, modelling the functional behavior of the entire system.

Looking back at the problem statement and questions, I think the entire system is intended, which includes the non-electrical components.

Last edited: Nov 25, 2011
14. Nov 26, 2011

### Femme_physics

I noticed that somehow my attachment after saying "done" didn't get through.

That was supposed to be it:

http://img705.imageshack.us/img705/2407/blowblowd.jpg [Broken]

After you said "Well, a block diagram is nothing other than that you draw each component in your original diagram as a block.
"

What do they stand for, exactly, I wonder?

So I'll change my answer to just "fuel"

Last edited by a moderator: May 5, 2017
15. Nov 26, 2011

### I like Serena

Good!

But how is the blowtorch connected to the scale?

The blowtorch should be connected to the temperature sensor, because the blowtorch gives of heat, and due to the heat the temperature sensor will measure a higher temperature.

Then your problem asks for a block diagram with as input the desired temperature.
Where is it?
It should come in as an arrow from the left.

And your problem asks for an output in the form of the existing temperature.
This would be the signal coming from the temperature sensor.

U stands for voltage.

Ur is the "reference" voltage indicating Tr, which is the reference temperature (aka desired temperature).

Ue is the error voltage, which is the difference between the inputs (indicating reference temperature and measured temperature).

Um is probably the "magnified" voltage, which is the input for the electro-magnet.

Ub is the "feedback" voltage, not sure why it's a "b" though.
This represents the measured temperature.

That's okay.

Last edited by a moderator: May 5, 2017
16. Nov 30, 2011

### Femme_physics

Makes sense, but I did do what you told me-- to just put the words in blocks. Now it appears it's more of a game of engineering logic (which I have not yet acquired [soon! :) ]) than just writing what I see.

Hmm.... so
After blowtorch -> temp' sensor

and from temp sensor there is an arrow to the left for scale. Yes?

Thanks. I'm not entirely sure how to place them yet. I'll do a sketch at home.

Good :)

17. Nov 30, 2011

### I like Serena

The first step is to put the words in blocks.
The second step is to put the lines that are between the words also between the blocks.

And then an extra line to represent that someone turns the knob to set the desired temperature (the "scale" is the knob).

And another extra line to represent what comes out of the system (hot water, or rather the measured temperature).

Yes. :)

Can you check the original diagram?
The temp sensor is connected the another word there.

I'll be waiting!

18. Dec 1, 2011

### Femme_physics

Last edited by a moderator: May 5, 2017
19. Dec 1, 2011

Looks good!