NC Eng Systems. Block Diagrams. Learning Outcome 1. MjD

Transcription

1 NC Eng Systems Learning Outcome 1 Block Diagrams MjD Sep 2013

2 Block diagrams are a type of tool used by engineers to help them describe or visualise the way that an Engineering System works or operates. Block diagrams come in many different formats from the simple box & line drawing to the more complex 3-D drawing or even animated drawings, although the latter only work in the multimedia domain, not really on paper. Let s view a block diagram in its simplest form; that of a single block. Figure 1: Simple (single) block. Figure 1 above shows that an input or inputs to the system are from the left hand side [l.h.s.] of the block; the block itself shows the process and finally on the right hand side [r.h.s.] of the block is the output or outputs. If we add a bit more information we see just how block diagrams work: Figure 2: Simple block, more detail. What we see shown in Figure 2 above is an amplifier probably part of a larger more complex block diagram. It shows the input signal (a voltage of 20mV or 20x10-3 V) enters the block from the l.h.s. the bock process states multiply the input by one hundred (20x10-3 x 100) which results in an output on the r.h.s. of the block of 2Volts. What is hidden by this block approach is all the detail of the underlying electronic circuitry as can be seen in Figure 3 below. [falstad circuit simulation see appendix for Figure 3 sim-file] Figure 3: Electronics for x 100 amplifier.

3 Let s take a look at a simple block diagram that represents a generic power supply: Figure 4: Simple block diagram of a power supply. Here, shown in Figure 4, each box represents a description of the process contained within the box. In other words the box hides the actual process and only shows the generic action carried out by that box. Between the boxes are arrows that show the direction of flow from one process to the other. It is disputable whether the first block, in the diagram above, should be there as a process but because this block diagram is generic, it contains no detail all we know is that it represents a power supply, we do need the block diagram to start with something we could remove the box and leave the text as the input, this would also work well. Every block diagram should be read from left to right and top to bottom. We start at the left and follow the arrows to read and understand the process flow. Another block diagram, of exactly the same process is shown in Figure 5 below. Figure 5: Block diagram showing a bit more information. Again this diagram is generic, there is no finite information shown. What is different about this diagram is that it shows the output or input from or to each of the blocks in turn. In this case it is showing the signals that are being passed from left to right. Block-1. Shows us that we are looking at an a.c. supply. This block has no input as there is no arrow to the left hand side of the block. But it has an output, indicated by the arrow from the right hand side of the block pointing to the next block. Above the arrow we can see a picture depicting the type of signal at the output of Block-1. Here we can see that it is the form of a sine wave. This signal is the output of Block-1 and is the input to Block-2. Block-2. States that we are looking at a process that reduces the amplitude of the waveform. Again the picture shows us this the amplitude is lower than the previous picture. This signal is the output of Block-2 and the input to Block-3. Block-3. Tells us that the process is that of a bridge rectifier and the picture shows that the process removes the negative ½ cycle and flips it up onto the positive side of the waveform as we saw in the Test and Measurement Unit. This signal is the output of Block-3 and is the input to Block-4.

4 Block-4. The last block, shows us that the final process converts the lumpy d.c. signal into a steady state d.c. signal. As there are no more blocks in the diagram then this must be the final output of the system. Figure 6: Regulated 12V Power Supply. Figure 6 shows us, by addition of input and output text, that we can easily describe a complete power supply from its a.c. input requirements to the final D.C. voltage level at its output with four boxes. OK, can you see how it works? What is the benefit of this type of diagram? Well, we can examine the diagram and understand the overall operation of the processes without having to understand all of the minute detail hidden by the process boxes. This makes it easier to follow even very complex processes. Let s have a look at a slightly more complex block diagram. Computer PLC PLC PLC PLC PLC PLC PLC PLC Store B Store A Make B Make A Assembly Inspection Packaging Warehouse Figure 7: Computer / Programmable Logic Controller [PLC] control of a Manufacturing Process. Figure 7 shows the automated control of a manufacturing process using a single master computer and slave PLC s. Information flows from the master computer to each of the process control PLC s in the layer below. This information is sent to the PLCs depending on the status of the manufacturing process, indicated, probably by sensors, from the layer(s) below the PLCs. The status of the processes are then returned through the layers to the master computer. The process could go something like this: Send the parts from Store-B to Make-B station When Part-B is complete send it to the Assembly station Send the parts from Store-A to Make-A station

5 When Part-A is complete send it to the Assembly station When Part-A and Part-B are present at the Assembly station then assemble the final part When the final part is assembled send to the Inspection station If the part passes the tests at the Inspection station then send the part to the Packaging station Once the part has been packaged send it to the warehouse Store until orders for that part come in Ship to customer Note: Within Figure 7 there is no extra meaning portrayed by the various colours used for the blocks. This is not necessarily always the case. Here is another block diagram that you may well come across again later on in this course. The block diagram of a digital thermometer. Figure 8: Digital thermometer block diagram. This block diagram does use colour as a means of identification within the overall process. Where this is the case we must always supply a Key indicating the meanings of the colours. The Key for Figure 8 is shown here on the right. Examining the block diagram reveals the following: note that a separate ADC blocks. Block-1: Shows we have heat as an input The process shows us that it is a thermistor [THERM The output is a changing resistance Block-2: Input changing resistance Process: a Resistance to Voltage convertor The output is a changing voltage Block-3: The input is a changing voltage Process: an amplifier [or signal conditioning] The output is a changing voltage at a higher level note that a separate explanation exists for the R to V and HERMal resistor resistance changes with temperature]

6 Block-4: The input is a changing voltage Process: Analogue to Digital Conversion [ADC] The output is a Digital or Numeric value Block-5: The input is digital or numeric value The process is a microcontroller, running a program, converting the numeric value to temperature - again as an alphanumeric code The output is an alphanumeric code [may be ASCII American Standards for Computer Information Interchange] Block-6: The input is an alphanumeric code The Liquid Crystal Display processes the code Output is the text displayed on the LCD screen

7 Control Engineering Another area of engineering that uses block diagrams to describe system operation is Control Engineering. Here the diagram blocks are used to show how an overall system, including all of the external influences on the system, interacts. Figure 9: General Block Diagram of a Control System. Figure 9 shows the general block diagram for a control system. The diagram shows, from left to right a set point (SP) being fed in to a comparator, we will come back to this, the signal from the comparator is fed into the control element which controls the process and this feeds to the system output. In the meantime some information about the system output is fed back to the comparator where it is subtracted from the set point and produces the error signal for the control system. How does it work? Let s suppose that the control system diagram represents an industrial automated weighing process. The system output is the means of feeding product into a bag or sack, the feedback element is a loadcell which feeds back information about the weight of the product in the bag or sack. For arguments sake let s say that the bag or sack is to be filled with 50kg dog meal [SP=50kg]. 1). As the process starts there is no dog meal in the bag so the feedback element is zero [FB=0kg] 2). This means that the error [e] signal being fed to the control process is [e = SP-FB] or [e = 50 0] 3). This tells the control system that it can switch the output on at full also known as the control effort [CE] 4). Some time later when the feedback element is reading 45kg this is fed back to the comparator

8 5). The error is now only 5kg [e = SP-FB] [e = 50-45] and the bag or sack is nearing it s set value 6). The controller can now set the system output to slow down the amount of dog meal being fed into the bag or sack 7). This means with good control that the bag should never over or underfill.

9 Figure 10 shows the control diagram for a domestic heating system.? Figure 10: Domestic; Room Temperature Control System. See if you can describe the control process:

10 Figure 11 shows the diagram for the cruise control of a car.? Figure 11: Cruise Control in a Car. See if you can describe the process:

11 The Medical Profession also use Block Diagrams. Motor control of ligaments ECG Electrocardiogram Data movement between patient and care givers.

12 The Microcontroller The microcontroller is another area in engineering where block diagrams are used to hide layers of complexity, although at first glance Figure 12 looks extremely complex. Figure 12 shows the Infineon SAB 80C166 Microcontroller in functional block diagram format. What this diagram shows is the electrical connections [busses] between each of the functional blocks along with the size of the bus. The bus size is shown as a number with a diagonal through the interconnection e.g. 16/, this means that that bus is 16-bits wide. The diagram also shows the interaction between the blocks. The actual datasheets for this device runs into the hundreds of pages and is very complex. Figure 12: Block diagram of the Infineon SAB-80C166 Microcontroller Another Microcontroller the mbed Microcontroller are a series of ARM based development boards designed for fast, flexible and low-risk professional rapid prototyping. Figure 13 shows the functional block diagram of the mbed microcontroller. Here the diagram shows the functions allocated to each of the pins on the development board. For example, the diagram shows where the two USB pins [D-] and [D+] are brought out to the outside, or the pin allocations for the serial communications ports. Figure 13: mbd Functional Block Diagram

13 Flow-Charts or Flow-Diagrams Software engineers also use a more stylised block diagram this is known as a flow-chart or flow-diagram and is used, not only to view the program flow, but also to help visualise the logic or decision making during the course of program execution. This type of block diagram uses a set of blocks that carry out different processes that depend on their shape see Figure 14. In all the other block diagrams that we have looked at there are only a few basic shapes that are used. With flow-charts or flow-diagrams there are a number of different shaped blocks. A shortened list of which are shown here Figure 14. This type of diagram also differs from the standard block diagram in that it normally runs vertically from the top of the page downward it always has a START indicator at the top and a STOP indicator at the end. Figure 14: Flow-Chart symbols. Just like standard block diagrams the flow is indicated by arrows. So what does a Flow-Chart look like? Figure 15: Are we there yet? This flow-chart, Figure 15, shows what happens on a long boring drive when you have a child in the back seat, or at least what the child s actions are. Start the journey Ask the question If yes END If no wait a while Then ask the question again Repeat until we ARE there.

14 In fact, this style of block diagram has actually been turned into a programming language for microprocessors (flow code) see Figure 16 for an example. Figure 16: Flow Code example Carry out a few class-room examples of flow-diagrams.

15 Apendix Falstad Figure 3 sim-file $ 1 5.0E r r a a w w w w w w g w w w w w w w w w v w w r r w w w w w w w o E o E-5 1-1

16 This diagram is a flow-chart of a computer based data capture system. The system has been designed to monitor the temperature of an industrial process over a 10 minute period. It has to take a single data sample every 10 seconds and at the end of 10 minutes it is to graph the results on the PC screen. A B A] B] C] D C D] E] E F] G] H] G F I] H I F