EEE6207 – Assignment – 2020/21 P. Rockett 12th December 2020 Note This assignment will almost certainly require you to do some searching to identify suitable methods of solving the problem(s). The Linux API—the system to be used here—contains so many functions that it is impossible to learn all the details. Therefore, real systems program- ming frequently involves looking through the documentation. However, by this stage in the course, you should understand enough about operating systems to narrow your search down to the relevant OS features. 1 Introduction Computers are increasingly being used in control applications. Consider the case of a computer- based controller where the system generates an updated control input every 10-15 minutes—in other words, this is a real-time controller, but the time scale is not computationally very demanding. Such systems are common in, for example, chemical engineering where the characteristic response time of a physically massive chemical reactor is very slow due to inertia. Looking at the simple example program below, the main program comprises an infinite loop (since control is ongoing). The value of the input to the control variable u is determined by a simple function called control function that takes the time index t as an input. Notice that in this trivial example, the printf statement mimics sending the value of u to the plant at which point the process sleeps for some period equal to the update time for the controller—maybe 10-15 minutes might be used in practice for a large, slowly-responding plant1. // Example controller program — pir #include #include #include #include int control_function(const unsigned k, double* u) { *(u) = sin((double)k / 50.0); // Calculate input to plant return 0; } // control_function() 1More elegantly, the process should calculate the absolute time in the future when the next control output needs to be calculated, and set an alarm for this time. In this way, if the calculation of the control signal takes a significant amount of time, the sampling interval is held truly constant. But the simple sleeping solution is adequate here. 1 int main() { unsigned t = 0; double u; while(1) { if(control_function(t, &u) == -1) { printf(“control_function failed\n”); exit(-1); } printf(“%lf @ %u\n”, u, t); // Mimics applying controller input u to plant t++; // Increment time index sleep(1); // Sampling time… maybe 10-15 minutes in practice } return 0; // Should never reach here! } // main()) Note: The control function here is trivial. . . in fact, it’s not really a controller at all! This assignment is not about control rather the effective implementation of computer control by ex- ploiting operating system concepts. In practice, it is often necessary to update the control function function to accommodate changes to the plant characteristics, modifications to the plant operating protocols, etc. The simplest way of doing this would be to stop the controller program, replace it with an updated program, and restart the program. But stopping the controller is undesirable—the (brief) loss of control can cause product loss or deviations. Also, some industrial processes are unstable so removing control—even momentarily—can have bad consequences. Finally, the control program may be running on a small embedded computer that does not even have a terminal so any updates need to be made remotely. ©P.Rockett, 2020 2 TheAssignment Part 1: What is required is a means of updating the controller functionwithout stopping the controller program. Thus the first part of the task is to modify the above C pro- gram to: 1. Change the function called within the infinite loop to some a completely arbitrary function; the function prototype, however, should remain unchanged. As a demon- stration, you can use something like a function almost identical to the existing control function but one that calculates u = 2.0 * sin((double)k / 20.0) although this is just a simple example—in practice, the new function could be absolutely anything. 2. The controller program should only make this change when instructed to do so by an external command; having the controller program read a user input with a scanf in- struction or similar is not an acceptable way of initiating the update—the controller may be running on an embedded computer with no terminal. 3. To conform to good programming practice, your solution should include appropriate error checking and all possible recovery mechanisms. For example, if your procedure to substitute a new controller function goes wrong for some reason, the existing function should continue to be used so that the plant is still under control. This may no longer be optimal, but it is better than the control of an exothermic chemical process ceasing at 4am on a Sunday morning! Assignment Part 2: Unless you are very lucky, the output of the new control function will not be equal to the output of the old control function at the point of switchover. This may result in the plant being subjected to an abrupt step input, which is highly undesirable for a number of reasons. It is therefore normal when switching between controllers to avoid such a step (or ‘bump’) in the control inputs by implementing so-called bumpless control. Instead of changing control algorithms abruptly, the action of the new controller is gradually introduced by using a system input of: u = λunew + (1− λ)uold where the value of λ is gradually increased from 0 to 1 over some number of time steps—say, 10—and thereafter the old controller’s output is ignored. This has the effect of ‘fading in’ the action of the new controller and ‘fading out’ the old one to achieve a bumpless transition. Further modify the above program to implement a bumpless transition when you introduce the new control function; demonstrate that you have achieved this with a suitable plot of controller outputs covering the switchover between controllers clearly indicating the point at which the new control function was made active. (Strictly, bumpless control is not really an operating system topic so this section just ‘tidies up’ the program; for that reason, it attracts a fairly low proportion of the marks.) ©P.Rockett, 2020 3 Report Submission • This assignment will contribute 25% to the overall module mark. (You will need to pass each of the three assessment elements, of which this is the first, to successfully pass the course.) • The report should comprise up to 4 pages of A4 submitted via the EEE6207 course page on Blackboard by 8th February 2021. • You should discuss the design alternatives and trade-offs you have considered, and justify your final choice. These reasons might include, for example, that your preferred solution was sim- pler to code (and therefore less likely to contain bugs). Or maybe that your solution makes it possible to also do x, y and z. • The report should include your code—or at least sufficient of it to fully demonstrate your im- plementation, as well as evidence that it actually works. (A screen grab of the terminal output may be adequate.) • Part 1 will be worth 80% of the total marks—45% of which will be awarded for the design and discussion of design alternatives, and the remaining 35% for describing and demonstrating a working program. • Part 2 will be worth 20% of the report mark awarded for describing and demonstrating a pro- gram that implements bumpless transition between controllers. • On the subject of code, wud U. rite th:E Boddiy OF /tˆhe ripport lYke thiSS? No? So why would you submit a program written like this? See the interesting article on Technical Debt (https://www.parkersoftware.com/blog/what-is-technical-debt-and-what -should-you-do-about-it/). That students frequently submit difficult-to-follow code that is totally devoid of comments is probably understandable—‘success’ in a programming assign- ment is seen as getting the program to work. End of story. But a properly presented program makes it clear to me what you have done, how you have done it, and therefore makes it easy for me to award yo
u marks! If you force me to work through contorted logic with variables named ‘d’ , ‘dd’, ‘ddd’, etc.2, I may fail to grasp your ingenious, working solution to the problem, and award a low mark because I cannot understand what you have done. Core principle: always help the examiner to award you marks. 2I once had a colleague—now no longer in the department—who would write Fortran programs in which the first variable was named d’ (for data). If he needed a second variable, he would call it ‘dd’. A third variable would be called ‘ddd’. And so on. I really wish I was making this story up. . . ©P.Rockett, 2020 4 欢迎咨询51作业君
