INTRODUCTION TO CONTROLLER PROGRAMMING

It is beyond the scope of this text to present a detailed discussion of how to program a microprocessor in machine language. Still,

it is useful to investigate in a general way what the software must do. A digital controller is a computer operating in real time.This means that the program is running all the time—repeatedly taking in the newest sensor data and then calculating a new output  for the actuator.
The basic structure of a controller program is a loop. In a loop structure, the same sequence of instructions is executed over and over again, and each pass through the loop is called an iteration, or scan. Figure 14 shows a generalized controller program,and an explanation of the program follows:

1. The program reads in the set-point data (recall that the set point is the desired position of the controlled variable). This data  could be read in from an input port or from memory.
2. The program directs the computer to read (from a sensor) the actual value of the controlled variable.
3. The actual data are subtracted from the set point to get the error.
4. Based on the error data, the computer calculates a new actuator control signal.
5. The new output is sent to the actuator.
6. The programs loops back to step 1 and starts over again.

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INTERFACING TO A MICROPROCESSOR CONTROLLER – The Serial Interface

In a serial interface, the data are sent 1 bit after the other on a single wire. There are a number of good reasons for doing this. First, the cabling is simpler because only two wires are needed (at a minimum), those being “data” and “return.” Second, shielding a small group of wires, which is often necessary in an electrically noisy industrial environment, is easier. Third, serial data can make use of existing single-channel data lines such as the telephone system (which may require using a modem). For these reasons,
serial data transfer is usually recommended for distances greater than 10-30 ft.
Because data always exist in a parallel form inside the computer, it must be converted to serial data before coming out the serial port. This is accomplished with a special
parallel-to-serial converter IC called a universal asynchronous receiver transmitter (UART). On the other end of the line, a receiver must convert the serial data back into parallel data, which is done with another UART. Figure 10 shows the basic serial data circuit.
Serial data are classified as being either synchronous or asynchronous. Synchronous data require that the data bytes be sent as a group in a “package.” It is used in sophisticated communication systems that move a lot of data and will not be further discussed here. Asynchronous data transfer is the more common (but slower) type of serial transfer and allows for individual bytes to be sent when needed.
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INTRODUCTION TO MICROPROCESSOR OPERATION

The microprocessor works by executing a program of instructions. Creating the program is similar in concept to programming in BASIC, C, or any other high-level computer
language. Each type of microprocessor has its own instruction set, which is the set of commands that it was designed to recognize and obey. Microprocessor instructions are very elemental and specific, and it usually takes more than one to accomplish what a single, high-level language instruction would. Many microprocessor instructions simply move data from one place to another within the computer; others perform mathematical or logic operations. Still another group of instructions control program flow, such as jumping forward or backward in the program. Each instruction in the instruction set is assigned its own unique operation code, (which is typically 8 bits long and
referred to as the op-code). The CPU uses this 8-bit number to identify the instruction. All microprocessors have at least one accumulator [Figure 4(a)], which is a dataholding register in the CPU. The accumulator acts as a “staging area” for data.
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INTRODUCTION TO MICROPROCESSOR SYSTEM HARDWARE

A computer is made up of four basic functional units: the central processing unit (CPU),  memory, input, and output (I/O). The central processing unit does the actual computing  and is composed of two subparts: the arithmetic logic unit and control sections  Figure 2. The arithmetic logic unit (ALU) performs the actual numerical and logic  calculations such as addition, subtraction, AND, OR, and so on. The control section of
the CPU manages the data flow, such as reading and executing the program instructions.  If data require calculations, the control section hands it over to the ALU for processing.  In a microprocessor-based computer, the microprocessor is the CPU.

CLASSIFICATIONS OF CONTROL SYSTEMS – Motion Control

Motion control is a broad term used to describe an open-loop or closed-loop electromechanical  system wherein things are moving. Such a system typically includes a  motor, mechanical parts that move, and (in many cases) feedback sensor(s). Automatic  assembling machines, industrial robots, and numerical control machines are examples.
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CLASSIFICATIONS OF CONTROL SYSTEMS – Process Control

Process Control

Process control refers to a control system that oversees some industrial process so that a uniform, correct output is maintained. It does this by monitoring and adjusting the  control parameters (such as temperature or flow rate) to ensure that the output product  remains as it should.

Figure 7  A closed-loop oven-heating system.

The classic example of process control is a closed-loop system maintaining a  specified temperature in an electric oven, as illustrated in Figure 7. In this case, the  actuator is the heating element, the controlled variable is the temperature, and the sensor is a thermocouple (a device that converts temperature into voltage). The controller  regulates power to the heating element in such a way as to keep the temperature (as reported by the thermocouple) at the value specified by the set point.
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ANALOG AND DIGITAL CONTROL SYSTEMS

In an analog control system, the controller consists of traditional analog devices and circuits, that is, linear amplifiers. The first control systems were analog because it was  the only available technology. In the analog control system, any change in either set point or feedback is sensed immediately, and the amplifiers adjust their output (to the actuator) accordingly.
In a digital control system, the controller uses a digital circuit. In most cases, this circuit  is actually a computer, usually microprocessor- or microcontroller-based. The computer executes a program that repeats over-and-over (each repetition is called an iteration  or scan). The program instructs the computer to read the set point and sensor data and then  use these numbers to calculate the controller output (which is sent to the actuator). The  program then loops back to the beginning and starts over again. The total time for one pass  through the program may be less than 1 millisecond (ms). The digital system only “looks”
at the inputs at a certain time in the scan and gives the updated output later. If an input  changes just after the computer looked at it, that change will remain undetected until the  next time through the scan. This is fundamentally different than the analog system, which  is continuous and responds immediately to any changes. However, for most digital control  systems, the scan time is so short compared with the response time of the process being controlled that, for all practical purposes, the controller response is  instantaneous.
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Transfer Function

Physically, a control system is a collection of components and circuits connected together to perform a useful function. Each component in the system converts energy from one form to another; for example, we might think of a temperature sensor as converting degrees to volts or a motor as converting volts to revolutions per minute. To describe the performance of the entire control system, we must have some common language so that we can calculate the combined effects of the different components in the system. This need is behind the transfer function concept.
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Introduction to Control Systems

INTRODUCTION

A control system is a collection of components working together under the direction of some machine intelligence. In most cases, electronic circuits provide the intelligence, and electromechanical components such as sensors and motors provide the interface to the physical world. A good example is the modern automobile. Various sensors supply the on-board computer with information about the engine’s condition. The computer then calculates the precise amount of fuel to be injected into the engine and adjusts the ignition timing. The mechanical parts of the system include the engine, transmission, wheels, and so on. To design, diagnose, or repair these sophisticated systems, you must understand the electronics, the mechanics, and control system principles.In days past, so-called automatic machines or processes were controlled either by analog electronic circuits, or circuits using switches, relays, and timers. Since the advent of the inexpensive microprocessor, more and more devices and systems are being redesigned to incorporate a microprocessor controller. Examples include copying machines, soft-drink machines, robots, and industrial process controllers. Many of these machines are taking advantage of the increased processing power that comes with the microprocessor and, as a consequence, are becoming more sophisticated and are includ- ing new features. Taking again the modern automobile as an example, the original moti- vation for the on-board computer was to replace the mechanical and vacuum-driven subsystems used in the distributor and carburetor. Once a computer was in the design, however, making the system more sophisticated was relatively easy—for example, self- adjusting fuel/air ratio for changes in altitude. Also, features such as computer-assisted engine diagnostics could be had without much additional cost. This trend toward com- puterized control will no doubt continue in the future.
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