Systems Diagrams Most industrial product design is solved by the systems approach.

This approach involves studying the desired function of the product, and then breaking this function down into a series of subsystems.

INPUT

CONTROL

OUTPUT DRIVER

OUTPUT

When applied to control systems, a systems diagram is a useful way of visually representing the desired function of the system. The systems diagram is a form of block diagram that contains all the subsystems within a dashed box, called the systems boundary. The systems boundary indicates the extent of the control system. The 'real world' input and output conditions of the system are shown as arrows entering, and leaving, the systems diagram. e.g. Dirty Clothes

Washing Machine

Clean Clothes System Boundary

A Block Diagram shows the individual sub systems in a whole system

Drum Washing Machine Heater Pump

Temperature Sensor Program Switches

A Control Diagram shows the the feedback loop and error detector. Feedback Sensor

Set Value

Amp

Output

Const Value

Basic Control Function All systems require some controlling function to ensure that the system will carry out the desired task. Moreover the controlling function has to ensure that the task is carried out within the required specification for safety, accuracy, repeatability and speed. A control operation requires an input. The input expresses the desired outcome from the control function. The control function will process this input and hopefully produce the desired output. Examples of input - control - output: a) a person observes the flow of traffic (input) checks for a safe time to cross (control) crosses the road (output) places ingredients for making a cake in a mixing bowl (input) mixes ingredients for a specified time (control) transfers mix to baking bowl (output) set an electric drill input for a specified speed (input) input processed by control section (control) output to motor for desired speed.

INPUT

CONTROL FUNCTION

OUTPUT

b)

c)

The same principles are applied to an industrial robot. The robot requires an input to know what it has to do. It requires a control section to interpret and process this input into the required output signal. Finally it requires the proper output devices to respond to the output signal and produce the desired effect. Sequence Control Sequence control is applied to many industrial processes, including mechanical handling and packaging. Sequence control means carrying out a series of tasks, one after the other and in a prescribed order. There are two forms of sequence control 'event-based' and 'time based'. Event based In an event based system each task in the sequence of tasks can only be initiated when the previous task has indicated completion. Sensors are required in an event-based system to indicate when a task is completed. An example of an event-based system is shown. It is simply an automated system to pick up items, take them to a drill to drill a hole and then remove the completed item to a box. The rectangles represent tasks or actions to perform in sequence from top to bottom. Each short horizontal line provides a sensor to indicate when the previous action has completed. Only when the condition at the horizontal line has been satisfied can the system proceed to the next task.
Pick up item item picked up Move item under drill item under drill Drill hole in item hole completed Drop item in box item in box

Time based Each task is still carried out one after the other but in a time based system each task is sequenced to time. There are no sensors to indicate when a task has been completed. In basic terms an action is scheduled to last for a set time. When that time has expired the next action is initiated and it will endure for a set time and so on. An example will best illustrate the time based system: a robot arm moves to picks up an item the robot arm moves item under drill a hole is drilled in item item is moved into a box return to pick up a new item. three seconds delay three second delay six seconds delay four seconds delay

Here we have repeated the identical sequence as before but substituted time delays for sensors. In such an application the time based system may work perfectly many times but it cannot be as accurate or reliable as the event based system. What happens if an item is not available for pick up? The time based system will not be aware of this and continue to operate as if an item was present. What happens if the robot slows down because of increased friction or low voltage to its motors? The time delay to allow it time to place the item under the drill may suddenly be too short and the system may attempt to drill with the item still moving in under the drill. The sensor based event system would wait until an item was available for pick up. If no item were available the system would wait until an item became available. If the robot arm slowed down for any reason the system would still function properly as it would wait until an item was placed under the drill before drilling could commence. Summary Event based systems are more reliable than time based systems but are more expensive due to the requirement to fit sensors for each event in the sequence. Time based systems are less reliable as they use time delays between events rather than perceiving that each event has occurred. In less critical applications such as mixing sequences (mixing various ingredients for various times) time based systems are ideal and less expensive (no sensors required). Open loop control The cheapest and easiest method of control to apply to a process is open loop control shown. ( no feedback)

Disturbances INPUT SIGNAL CONTROL FUNCTION OUTPUT DEVICE

Open Loop control requires an input. The input expresses the user's requirement from the process under control. The control function takes the input, processes it and produces an output effort. The output accepts the output effort and produces the desired effect on the process. We will comment on 'disturbances' shortly. Examples Let's look at an example of open loop control prior to examining it in greater depth. A new conveyor is installed in a factory to carry goods from the production machines to the warehouse. The conveyor will run at a steady speed on a particular production run but must be capable of running at four different speeds for different production runs. The speed will be set by an operator before the start of each run. Solution Assume that a DC motor has been selected to drive the conveyor. The control function will be a device which provides a variable DC voltage output, for a variable input, to run the motor at different speeds. Let's assume that a control dial is provided to vary the input to the controller. A commissioning procedure prior to start up would entail one technician measuring the speed of the conveyor with a hand held device and a second technician adjusting the speed with the control dial. Once the desired speed is attained the speed position would be marked on the dial. The same procedure would be done to mark the other three speed positions on the dial. The system is now set up and can be used easily by different operators on different shifts by merely setting the desired production speed on the controller dial. 3

Disturbances Certain factors are liable to occur which may prevent the conveyor running at the designated speeds on the dial. The bearings and rollers on the conveyor may develop increased friction due to poor maintenance. The DC drive motor will need to meet this increased load. To do this it has to slow down. The same slowing down effect occurs if additional load is placed on the conveyor (heavier items to be carried). Limitations of open loop These examples illustrate the main point about an open loop system. It is not a self monitoring system (no sensor to monitor the actual speed) and so its accuracy is limited because external disturbances can modify the desired output. Advantages of open loop Open loop control is relatively cheap to apply (no sensors required) and in many applications it may be a satisfactory solution where high accuracy of final output is not essential. In most applications of open loop control the response to a step input (instant change of input signal) is a smooth change from one stable state to the next. In the conveyor example it may be perfectly satisfactory to have a system which only maintains speed accuracy at 10%. Open loop control gives us the capability of running the conveyor at different speeds at a moderate cost. Closed loop control Closed loop control is a more accurate system of control and at the same time more expensive. It employs selfmonitoring where a sensor is used to read the condition being controlled. ( with feedback). The Figure shows a block diagram of a closed loop system. The process under control could be any of a range of industrial processes involving temperature, speed, force, pressure etc. The value we are trying to achieve is the input value set by the users and is often called 'desired value' or 'set point'. The sensor gives the actual value of the condition we are controlling and is often titled 'measured value' or 'actual value'. Closed loop control uses a comparator / diff amp. It compares two values by subtracting the actual value Desired from the desired value. Any value difference between the two produces an error which is fed to the controller. The controller generates a control action on the process to try and eliminate this error. On-off control The simplest form of closed loop control is on-off control. The system applies full corrective action if an error is present and none if the error is zero. A simple example is the temperature control of a water heater in a domestic situation. When the temperature is lower than the desired value, the thermostat contact closes and full heat is applied. When the temperature is equal to or greater than the desired value, the thermostat opens and the heater is turned off. Figure 7 shows the temperature control achieved with on-off control. Note the oscillating nature of the actual temperature around the desired value. The fluctuating nature of the control is acceptable in applications with slow response times. The unsteady nature of the measured value in on-off control is the result of the output either being full on or full off. This causes over correction to the measured value, especially with processes that have significant delays in their response to change. 4 Error Control action Controller Actual value Feedback sensor Process being controlled

Micro controllers are used nowadays extensively to control industrial and domestic systems. The choice of micro controllers for any particular purpose will depend on various factors: cost, stability,accuracy, onboard ADC or external ADC,, type of memory, size of memory, number of pins, type of integrated circuit. Type of memory OTP - one time programmable - as there name suggests they can only be programmed once and can not be changed if the the system changes Flash - renewable memory. can be reprogrammed any number of times making it more versatile when developing new products as the program can be easily changed. Integrated circuits SM DIL - Surface mount smaller packages therefore smaller PCB needed which is cheaper to produce and more easily embedded in the product. Easily handled by pick and place robotic for assembling. - dual in line packages- larger volume of package needing specific hole position and pattern in PCB to fit ICs. harder to position with by pick and place robots.

Assignment 1 Explain what is meant by 'sequence control'.

A robot arm picks up a metal part from a conveyor. When grasped the arm is raised to clear obstacles. When raised the arm moves the part over a container. When over the container the arm lowers into the container. When in the container the arm releases the part. When released the arm is raised to clear the box. When clear the arm moves back to the conveyor. a) Describe two ways in which these tasks could be executed. Select the best method for the task, giving a reason for your choice and why the alternative method may not be as suitable.

b) Suggest a suitable application for the alternative control method.

A D.C. electric motor is chosen to drive a section of a production line. Whether to control the motor speed by open or closed loop has not yet been decided. Describe how each methods works and comment on the suitability of each in terms of: a) how easily they could be applied

b) the cost of each method

c) the accuracy of each technique

d) the stability. 5

GEORGE WATSONS COLLEGE

ADVANCE HIGHER GRADE

TECHNOLOGICAL STUDIES

SYSTEMS AND CONTROL

OUTCOME 1