Characteristics of Automated Manufacturing Systems
Automated manufactured systems have computerised controls built into the manufacturing equipment. Data are gathered through the sensors, processing occurs and a signal is sent to an actuator.
A manufactured system supports the production of a product in a number of ways:
- Tracking inventory: location of products and parts
- Record keeping: who, where, when, quality control
- Scheduling production: steps in the process
- Carrying out production steps.
The main users of the system are:
- Supervisors who oversee the operation
- People who supply information to the system.
What are block diagrams?
Block diagrams are used to represent the control system graphically. They are similar to a flowchart in that they represent the flow in an algorithm. Each block in the diagram represents a logical function in the system, including the controller, processes, actuators and sensors. All components are necessary, and a controller is essential in all block diagrams.
- Rectangles enclose the controller and each process
- Circles represent actuators and sensors
- Arrows show the direction of the flow of data.
Block diagrams should not look like a drawing of a series of toy cars on a road going in one way in linear fashion. It is a good idea to plan the drawing of a block diagram by first listing the components you have identified as necessary for the system you are describing. Draw the controller towards the centre of the page; arrange the processes around it, then place the sensors and actuators. When you joint the diagram with arrows in the direction of the flow of data, you will be checking that you have included all necessary components.
Why use automated manufacturing systems?
The reasons for automation are wide and varied.
| Reason | Explanation |
|---|---|
| Repetitive tasks | Automated manufacturing systems are able to accurately repeat actions, resulting in increased productivity and reduced costs. |
| Safety | Automated manufacturing systems can protect people who work in high risk environments. |
| Cost reduction | It is possible to run automated systems for many hours at little cost. |
| Quality control | Because actions are accurately repeated, a consistent level of quality is produced. |
| Precision and acceptable tolerance range | An automated robotic system is capable of performing repetitive tasks and processes to a high level of precision. Sensors and limit positioning devices ensure that the automated system completes its tasks within the acceptable tolerance and ensure that the automated system does not stray beyond its intended work space. |
| Productivity gains | An automated system results in higher productivity as it is capable of operating for an increased number of hours, at a greater production rate with lower levels of labour costs. |
| Gains through simulating and modelling | Such as:
|
Subsystems
Many computer-controlled systems can be broken down into subsystems. This means that the process identified when you first described how the system operated could itself have a controller, processes, actuators and sensors.
Open and closed systems
In an open system the controller never receives a signal that tells it the result of its actions. Most important in this type of system are the timing and the sequence of the actions. These activities could rely heavily on human interference or monitoring to ensure that the steps and stages of the activity are being carried out.
One example is the system by which an oven timer keeps thermostat operating until a period of time has elapsed and then turns the oven off.
In a closed system the controller is given feedback about its actions from sensors. These sensors gather data that allow the controller to regulate the process. A closed system requires planning and knowledge of the ways in which sensors and controllers interface.
For example, the sensors on automatic doors allow the doors to open only when a person wishes to enter, but don't open to the movement of a bird or a dog. This is an example of a closed loop.
Closed loop systems are able to react to changes in the environment, because they can sense that environment and react to it. These sensors attempt to simulate the human senses of sight, touch, hearing and smell. They do this by measuring the environment. The data are then passed to the controller which operates under a given set of rules that define the parameters for action under the described conditions.
Feedback
In a closed loop system, output is often fed back into the system to become part of the input for a new or different action.
Feedback can be positive or negative: added to the input signal (positive) or subtracted from the input signal (negative). In a situation where feedback is given, damping may be necessary.
Damping
Because the components of a system will need to be protected during their operation in order to extend their life, and because it is not always appropriate to have the system turning on and off until the stable condition is met, it is necessary to dampen the feedback signals. This will remove instability in the system.
Critical damping is the ideal situation, where the desired result is achieved in the shortest possible time, without any overshooting of the target condition.
Overdamping occurs when the system achieves the desired result without overshooting, but takes longer than necessary to do so, as the critical elements require a slow response time.
Underdamping occurs when the desired effect is achieved through the system seesawing through the critical condition, taking a smaller swing each time until the desired value is reached. These systems are not as stable as overdamped systems.
Sensors
Sensors are devices that will generate a current, voltage, or other value that changes in response to a change in the environment in accordance with well defined principles of physics. They therefore provide a number that is the value of a measurement made as a result of a condition in the environment.
Sensors can measure temperature, pressure, sound, liquid flow, magnetic fields, light, humidity, radioactivity, gravity, the proximity of substances anything that can be measured. They provide the data for the controller to respond to.
Some sensors provide data continuously, while others are discrete; that is, they turn on or off under certain conditions. Examples of discrete sensors are light sensors, micro-switches, reed switches and float switches.
Signal conditioning
The data from many sensors are in analog form, and need to be converted to digital form for a computer controller to use. An analog-to-digital converter is needed for this to happen. The conversion between analog and digital signals is carried out by an interface.
This process is called signal conditioning, and it may also be needed in reverse to allow the controller to activate the actuators.
Another situation where signal conditioning may be necessary could be the amplification of a signal, or the filtering of the signal in order to remove "noise" or interference from the needed signal.
Noise is unwanted electrical signals. Some noise can be eliminated by careful design, such as ensuring that the power supply does not hum within electrical sensing circuits, or using shielded cables. Filters that remove all signals except those in a specified frequency may be necessary, as may be timing controls or artificial intelligence concepts to determine whether the signal is valid. Feedback can also be used to remove unwanted signals.
Actuators
Actuators may sometimes be called effectors. They are the components. That effect the desired change on the environment in which the system has been placed.
The output from the sensor in a process becomes the input for a machine called an actuator. These carry out the activity that the conditions require according to the rules under which the controller is operating. Some examples are solenoids, relays, VDUs, safety buzzers, or motors. Actuators will be used to convert energy from one form to another.
The general design for some actuators is similar e.g. piston, when a linear action is required, and some form of turbine-like device, when a rotary action is required. Control can be maintained by the addition of a servo valve on the input line. If the power transmission is hydraulic, the advantages are that the hoses providing the linkages can be located some distance from the power source, and that the linkages are extremely flexible. Hydraulic or pneumatic systems are efficient when compared with the conversion of power in electrical to mechanical systems, and also have an advantage of the relationship between low amount of pressure in the compressor becomes high pressure in the actuator. These are the reasons why pneumatic actuators are favoured in some industrial robotics applications.
A servo motor is an electrical motor that is combined with an angular position sensor. It operates on the length of the control signal to determine the angle of the motor shaft, for example, the movement of a flap on a model aircraft. The angular movement can be measured using magnetic flux effects, such as a Hall Effect Sensor, rather than resistance or light variance.
A solenoid is an electro-mechanical device that can turn a device on. It uses an electromagnet to move an arm or plunger through a small movement.
A relay is often used to switch the current from one circuit to another with a different rating. Relays come in two types: electromechanical and solid state. An electromechanical relay is made up of a heavy-duty switch and an electromagnet. When a small current passes through the electromagnet, the magnets are pulled together; these are the switch contacts for the differently rated circuit.
Solid state relays work without electro-mechanical components, switching and blocking at molecular level. They are becoming more common than mechanical relays because they are faster and eliminate bounce and arcing, which causes interference. They also work on lower input voltages.
A step (stepped or stepper) motor allows a precisely controlled movement of the output shaft of the motor as it turns in small, precise steps. The number of steps will vary, with more steps relating to a higher quality. A cheap motor might have 12 steps (a movement per step of 30°) while an expensive one might have 360 steps. Step motors are powered by pulses rather than a continuous flow of power. Because such pulses of power can be regulated directly by the controller, a computer or microprocessor can keep direct track of a step motor without the need for a sensor tracking position. A step motor must be used within its tolerances, otherwise the microprocessor will "think" that the turn has been, say, 36° when, because of the torque, the movement was only, say, 20°.
Resistance actuators generate either light or heat. There is no mechanical activity when they are used, and no switch is involved in turning them on as they respond directly to the current.
A word on temperature
Thermostats are not actuators. They change resistance on the basis of the temperature and in that role are a sensor and resistor.
Thermostats are units with variable sensitivity designed to adjust to the environmental parameters. They are a subsystem within a control situation and may fulfil the role of a controller.
Thermocouples are bi-metallic strips, the components of which expand at different temperature rates causing the strip to bend. It then acts as the contacts in a switch. It undertake the role of both sensor and actuator in one unit.
Magnetic field actuators cause a movement directly within a fluctuating magnetic field. For example, a magnetic field resonance image is an image built by measuring the magnetic changes in each cell of the body after they have been polarised by a strong magnetic field. The image built up is extremely detailed and flexible, and gained without the use of radiation.
Controller
This is the device that regulates the operation of a system. At an airport, the air traffic controller regulates the flow of aircraft as they land, take off and fly over the land around the airport.
Systems with a microprocessor regulating the system are microprocessor-controlled, a more accurate term than computer-controlled.
In a computer-controlled system, the computer program will analyse the data gathered by the sensors and use this information to maintain or improve the performance of the system. It is then able to apply the rules of the program, controlling the conditions of the system and sending signals to the actuators to effect a response in the environment to the signals from the environment.
Control systems
Control systems differ to suit different conditions.
A continuous system has no starting or finishing points, and the system continuously performs the same overall job. The alarm system in a home or school, once it is turned on, needs to be continuous to be effective.
A batch system is characterised by separate runs, with the system able to be modified in order to carry out different tasks that are similar to each other. In a fruit processing plant, canning apples is similar to but different from canning peaches, apricots or pears. The system that handles these processes uses the same components operating within different parameters to cater for the different fruit runs, as each is batched through the system. Note that the system does not sort the fruit, but processes each type of fruit as a batch.
A discrete system deals with a single item and performs several tasks on that item. A morning tea maker linked to an alarm would be an example.
Design of control systems
Control systems must be the result of a careful analysis of the situation if managers are to make decisions on which type of system is most appropriate, and the parameters under which it will operate in order for it to function efficiently and safely. Once input and output types and parameters have been identified, sensors and actuators appropriate to the design must be chosen. The range of these is increasing, and it is possible that types not mentioned specifically above are available for specialised situations.
It should be possible to describe the system and its component subsystems in a set of related block diagrams.
