The first question is really "Why do you need a control system at all?” Consider the following: What good is an airplane if you are a pilot and you can't make it go where you want it to go? What good is a chemical products production line if you can't control temperature, pressure and pH in the process and you end up making tons of garbage? What good is an oven if you can't control the temperature? What good is a pump if you can't control the flow rate it produces?

The common denominator in all of these questions is that there is some physical quantity that must be somehow controlled in a way that ensures that the physical quantity takes on the value that is specified. What is clear is that if you want to control a system, you need to know what you want it to do, and you need to know how well it is doing. First, you need to know what you want the system to do. There are lots of ways you can do that. For example, in your home you set a temperature by dialling it into the thermostat. One way or another, the control system has to know what it has to do. The other thing that the control system has to know is how well the system is doing. You can use temperature sensors, pressure sensors, tachometers and many other sensors that measure physical variables to get a handle on system performance. One way or the other, the system has to measure or monitor its performance.

As you think about what you have to do to control a system, you realize that the information about how well a system is performing - usually taken at the output of the system - has to be fed back around the system to the input and compared somehow with the input - the information about what you want the system to do - and that comparison gives you the information you need to produce/develop and apply a control signal. Feeding back that performance information is what gives us the idea of feedback and feedback control systems. In a feedback control system, information about performance is measured and that information is used to correct how the system performs. It's common. It's used in the human body over and over again to correct body temperature, the amount of light that hits your retina, and lots of things you never have to think about.

They're everywhere, and they don't always happen naturally, so you need to learn about how to design them. You will often find electrical engineers who design control systems for aircraft of chemical plants. Designing control systems takes a person who can bring together various individual aspects of a system and make them work together, and often that process is highly analytical and mathematical. A control system designer often has to consider the safety or even the lives of the people using the systems they design. It's not always easy to predict how a system will behave. Analysis tools are not perfect, and systems are not always completely understood. Despite that, if the systems are going to be used, you - if you are the control system designer - need to do the best job that you can to ensure that your system performs well.

What other products can you think of which use a control system?

BLOCK DIAGRAMS Block diagrams are a schematic diagram used to represent a system. This is a blank block diagram which shows only the two elements of the system; the input signal and the output. The input signal is the information fed into the system by the user and the output is the result of that information having been fed into the system (usually resulting in something which can be measured in units). The input will usually be an ideal form of the output. Eg: We turn the thermostat to 22degrees and the air flowing from the air-conditioner comes out at 22degrees.

Now that the system has an input and an output it needs to be measured to ensure that the output is correct to the input and therefore the system is doing what it should be doing. The image above shows the ‘Control Effort’ which is produced by the input signal going into the System and flowing to the output (in this case a temperature). As well as flowing to the output it is also flowing to a sensor which is measuring the output and ensuring that the output matches the input signal. This reading is known as the measured output. This gives feedback to the operator as to how the system is performing.

The next component typically found in basic control systems is a Comparator. This component takes the information produced by the sensor and compares it to the input. This is done by subtracting the measured output from the input signal, resulting in an Error Signal which is the difference between the desired measure and the actual measure.

The final component found in most basic control systems is a controller. The controller acts on the error signal and uses that information to produce a signal to correct the system. The controller has two jobs: - Compute what the control effort needs to be. - Apply the computed control effort. Control system designers have to be sure that their controller has sufficient power to drive the system if the error is of a large degree. It is important to note that a controller does not have to be digital, it can also be mechanical.

EXAMPLE Let’s imagine this control system is found in a digital kettle. Control signal = Digital Control Panel Controller = Central Processing Unit (CPU) System = Heating elements Sensor = Thermostat Comparator = Central Processing Unit (CPU)

There are 4 main types of control systems: Electronic Hydraulic Pneumatic Mechanical

Electronic Control Systems: These control systems can be found in almost every major industry from manufacturing to domestic and military. These systems used a vast range of electronic components in circuits to input, monitor and control electronic signals, levels and actions. Electronic control systems can be very simple like in a kettle or extremely complex like the flight control systems in a Lockheed F-117 Knighthawk. The F-117 uses fly-by-wire technology which uses over 20 electronic computer control systems just to keep the aircraft in the air by inputting data, error correcting and adjusting thrust to its two turbines.

Mechanical Control Systems: These control systems are also quite common throughout a number of industries by are generally prevalent in the automotive industry. These systems control a task through forces and movement (much like in your mechanism design task). It is often harder to indentify and correct error in a mechanical system without the aid of electronic components. This is why many mechanical systems in modern vehicles work in conjunction with computers to monitor and control them. This is not to say that mechanical control systems cannot do this on their own. In earlier times many steam engines used mechanical sensors and human interaction to control and adjust systems. A common mechanical control system found in our modern cars is the cooling system which flows around the engine, through the radiator and uses a thermostat valve to regulate the flow and cooling of the fluid in the system.

Hydraulic & Pneumatic Control Systems: These control systems share many similar characteristics but operate on two different mediums. Hydraulic systems use fluids to generate forces and movement while pneumatic systems use air and gasses to perform the same task. Generally smaller applications might use Pneumatic systems while larger industrial systems will use Hydraulic systems. In these systems the units measured at typically pressures generated by pumps or mechanical inputs. Typical application for these types of systems are in industrial equipment such as vehicle lifts, cranes and manufacturing equipment.