Open and Closed Loop Control System

Closed Loop Control System

The closed loop control system is rather more expensive to implement than that of its counterpart. This is down to the complexity of the functions that it can complete. The closed loop does have some form of monitoring capabilities and can in fact use sensors ad artificial intelligence well. An example of a closed loop control system would be that of the sprinkler system, where the user sets the desired time for the sprinkler to come on, but instead of juts doing this when set it could read the grass moisture in the first place and then make a split decision in whether it needed to be implemented or not.

Control Loop Operations

Normally an open loop control system comprises of the following three things,

1. Sensor – This is used to measure a value before being passed in to the process
2. Decision – Decisions usually follow information from the sensor being collected and analysed. This allows the system to remain up to date and make any appropriate changes accordingly
3. Action – This is the action of the computer or comparator altering settings

There are two differing types of systems that can be used for control and they are Analogue or digital. The two shall be explained in greater detail below.

Digital and Analogue Control Systems

The digital control system is used in the form of a micro controller to control a computer system. A digital controller is usually cascaded with the plant in a feedback system. The rest of the system can either be digital or analog. Some examples of analog systems with a digital feedback controller are:

• Aircraft
• HVAC
• Electric motors
• PID controllers
• Radar
• Robotics
• Typically, a digital controller requires:

For mixed control types such as digital and analogue systems we need a process of conversion, for example digital to analogue converters (these are called DACS). The opposite can take place, by using an analogue to digital conversion process called ADC.

All of these analogue control systems are embedded with micro controllers. These micro controllers have the power to run the systems and comprise of fairly common hardware properties. The typical hardware properties that can be found in the control systems micro controller can be seen below;

• CPU – This is the central processing unit
• EPROM – Non volatile memory source that doesn’t loose its data when the power is switched off
• RAM – This is the random access memory
• I/O – These are the input output devises

A micro processor will control the control system and process the information in order for it to function correctly. For example, if we have a sprinkler system in the garden as previously described, the inputs and outputs would need to be managed by the micro controller. If the ground was wet, it could check its EPROM (erasable program read only memory) to see what the water density was, and then the process could make a decision and process it appropriately. Below is a list of the inputs that could be put in to this system.

• Time for eater to be sprinkled
• Required water density in the ground
• Temperature of the ground
• Duration timer

Transducers (analogue systems)

Usually the forms of input found in the analogue systems are called transducers. There is a specific purpose to doing this and its main reason is down to conversion. If for example we have an electrical input and we want to display it as a picture on the page we must use a television. The transducer in this will be the Cathode ray tube (CRT) which is electromagnetic.  

When using sensors or transducers the signal must be converted in to an electrical measurement (volts). Below is a list of the possible input devises we can have within a system or a circuit.

• Geophone – Converts earth tremors in to electrical voltage
• Geiger-Muller tube used for measuring radioactivity
• Cathode ray tube (CRT) - converts electrical signals into visual form

When the electrical charge is received it then needs to be converted in to the correct signal type using one of the transducers above, however for these signals to be used correctly we need to obtain the appropriate components.

• Lamp – The input for a lamp is electricity and this is then output as a light source after being converted. This type of power source is used for many things
• Speaker – Gathers electricity from the data source and then is output as a volume so people can hear it. This also has a large application/usage basis
• Heat – The input again is electricity, this is then converted and output as heat. This is normally found on things like electric fires and car cigarette lighters
• Motor – This is input of electricity and then is output as movement energy. This type of system is used within moving doors