In this assignment I shall be giving an outline to the different types of control sensors we get in control systems. There are many types but I shall be explaining a selected few, they are the control systems found within;
a. Linear and Angular position control
b. Pressure/force control.
c. Temperature, light, humidity control
d. Digital devices e.g. switches, relays etc
The idea of a pressure sensor is to ultimately measure and monitor the pressure of either gasses or liquids. But, what is pressure? Well, pressure ultimately is an expression used to describe the process of the force that is used when restraining a gas or a liquid from expanding. It has a certain measurement normally in the force per unit area. When the pressure is measured it can be monitored in a number of ways, but, most typically it is done with an electrical signal.
We can see many examples of pressure monitors/switches in appliances all over the world today. Pressure sensors are found in items such as cars and lifts. For example, a person walks up to a door and they wish for it to be opened on arrival, with a pressure switch located in the floor as soon as the pressure switch is activated the door will open, this form of switch will tend to require a form of displacement. Another example if where a pressure switch/monitor may be found is within a car. When the tyres reach a certain pressure the control system kicks in and then they are warned to pump them up. If we consider large industries such as factories with combustion engines then we need to bear in mind that there needs to be some form of indication when present conditions change within the mechanism, this is also where a pressure sensor will come in handy. The picture on the top left of this paragraph is similar to what may be found in an industrial placement.
A Rotary Switch
A rotary switch is used when we have a devise that needs to operate or change to two or more different states, for example, a rotary switch may be used for;
* A fan that regulates heat at different speeds
* A radio that needs to operate on different frequencies
The most common type of rotary switch is the rotary light switch, and the way in which this works is pretty straight forward.
A rotary switch is a device that has a rotating shaft connected to one terminal capable of making or breaking a connection to one or more other terminals (the rotating shaft can be seen in the picture on the left) to many other types of electrical switching mechanisms, the electromechanical rotary switch provides a desirable means to control large numbers of circuits over a wide range of currents, voltages and power requirements. Rotary switches provide electrical control for the following types of equipment;
* Medical equipment,
* Aircraft, computers,
* Industrial controls,
* Ground support equipment
Different types of rotary switches are used in a variety of electric devices. They are in the form of at least two parts and have a rotatable switching means arranged in a grouping and a rotary knob having a coupling element which can be pushed into or onto the switching means. The rotary switches are provided with a contact/sensor system and with a detent mechanism. The detent mechanism determines the number of possible switch positions. Rotary switches may feature different switch positions that can be set by rotating the switch spindle in one or another direction. Rotary switches may convert positions into binary numbers; such switches are known as coded switches. A rotary electromechanical encoder includes the overall characteristics of a rotary switch, but has additional mechanical movements.
A pyrometer comes in very useful in industrial places where measuring and controlling inputs and outputs of a computer system become far too dangerous for a human to undertake the responsibility. It is often used to determine and control the temperature; however, it can also be used for radiation purposes. The word pyrometer is derives from the Greek word for fire, “???” and the Greek word for measure, which is “meter”.
As previously said, pyrometers are optical aids and although now days there are many forms of pyrometer as the need for more specific, demanding control systems are required the most basic still works on the principle below;
On the previous page we can see how a basic optical pyrometer works, the only problem being that it can display temperatures that are high (up to around 700oC, but is inefficient for very low temperatures. As temperature inside the work place/furnace increases, so does the filament inside the pyrometer giving off a red colour, indicating the heat capacity. Using digital readouts the user can use the pyrometer to calculate the temperature of the material and also use the pyrometer to heat the material or furnace, or wherever the control system is placed to whatever they require.
A thermocouple is a very simple heat/temperature sensor and comprises of two components. One end is joined tightly and the other end is separated. It is the end that is separated that is considered to be the output of the control system, and this is the end that actually generates the voltage. This voltage is then proportional to the heat or temperature to which it is measuring. This then means that the hotter the temperature the more voltage is output. Below can be seen the two most commonly found applications of thermocouples;
* Measuring room temperature
* Monitoring the presence of a pilot light
It is the actual materials used within the control system that determines the application. There are then thermocouple types derived which can be used for differing purposes. Thermocouples are easy to use and require no batteries or fancy electronics. However, the voltage signals generated are very small so an amplifier may be required if you wish to read the temperatures into a computer
Control System Thermometers
Control system thermometers consist of a normal day to day thermometer that’s connected to an electric circuit. Then, when the heat/temperature reaches a certain level it can be indicated or displayed in a number of ways, for example if connected correctly we could have it so that the temperature, when it hits 20, displays a red light located somewhere else, and, then when it falls below the light goes off.
The idea of an inductosyn is to read the measurements of current within a system and then make extremely accurate measurements or movements based upon these readings. The principal in which it works is as follows.
An inductosyn has a metal block in which grooves are cut out. Along these grooves are continuous strips of magnetic metal, then, a current is passed from one end to the other. The sliding block has an identical strip set in to it completes the circuit when connected with the other strip. Now we have the possibility of resistance change, by sliding the sliding block from one end to the other we have a system that changes the resistance of the circuit. This then increases and decreases the current accordingly. This is then output to a display which the user can see. This relationship between current and movement can then be used to move objects very small distances with a tremendously high degree of accuracy. This is all based on the current.
A thermistor is a type of resistor that varies resistance in a computer control system according to the temperature readings. Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance when subjected to a corresponding change in temperature.
A temperature sensor application would be compared to that off the one found in a central heating system. A sensor would detect the temperature in the air, by using a thermometer and then change as desired using something like a thermistor. The thermistor would act as the thermostat. Therefore, when the thermostat is changed, we can change the output of the heating system accordingly. The thermistor is widely used and often unknown to the untrained eye.
Linear Limit Switches
The theory behind the linear limit switch is very straight forward. Basically the linear limit switch is a button, that when activated starts a process. In relation to computing it’s like an event drive operation. For the linear limit switch to be activated it must be pressed. For example, in a lift, how do we get the lift to stop going up, or, stop going down, or stop on the right floor.
It would be of my opinion that a linear limit switch is implemented, so that when the lift reaches the required floor, a switch is activated and the lift stops and the doors open. However, its not limited to lifts, the linear switch is a very common device.
In the picture on the left we can see a clear example of a linear limit switch. The operation needed to be performed is for that of the swimming pool cover, when the cover reaches the end of the pool the switch is pressed and the motor stops working, this then stops the problem of the cover falling off of the reel.