MECHATRONICS SENSORS

Sensors and actuators are two critical components of every closed loop control system. Such a system is also called a mechatronics system. A typical mechatronics system as shown consists of a sensing unit, a controller, and an actuating unit. A sensing unit can be as simple as a single sensor or can consist of additional components such as filters, amplifiers, modulators, and other signal conditioners. The controller accepts the information from the sensing unit, makes decisions based on the control algorithm, and outputs commands to the actuating unit. The actuating unit consists of an actuator and optionally a power supply and a coupling mechanism.

Sensors and Transducers Sensor is a device that when exposed to a physical phenomenon (temperature, displacement, force, etc.) produces a proportional output signal (electrical, mechanical, magnetic, etc.). The term sensor is used for an element which produces a signal relating to quantity being measured. The term transducer is often used synonymously with sensors. Ideally, a sensor is a device that responds to a change in the physical phenomenon. On the other hand, a transducer is a device that converts one form of energy into another form of energy. Sensors are transducers when they sense one form of energy input and output in a different form of energy. For example, a thermocouple responds to a temperature change (thermal energy) and outputs a proportional change in electromotive force (electrical energy). Therefore, a thermocouple can be called a sensor and or transducer.

Table shown listed various types of sensors that are classified by their measurement objectives.

Sensors Sensor outputs are classified into two categories Discreet(digital or logic) Proportional(analog) Discreet sensors provide single logical output(a zero or a one) do not provide information about current value of the parameter being sensed. They only decide if the parameter being sensed is above or below the preset value. Eg. The thermostat that operate the airconditioning unit will outputs to zero if the room temperature is below the thermostat’s setpoint and outputs to a logical one when the room temperature rises above the setpoint and turn on the compressor in the airconditioning unit. Propotional sensors Provide an analog output in the form of voltage, current, resistance or even a digital word containing a discreet value. Measure the value of the parameter, converts its to a signal that is propotional to the value, and outputs that value. Eg. The fluid level sending unit in the fuel tank of an automobile that send a signal to operate the fuel level gage. The sensing unit is generally a potentiometer that is operated by a float.

Proximity Sensors Proximity sensors are discreet sensors that sense when an object has come near to the sensor face. A proximity sensor gives a switching signal when objects pass at a close distance to it or within a rated sensing distanced. There are several ways of producing a switching signal including magnetic fields, acoustical(ultrasonic) reflection, high frequency interference by inductive or capacitive. Hence; four fundamental type: Inductive sensors Capacitive sensors Optical(photoelectric) Proximity sensor Ultrasonics

Inductive sensors Inductive sensors ~ operate on the priciple that inductance of a coil and the power losses in the coil vary as a metallic(conductive) object is passed near it. When a moving object passes through a high frequency magnectic field produced by the sensor, the circuit inductance will change. The change in circuit current will operate a relay to produce a switching signal. Inductive proximity sensors are only used for sensing metal(coductive) objects and will not work with non-metallic objects.

Capacitive sensors Capacitive sensors ~ the principle of operation is that an internal osillator will not oscillate until a target material is moved close to the sensor face. The target materials varies the capacitance of a capcitor in the face of the sensor that is part of the oscillator circuit. Two types of capacitive sensor: Dielectric capacitive sensors – have two side-by-side capacitor plates in the sensor face. The external target acts as the dielectric. As the target move close to the face, the change in dielectric increase the capacitance of the internal capacitor to produces aswitching signal. Reliable in detecting both metallic and non-metallic materials. However, materials must have a high density. Conductive capacitive sensors – have one capacitor plate, the target becomes the other plate. It is best if the target is an electrically conductive material( metal or water-based). Ideally suited for sensing metal and conductive liquids. Capacitive proximity sensors will sense metal objects just as inductive sensor will. But capacitive sensors are much more expensive. Potential applications for capacitive sensors: Non-contact, liquid-level sensor – be placed outside a container to sense the liquid-level inside such as for bottling operations of milk, juice or carbonated-drink. As replacements for pushbuttons and palm switches. Mounted inside liquid tank to sense the tank-fill level such as in petrol station.

Optical(photoelectric) Proximity sensor Optical(photoelectric) Proximity sensor ~ sensing method uses light, a photoelectric sensor will give a switching signal when a preset level of light is received. They are capable of sensing any objects that are opaque, operate over long distances, will sense in a vacuum, and can sense any type of material wheter metallic, conductive or porous. Fundamentally three type: Thru-beam(Separate) type: a light at one location fovused on and shinning into a photoelectric receiver at another location. An object moving between the light source and the receiver interrupts the light and trips the receiver. The receiver switches on its output. Work as long as the object to be sensed is not transparent. Have excellent maximum operating range but requires wiring on both sides of transport system(conveyor) which may cause inconvenient or impossible. Retro-Reflective(Reflex) type: the sensor contains both the emitter(light source) and receiver housed in a single unit. A reflector(mirror) is placed at another location behind the moving objects which reflect the light source back to the receiver. An object moving between the mirror and the sensor interrupts the light beam. Diffuse Reflective type: the sensor contains both the light source and the photo pickup. Normally the light beam is shinning out into space and is not reflected back. An object moving through the light beam will reflect light back into the photo sensor to give a switching signal.