2.  Potentiometers are a convenient method of converting the displacement in a sensor to an electrical variable.  The wiper or slider arm of a linear potentiometer can be mechanically connected to the moving section of a sensor.  Where rotation is involved, a single or multiturn (up to 10 turns) rotational type of potentiometer can be used.  For stability, wire-wound devices should be used, but in environmentally- unfriendly conditions, lifetime of the potentiometer may be limited by dirt

3.  Linear variable differential transformers (LVDT) are devices that are used for measuring small distances and are an alternative to the potentiometer.  These devices are not as cost- effective as potentiometers, but have the advantage of being noncontact.  The outputs are electrically isolated, accurate, and have better longevity than potentiometers.  A variety of LVDTs is  available, with linear ranges

4.  Hall effect sensors detect changes in magnetic field strength and are used as a close proximity protector.  The Hall effect occurs in semiconductor devices.  Without a magnetic field the current flows directly through the semiconductor plate and the Hall voltage is zero.  Under the influence of a magnet field, the current path in the semiconductor plate becomes curved, giving a Hall voltage between the sides adjacent to the input/output current.

5.  There are numerous applications for ultrasonic distance sensors in industrial control. Ultrasonic distance sensors are used in all industries for measuring the distance to or size of material objects.  Ultrasonic distance sensors measure the distance or presence of target objects by sending a pulsed ultrasound wave at the object and then measuring the time for the sound echo to return. Knowing the speed of sound, the sensor can determine the distance of the object.

6.  The armature of dc tachogenerators is kept in permanent magnetic field.  The armature of the tachometer is coupled to the machine whose speed is to be measured.  When the shaft of the machine revolves, armature of the tachometer revolves in the magnetic field producing an emf proportional to the speed.  The polarity of the output voltage indicates the direction of rotation

7.  The opt electric tachometer is a non contact type tachometer which also called digital tachometer.  It uses a principal of retro refractive scanning, in this scanning the light source and photo sensor are placed in common housing.  Light source sends a light towards a retro refractive target from which it gets reflected.  In photo electric tachometers, the photor detector detects the reflected ligh and produce electrical pulse.