Sensors for Distance Measurements

There are a wide range of sensors available for distance and position measurement,
for applications requiring continuous position measurement and control.
The output of these sensors is an analog signal (voltage or current) which is related
to the distance from the sensor and the measuring object.

Linear Potentiometer




Inductive Distance Sensor




Optical Sensor



Ultrasonic


Linear Variable Differential


Varable Capacitance Sensor

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Inductive Analog Distance Sensor

The output voltage signal depends on the electrical conductivity of the metal to be detected and the distance between the sensor and the object.

Potentiometer
Using a potentiometer is the simplest form of analog position sensing. The function of a linear potentiometer is based on the voltage divider principle.
Moving a wiper/contact will change the value of the resistance.
Moving the wiper upward will increase R2 and decrease R1 because the value of a wired-resistor is proportional to the length of the wire , the total resistance (R1+R2) will not change.
This is the ideal linear relationship, V2 is proportional to R2 which is proportional to the distance moved. However if a considerable high current is drained from the sensor, it will cause some errors. For this reason, an amplifier is attached to the output of the potentiometer when it is used in a practical measurement or a control system.

Ultrasonic Sensors
Sound frequency which is above the limit of human hearing (>20KHZ) is described as ultrasound.
Ultrasonic transceivers are made up of two parts:
Transmitter: which can be used to generate (transmit) ultrasound waves.
Receiver: receives ultrasonic sound from the transmitter and measures a signal in volts which is proportional to the distance between the two devices.
The majority of ultrasonic receivers determine the timing between the sound generated signals and the echo received signals and since the sound speed is known,the distance can be calculated.

Other devices use the same principle of sending a signal and measuring
time of flight or time of receiving reflection, but they send different type of signals.
• RADAR (radio detection and ranging) uses radio waves.
• SONAR: uses sound waves.
• LIDAR uses LASER light.

Advantages of ultrasonic sensors
• Ultrasonic waves can reflect easily from structures.
• Ultrasonic sensors are cheap, small and simple.

Optical Analog Sensors
There are several methods of optically sensing distance, such as laser interferometers and diffuse sensors with or without fiber optics.

Variable capacitance

The capacitance level sensor measuring principle, is based on the physical
properties of a capacitor formed by the sensor (probe) and the tank wall, the
capacitance is affected by the dielectric value of liquid in the tank.
Distance Change --> Capacitance change --> Frequency Change --> Voltage Change.
The capacitance between two metal plates changes if the distance between these
two plates changes.

In order to convert the capacitance to a voltage signal, it is necessary to connect
the capacitor to a circuit called oscillator.
The oscillator is a circuit which produces an output signal of some frequency. The
frequency of the oscillator can be changed by changing the value the capacitance.

A circuit called frequency discriminator converts the changes in frequency
into changes in voltage.
Advantages of capacitive sensors
• High resolution, which means very small displacement can be sensed.
• Cheap and small size.
Capacitive sensors are not good choice in these conditions:
• Dusty or wet environment.
• If the distance between the sensor and the object is large.
Application
Capacitive sensors can be used to measure the changes in liquid level in a tank.