17-01-2013, 04:33 PM
BASIC SENSOR THEORY & OPERATION
[attachment=47406]
Objectives of ST120
With the fast paced industrial world looking for ways to manufacture products better, faster and more economically, it is important to be knowledgeable of sensor technologies.
This class is designed to give you a fundamental understanding and working knowledge of different sensor technologies. We will achieve this through the study of sensor technologies, terminology explanations, lab exercises and application examples.
What Is a Sensor
According to Merriam-Webster's dictionary, it’s a device that responds to a physical stimulus and transmits a resulting impulse.
Advantages of Electronic Sensors
Non-contacting
Wear and tear is kept to a minimum
A must for fragile equipment
Odd shapes (not a problem)
Reliable
Last a long time (Solid State)
A mechanical switch has a definite life span
Few maintenance problems
Longer/better reliability
Detector Circuit
The Detector Circuit converts the
amplitude of the oscillating signal to a
DC RMS level from which the ON and
OFF thresholds are used to trigger the
output circuit.
Shielded and Unshielded
“Shielded” and “Unshielded” are two terms used to describe these sensors. The basic difference in these two types arrive from their mechanical construction.
In general, these two construction styles yield slightly different sensing performances. In summary, the unshielded style will provide approximately twice the sensing distances with an inherent drawback of specific mounting and installations requirements.
Capacitive Proximity Sensors
A Capacitive Proximity sensor is similar to an inductive sensor in that it has a Oscillator, Detector and Output circuit. It differs in that it uses a plate shaped Electrode instead of a wire-wound core. In operation, it forms an electrostatic capacitive field formed between it and the the sensors ground. (In practice the supply line is in effect the ground.)
Dielectric Constants
The measure, or unit of Dielectric Constant, is the ability of a material to concentrate electrical flux. Its numerical value is specified as the ratio of flux in the material verses the flux in air or vacuum. The dielectric constant of air or vacuum is 1 – since it is the reference.
Possible Problems
Even though color and shape do not affect this sensor type, the sheen or glossiness of the item may have an adverse affect. If the object is extremely shiny, or highly reflective, it could reflect more light back to the receiver than the reflector does on its own. It this case the object could/would pass by undetected. As a commonly found issue, there is a solution for these applications.
Diffuse Reflective – Lab Exercise
Hook up the sensor and verify its operation by placing your hand in front of the sensor to activate the output.
Set the sensitivity of the sensor to its maximum setting. Position a piece of the colored paper in front of the sensor and slowly back it away to find the maximum sensing distance. Repeat this process for each test piece and document your results.
Modulation
Modulating an LED means turning it on and off at a set frequency. This is generally done at high frequency with short bursts of voltage. By applying power in this manor the circuit is able to generate a very high intensity light at a certain spectrum. Because the LED is not on continuously it does not suffer from the adverse affects of heat.
[attachment=47406]
Objectives of ST120
With the fast paced industrial world looking for ways to manufacture products better, faster and more economically, it is important to be knowledgeable of sensor technologies.
This class is designed to give you a fundamental understanding and working knowledge of different sensor technologies. We will achieve this through the study of sensor technologies, terminology explanations, lab exercises and application examples.
What Is a Sensor
According to Merriam-Webster's dictionary, it’s a device that responds to a physical stimulus and transmits a resulting impulse.
Advantages of Electronic Sensors
Non-contacting
Wear and tear is kept to a minimum
A must for fragile equipment
Odd shapes (not a problem)
Reliable
Last a long time (Solid State)
A mechanical switch has a definite life span
Few maintenance problems
Longer/better reliability
Detector Circuit
The Detector Circuit converts the
amplitude of the oscillating signal to a
DC RMS level from which the ON and
OFF thresholds are used to trigger the
output circuit.
Shielded and Unshielded
“Shielded” and “Unshielded” are two terms used to describe these sensors. The basic difference in these two types arrive from their mechanical construction.
In general, these two construction styles yield slightly different sensing performances. In summary, the unshielded style will provide approximately twice the sensing distances with an inherent drawback of specific mounting and installations requirements.
Capacitive Proximity Sensors
A Capacitive Proximity sensor is similar to an inductive sensor in that it has a Oscillator, Detector and Output circuit. It differs in that it uses a plate shaped Electrode instead of a wire-wound core. In operation, it forms an electrostatic capacitive field formed between it and the the sensors ground. (In practice the supply line is in effect the ground.)
Dielectric Constants
The measure, or unit of Dielectric Constant, is the ability of a material to concentrate electrical flux. Its numerical value is specified as the ratio of flux in the material verses the flux in air or vacuum. The dielectric constant of air or vacuum is 1 – since it is the reference.
Possible Problems
Even though color and shape do not affect this sensor type, the sheen or glossiness of the item may have an adverse affect. If the object is extremely shiny, or highly reflective, it could reflect more light back to the receiver than the reflector does on its own. It this case the object could/would pass by undetected. As a commonly found issue, there is a solution for these applications.
Diffuse Reflective – Lab Exercise
Hook up the sensor and verify its operation by placing your hand in front of the sensor to activate the output.
Set the sensitivity of the sensor to its maximum setting. Position a piece of the colored paper in front of the sensor and slowly back it away to find the maximum sensing distance. Repeat this process for each test piece and document your results.
Modulation
Modulating an LED means turning it on and off at a set frequency. This is generally done at high frequency with short bursts of voltage. By applying power in this manor the circuit is able to generate a very high intensity light at a certain spectrum. Because the LED is not on continuously it does not suffer from the adverse affects of heat.