Proximity Sensors for Robots
Presented by
Pradeeshkumar P. D
Roll no: 38
S-7 Mechanical
Department of Mechanical Engineering
Rajiv Gandhi Institute of Technology, Kottayam
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Abstract
In order to perform the task in a satisfactory way the robot must have certain humanlike senses and capabilities. These include vision and hand-eye coordination, touch and hearing. This is achieved by using sensors. The types of sensors used in robotics include vision, voice, tactile, proximity etc. proximity sensors measure the relative distance between the sensors and objects in the robot's environment. This sense keeps a robot from running into things. This paper describes the various proximity sensors employed in robots.
Contents
1. Introduction ……………………………………...4
2. What is a sensor……………………………………5
3. Proximity sensors …………………………………6
4. Working principles ………………………………..7
5. Sensor input to the robot ………………………….16
6. Other applications ………………………………..17
7. Conclusion ………………………………………..18
Introduction
Robot, once a creature of science fiction, is today a reality. It is the off-shoot of the second industrial revolution.
An industrial robot is a general purpose, programmable machine which possesses certain human like characteristics. The robot’s most functional element, its arm, together with its capability to be programmed makes it ideally suited to a variety of tasks in the field of industry, military, medicine etc. The robot can be programmed to a sequence of mechanical motions and it can repeat that motion sequence over and over until reprogrammed to perform some other job.
To carry out its task, a robot must have access to information on predetermined parameters of the environment. Sensors are used to provide this information. It must be remembered that best sensory power has been bestowed by nature in the homomorphic creatures. It is the aim of engineers to attain similar perfection for robots. In order to enable the robot to perform its tasks by understanding the environment around it, sensors provide the information like
(a) recognition data – to understand the shape, size and features of the object.
(b) orientation data – the position of the object in relation to the robot arm
co-ordinates.
© physical interaction data- to understand the intensity interaction between the
end effectors and the object.
The various types of sensors used for this purpose are
1. Vision sensors – to sense the presence of an object and its position and orientation.
2. Voice sensors - to sense the sound in environment and for programming.
3. Tactile sensors - to respond to contact forces between itself and other objects within the environment.
4. Proximity sensors- to sense when one object is close to another object.
What is a sensor?
The basic function of an electronic sensor is to measure some feature of the world, such as light, sound, or pressure and convert that measurement into an electrical signal, usually a voltage or current. Typical sensors respond to stimuli by changing their resistance, changing their current flow, or changing their voltage output. The electrical output of a given sensor can easily be converted into other electrical representations.
There are two basic types of sensors: analog and digital. The two are quite different in function, in application, and in how they are used with the Robots. An analog sensor produces a continuously varying output value over its range of measurement. For example, a particular photocell might have a resistance of 1k ohm in bright light and a resistance of 300k ohm in complete darkness. Any value between these two is possible depending on the particular light level present. Digital sensors, on the other hand, have only two states, often called "on" and "off." Perhaps the simplest example of a digital sensor is the touch switch. A typical touch switch is an open circuit (infinite resistance) when it is not pressed, and a short circuit (zero resistance) when it is depressed.
Proximity sensors
Proximity sensing is the ability of a robot to tell when it is near an object, or when something is near it. This sense keeps a robot from running into things. It can also be used to measure the distance from a robot to some object. This sensing capability can be engineered by means of optical-proximity devices, eddy current proximity detectors, acoustic sensors or other devices.
Proximity sensors currently come in four flavors:
1. inductive
operates by detecting the eddy current losses when a material enters to an electromagnetic field
2. capacitive
operates by generating an electrostatic field and detecting changes in this . . . field caused when a target approaches the sensing face.
3. ultrasonic
Ultrasonic sensors detect objects by emitting bursts of high-frequency sound . waves which reflect or “echo” from a target.
4. optical
designed to be sensitive to different wavelengths of light
Working Principles
Principles of Operation for Inductive Proximity Sensors
Inductive proximity sensors are designed to operate by generating an electromagnetic field and detecting the eddy current losses generated when ferrous and nonferrous metal target objects enter the field. The sensor consists of a coil on a ferrite core, an oscillator, a trigger-signal level detector and an output circuit. As a metal object advances into the field, eddy currents are induced in the target. The result is a loss of energy and smaller amplitude of oscillation. The detector circuit then recognizes a specific change in amplitude and generates a signal which will turn the solid-state output “ON” or “OFF.”
A metal target approaching an inductive proximity sensor (below) absorbs energy generated by its oscillator. When the target is in close range, the energy drain stops the oscillator and changes the output state.
Target Correction Factors for Inductive Proximity Sensors
To determine the sensing distance for materials other than the standard mild steel, a correction factor is used. The composition of the target has a large effect on sensing distance of inductive proximity sensors. If a target constructed from one of the materials listed is used, multiply the nominal sensing distance by the correction factor listed in order to determine the nominal sensing distance for that target. Note that ferrous-selective sensors will not detect brass, aluminum or copper, while nonferrous selective sensors will not detect steel or ferrous-type stainless steels.
The correction factors listed below can be used as a general guideline.
Common materials and their specific correction factors are listed on each product specification page
(Nominal Sensing Range) x (Correction Factor) = Sensing Range.
Correction Factors
Target Material Approximate Correction Factor
Mild Steel 1.0
Stainless Steel 0.85
Brass 0.50
Aluminum 0.45
Copper 0.40
The size and shape of the target may also affect the sensing distance. The following should be used as a general guideline when correcting for the size and shape of a target:
• Flat targets are preferable
• Rounded targets may reduce the sensing distance
• Nonferrous materials usually reduce the sensing distance for all-metal sensing models
• Targets smaller than the sensing face typically reduce the sensing distance
• Targets larger than the sensing face may increase the sensing distance
• Foils may increase the sensing distance
Principles of Operation for Capacitive Proximity Sensors
Capacitive proximity sensors are designed to operate by generating an
electrostatic field and detecting changes in this field caused when a target approaches the sensing face. The sensor’s internal workings consist of a capacitive probe, an oscillator, a signal rectifier, a filter circuit and an output circuit. In the absence of a target, the oscillator is inactive. As a target approaches, it raises the capacitance of the probe system. When the capacitance reaches a specified threshold, the oscillator is activated which triggers the output circuit to change between “on” and “off.” The capacitance of the probe system is determined by the target’s size, dielectric constant and distance from the probe. The larger the size and dielectric constant of a target, the more it increases capacitance. The shorter the distance between target and probe, the more the target increases capacitance.
Shielded vs. Unshielded Capacitive Sensors
Shielded capacitive proximity sensors are best suited for sensing low dielectric constant (difficult to sense) materials due to their highly concentrated electrostatic fields. This allows them to detect targets which unshielded sensors cannot. However, this also makes them more susceptible to false triggers due to the accumulation of dirt or moisture on the sensor face. The electrostatic field of an unshielded sensor is less concentrated than that of a shielded model. This makes them well suited for detecting high dielectric constant (easy to sense) materials or for differentiating between materials with high and low constants. For the right target materials, unshielded capacitive proximity sensors have longer sensing distances than shielded versions. Unshielded models are equipped with a compensation probe which allows the sensor to ignore mist, dust, small amounts of dirt and fine droplets of oil or water accumulating on the sensor. The compensation probe also makes the sensor resistant to variations in ambient humidity. Unshielded models are therefore a better choice for dusty and/or humid environments. Unshielded capacitive sensors are also more suitable than shielded types for use with plastic sensor wells, an accessory designed for liquid level applications. The well is mounted through a hole in a tank and the sensor is slipped into the well’s receptacle. The sensor detects the liquid in the tank through the wall of the sensor well. This allows the well to serve both as a plug for the hole and a mount for the sensor.
Target Correction Factors for Capacitive Proximity Sensors
For a given target size, correction factors for capacitive sensors are determined by a property of the target material called the dielectric constant. Materials with higher dielectric constant values are easier to sense than those with lower values. A partial listing of dielectric constants for some typical industrial materials follows.
Dielectric Constants of Common Industrial Materials
Air 1.000264
Carbon Dioxide 1.000985
Cement Powder 4.0
Glass 3.7–10
Mica 5.7–6.7
Paper 1.6–2.6
Petroleum 2.0–2.2
Polyester Resin 2.8–8.1
Porcelain 4.4–7
Quartz Glass 3.7
Rubber 2.5–35
Sand 3–5
Water 80
Wood, Dry 2–7
Wood, Wet 10–30
Principles of Operation for Ultrasonic Proximity Sensors
The basic element is an electro acoustic transducer, often of the piezoelectric ceramic type. The resin layer protects the transducer against humidity, dust and other environmental factors; it also acts an acoustical impedance matcher. Since the same transducer is generally used for both transmitting and receiving, fast damping of the acoustic energy is necessary to detect objects at close range. This is accomplished by providing acoustic absorbers, and by decoupling the transducer from its housing. The housing is designed so that it produces a narrow acoustic beam for efficient energy transfer and signal directionality.
Ultrasonic sensors detect objects by emitting bursts of high-frequency sound waves which reflect or “echo” from a target. These devices sense the distance to the target by measuring the time required for the echo to return and dividing that time value by the speed of sound. This allows these devices to detect objects of any shape and material that can sufficiently reflect an ultrasonic pulse.
Analog models provide an output voltage proportional to the distance from the sensor face to the target, while digital/discrete output models change output state when this distance crosses a pre-set threshold. Because ultrasonic sensors depend on a reflected sound wave for proper operation, the correction factors and target requirements used for inductive proximity sensors do not apply.
Switching Frequency
The switching frequency is the maximum speed at which a sensor will deliver discrete individual pulses as the target enters and leaves the sensing field. This value is always dependent on target size, distance from sensing face, speed of target and switch type. This indicates the maximum possible number of switching operations per second.
Optical Proximity sensors
Optical proximity sensors are similar to ultrasonic sensors in the sense that they detect proximity of an object by its influence on a propagating wave as it travels from a transmitter to receiver. This sensor consist of a solid state LED which acts as a transmitter of infrared light, and a solid state photo diode which acts as a receiver. The cons of light formed by focusing the source and de4tectoron the same plane intersect in a long, pencil like volume. This volume defines the field of operation of a sensor since a reflective surface which intersects the volume is illuminated by the source and simultaneously seen by the receiver. It is possible to calibrate the intensity of these readings as a function of distance for known object orientations and reflective charecterstic, the typical application shown in figure, is in a mode where a binary signal is generated when the received light intensity exceeds a threshold value.
Sensor Inputs to the Robot
The Robot control system contains input ports for both analog and digital sensors. While both types of ports are sensitive to voltage, each type interprets the input voltage differently and provides different data to the microprocessor. The analog ports measure the voltage and convert it to a number between 0 and 255, corresponding to input voltage levels between 0 and 5 Volts. The conversion scale is linear, so a voltage of 2.5 volts would generate an output value of 127 or 128. The digital ports, however, convert an input voltage to just two output values, Zero and One. If the voltage on a digital port is less than 2.5 Volts, the output will be 0, while if the input is greater than 2.5 Volts, the output will be 1. Thus, the conversion is very nonlinear.
Other applications
• Grinding Machine
• Plating line
• Machine tools
• Wood industry
• Petroleum industry-Valve position
• Conveyor belts
• Food industry
• Food processing
• Printing
• Stainless steel sheet welder
• Online parts soldering
• Railroad Yard position sensing
Conclusion
With the help of sensors, automation in robots, its navigation, operational capabilities like packing, assembling, loading etc, and its integration with human workers have been achieved. Sensors present a wide range of applications in future automation technologies. They help in exploration of space, defence applications, medical applications, research fields etc, there by acquiring complete automation in all respects. Thus the future technological advancements greatly depend on sensors.
References
• Robot Technology……………………………..James. G. Kerman
Thomas Delmar Publications
• Robotics……………………K.S.Fu, R. L. Gonzalez, C. S. G. Lee
Mc-Grow Hill international
• CAD/CAM…………………………………..Groover, Zemmarick
Pearson Education
• sensorsmag.com
• robottechnology/sensors/proximity.com
