In a flexible manufacturing system, it is of vital importance that the devices that act as integrating elements of the same, offer a level of security that allows to guarantee the complete development of the process in execution.
In industries such as food, refreshment, manufacturing, commercial, extractive, in the same way in places such as museums, banks, among others.
In this sense, the inclusion of some sensors, in the robot manipulators, which are part of the Flexible Manufacturing System in the CAP Process Automation Center, is favorable.
As we know, a sensor is a device capable of detecting different types of materials, in order to send a signal and allow a process to continue, or to detect a theft; depending on the case that this is.
Within the selection of a sensor, different factors must be considered, such as: the shape of the housing, operating distance, electrical data and connections.
Similarly, there are other devices called transducers, which are elements that change signals, for the best measurement of variables in a certain phenomenon.
What is a Transducer?
A transducer is a device that transforms one type of physical variable (for example, force, pressure, temperature, speed, etc.) into another.
A sensor is a transducer that is used to measure a physical variable of interest. Some of the most frequently used sensors and transducers are strain gauges (used to measure force and pressure), thermocouples (temperatures), and speedometers (speed).
Any sensor or transducer needs this calibration to be useful as measurement devices. Calibration is the procedure by which the relationship between the measured variable and the converted output signal is established.
Transducers and sensors can be classified into two basic types, depending on the shape of the converted signal. The two types are:
- Analog transducers Digital transducers
The analog transducers provide a continuous analog signal, such as voltage or electric current. This signal can be taken as the value of the physical variable that is measured.
The digital transducers produce a digital output signal in the form of a set of status bits in parallel or in a series of pulses that can be counted. In one way or another, digital signals represent the value of the measured variable. Digital transducers often offer the advantage of being more compatible with digital computers than analog sensors in automation and process control.
Desirable characteristics of transducers
Accuracy
The accuracy of the measurement should be as high as possible. Accuracy means that the true value of the variable can be detected without positive or negative systematic errors in the measurement. Over various measurements of the variable, the average error between the true value and the detected value will tend to be zero.
Precision
The precision of the measurement should be as high as possible. Precision means that there is or is not a small random variation in the measurement of the variable. The dispersion in the values of a series of measurements will be minimal.
Operating range
The sensor must have a wide operating range and must be accurate and precise throughout the range.
Response speed
The transducer must be able to respond to changes in the detected variable in a minimum time. Ideally, an instant response would be.
Calibration
The sensor should be easy to calibrate. The time and procedures required to carry out the calibration process should be minimal. Also, the sensor should not need frequent recalibration. The term drift is frequently applied to indicate the gradual loss of sensor accuracy that occurs with time and use, necessitating recalibration.
Reliability
The sensor must have high reliability. It should not be subject to frequent failure during operation.
Sensors selection in automation
The selection is based on the decision on which is the most suitable sensor. This depends on the material of the object which is to be detected.
If the object is metallic, an inductive sensor is required. If the object is made of plastic, paper, or if it is liquid (based on oil or water), grainy or powder, a capacitive sensor is required. If the object can carry a magnet, a magnetic sensor is appropriate.
To choose a suitable sensor, follow these four steps:
- CASE FORM OPERATING DISTANCE ELECTRONIC DATA AND CONNECTIONS GENERAL
Case Shape
- -MATERIAL OF THE HOUSING
Materials available from standard housings.)
V2A stainless steel, Brass, nickel plated or Teflon coated.
Crastin, Ryton.
Crastin is a polybutylene terephthalate (PBT), which is reinforced with fiberglass. It is particularly resistant to shape changes, resistant to abrasion, heat and cold, and resists hydrocarbons (eg tricholo-ethylene), acids (eg 28% sulfuric acids), sea water, hot water 70 ° C etc.
For temperatures up to 150 ° C, Pepperl + Fuchs GmbH uses Ryton, a crystalline polyphenylene sulfide (PS), which maintains stability up to 200 ° C. The electronic components are immersed in an epoxy resin under vacuum molded resin.
CABLE MATERIAL.
- PVC (polyvinyl chloride). Standard grade of the electrical industry conditionally resistant to all oils and greases, solvents and does not weaken, with high resistance to abrasion.
- PUR (polyurethane). Resistant to all oils and greases, solvents, and with high resistance to abrasion.
- SILICONE. Ideal for high or low temperatures (-50 ° C to + 180 '”c) moderately resistant to corrosion, and to all oils, greases and solvents.
To avoid cable breakage, PVC and PUR cables must not be moved or manipulated in temperatures below -5 ° C.
Operating distance
It is the most important characteristic distance of a sensor. It basically depends on the diameter of the sensor (coil or capacitor). An additional influence has the dimensions and composition of the material, as well as the ambient temperature. With magnetic sensors, alignment and field strength must also be taken into account.
The definition of the operating distance, according to EN 60947-5-2, is valid for all types of sensors, with the exception of grooved and annular types. There are two possibilities to operate with a sensor:
By axial approximation
By radial approximation
The following definitions are valid for axial operation only.
USABLE OPERATING DISTANCE Your
The operating distance of an individual sensor, measured at an ambient temperature between -25 ° C and + 70 ° C and supplied with a voltage between 85% and 110% of the calculated operating voltage:
0.9 Sr <Su <1.1 Sr
Sensor Classification
Internal: information about the robot itself
- Position (potentiometers, inductosyn, optical…) Speed (electrical, optical…) Acceleration
External: information about what surrounds the robot
- Proximity (light reflection, laser, ultrasound…) Touch (rods, pressure, polymers…) Force (current in motors, deflection…) Vision (tube cameras)
Other classifications: simple / complex, assets / liabilities
According to the type of physical magnitude to be detected, we can establish the following classification:
- Linear or angular position. Displacement or deformation. Linear or angular speed. Acceleration. Force and torque. Pressure. Flow rate. Temperature. Presence or proximity. Touch. Light intensity. Artificial vision systems.
Another type of classification is to differentiate between active and passive sensors. Passive sensors require a power supply to perform their function, while active sensors generate the signal without the need for external power.
External sensors are the elements that allow the robot to interact with its environment in a flexible way. Although many of today's robots (especially those in industries) work in a pre-programmed way, the use of external sensors as support in the execution of tasks is increasingly widespread. External sensors give the robot greater independence from the specific environment in which it moves, which translates into a higher degree of "intelligence".
There are three types of external sensors that are generally used by robots for a wide variety of tasks. These are touch sensors, proximity or presence sensors, and range sensors.
Touch sensors are devices that indicate the contact of a solid object with themselves. They are usually used at the ends of the robot arms (grippers) to control the handling of objects. In turn, they can be divided into two types: contact and force.
Contact Sensors
Contact sensors simply tell us whether or not there has been contact with an object, regardless of the magnitude of the contact force. They are usually simple devices whose use is very varied.
They can be placed on the grippers of the robot arms to determine when an object has been picked up, they can be part of inspection probes to determine the dimensions of objects, or they can even be placed outside the grippers to probe an environment.
These sensors are usually limit switches or microswitches, which are simple electrical devices that change state when contacted.
Force sensors
The force sensors determine, in addition to whether there has been contact with an object like the previous ones, the magnitude of the force with which said contact has occurred. This ability is very useful as it will allow the robot to manipulate objects of different sizes and even place them in very precise places. To detect the force with which an object has been contacted there are various techniques
Force sensing wrist.
It consists of a load cell that is located between the wrist and the arm grippers. Its objective is to provide information on the three components of the force (Fx, Fy, Fz) and on its three moments in speed with which the arm moves is considerable, it is difficult to be able to control its movements fast enough so that it does not provoke no catastrophe (such as the crushing of an object).
Joint detection
This technique is based on the measurement of the torque of the joint. The measurement of this torque can be simple, since it is proportional to the current that flows through the motor that causes said torque.
Although this technique may seem simple and reliable, it has a major problem. The torque measurement is performed on the arm joints and not on the end effector (the gripper) as would be desirable, so said torque not only reflects the force that will be exerted on the gripper, but also the force used to move the joint.
Touch array sensors
It is a special type of force sensor as it is actually made up of an array of small force sensors. Due to this characteristic, they also allow to recognize shapes in the objects that are being manipulated. These types of devices are usually mounted on the grippers of the robot arms.
Each of the force sensors that make up the matrix is usually an elastomeric pad, which when compressed changes its electrical resistance in proportion to the applied force. Measuring that resistance is when we can obtain the information about the force. The resolution of this type of sensors will logically be given by the dimensions of the sensor array.
A very important factor that can be a problem when designing this type of sensor is the degree of wear on the contact surface.
Shielded and unshielded sensors
Shielded sensors.- They include a metal band that surrounds the ferrite core and the coil. This helps direct the electromagnetic field to the front of the sensor.
Shielded sensor.
Unshielded sensors.- They do not have a metallic band; however, they have a greater operating distance and have the ability to sense laterally.
Sensing range considerations (operating distance)
The operating distance (S) is basically a function of the diameter of the sensor coil. The maximum distance is reached with the use of a standard part. When using a proximity sensor, the part to be sensed must be within the assured range.
- Standard part: A 1mm thick square part (made of tempered steel) is used to determine the following operating tolerances: The length and width of the square is equal to either the diameter of the circumscribed circle on the sensing face (on the surface active), or 3 times the estimated operating distance (Sn), whichever is greater Operating distance (S) Estimated operating distance (Sn): Does not consider variations due to voltage or temperature Effective operating distance (Sr): 0.9 Sn <Sr <1.1 Sn Usable operating distance (Su): 0.81 Sn <Sr <1.21 Sn Insured operating range (Sa): 0 <Sa <0.81 Sn
Proximity sensors
They are devices that detect signals to act in a certain process or operation, having the following characteristics:
- They are devices that act by induction when an object is brought closer to them. They do not require direct contact with the material to be sensed. They are the most common and used in the industry. They are encapsulated in plastic to provide greater ease of assembly and protection against possible impacts.
APPLICATIONS:
- Control of conveyor belts, High speed control, Movement detection, Piece counting, Sensing of openings in security and alarm systems Control systems such as limit switches. (PLC's) Optical sensor.
Characteristics.
- Small in construction, but robust Longer operating distance Detect any material Long service life
Applications
- Barrier-type protection system in access grids in a hydraulic press, where operator safety is a priority Detection of parts traveling at very high speed in a production line (electronic industry or bottling plants) Detection of parts inside of tweezers, in this case the sensor is made up of an infrared emitter and receiver located opposite each other, in such a way that the interruption of the emitted signal is an indicator of the presence of an object inside the tweezers.
Inductive sensors
- It consists of a device made up of: A coil and a ferrite core An oscillator A detector circuit (switching stage) A solid state output.
The oscillator creates a field of high frequency of oscillation by the electromagnetic effect produced by the coil in front of the sensor centered with respect to the axis of the coil. Thus, the oscillator draws a known current. The ferrite core concentrates and directs the electromagnetic field at the front, becoming the active surface of the sensor.
When a metallic object interacts with the high frequency field, EDDY currents are induced in the active surface. This causes a decrease in the lines of force in the oscillator circuit and, consequently, the amplitude of oscillation decreases. The sensing circuit recognizes a specific change in amplitude and generates a signal, which switches (pilots) the solid state output "ON" or "OFF". When the metal object is removed from the senate area, the oscillator generates the field, allowing the sensor to return to its normal state.
Capacitive sensor
A capacitive sensor is suitable for the case of wanting to detect a non-metallic object. For metallic objects it is more suitable to choose an inductive sensor.
For distances greater than 40 mm, the use of this type of sensors is totally inappropriate, being preferable a detection with optical or barrier sensors.
Capacitive sensors work in a similar way to a simple capacitor.
The metal foil at the end of the sensor is electrically connected to an oscillator.
The object that is detected functions as a second sheet. When power is applied to the sensor the oscillator senses the external capacitance between the target and the inner foil.
Capacitive sensors work in the opposite way to inductive ones, as the target approaches the capacitive sensor the oscillations increase until reaching a limit level which activates the trigger circuit which in turn changes the state of the switch.
Typical applications
- Detection of practically any material Control and verification of level, tanks, tanks, buckets Distance measurement Control of the input-output loop of machines Tension-release control, expansion
Ultrasonic Sensors
There is a versatile line of sensors including 30mm sheet metal and plastic housings in two rectangular housing styles
It is narrow analog and widely yields to discrete devices, multiple sensor positioning sensing the environmental features of the robot environment.
The transparent whites
Ultrasonic sensors are the best choice for transparent targets. They can uncover a sheet of clear plastic film as easily as a wooden pallet.
The dusty environments
Ultrasonic sensors do not need the clean environment, needed by photoelectric sensors. The resin sealed piezoelectric transducer performs well in many dusty applications.
Uneven targets
Many applications, such as the discovery of inclined level or uneven materials. This is no problem for the ultrasonic sensor. This sensor offers 60 ° sonic cone angle. The wide cone angle allows a designated tilt of + -15 °.
Control Speed with Analog Performance.
The important feature is directly the analog current and voltage proportional to the designated distance. Analog performance for the fabric industry that processes applications such as lap tension and roll diameter for carpet, paper, textile or plastic.
The noise suppression circuitry.
Ultrasonic sensors are not affected by glass or metal signals, nor by motor-generated vibrations induced through the line.
Operating in difficult environments.
The sealed sensors, withstand temperatures from -25 ° to 70 ° C (-13 ° to 158 ° F) thus having a sensor ready for demanding applications.
White suppression in the background and foreground.
Ultrasonic sensors are provided with a potentiometer to adjust the far limit of the calibration window, most versions also offer a second potentiometer to adjust the near limit. This allows for the suppression of whites in the foreground and background.
The indicators.
All ultrasonic sensors have LEDs that indicate performance status. The designated presence on the sonic cone is also indicated.
Typical applications
- Control and verification of level, tanks, tanks Distance measurement Control of the input-output loop of machines Tension-release control
Author Ing. Iván Escalona
Logistics Consultant, Mobile Phone: 044 55 18 25 40 61 (Mexico)
Industrial Engineer
[email protected], [email protected]
Note: If you want to add a comment or if you have any questions or complaints about any published work (s), you can write to me at the emails indicated, indicating which work was the one you reviewed by writing the title of the work (s), also where are you from since you work (if you study, or work) Being specific, also the age, if you do not indicate them in the mail, I will delete the mail and I will not be able to help you, thank you.
- University Studies: Interdisciplinary Professional Unit of Engineering and Social and Administrative Sciences (UPIICSA) of the National Polytechnic Institute (IPN)
- Patoyac School Center, (Incorporated to UNAM)
Origin: Mexico
Bibliography
- Industrial Automation Techniques.
José J. Horta Santos.
Edit. Limusa
Mexico, 1982.
47-102 pp.
- Robotics: An Introduction
Mc Cloy
1st. Edition.
Edit. Limusa
Mexico, 1993
22-27 pp.
- www.yahoo.comwww.google.comwww.sensors.comwww.elhijodeputa.comwww.monografias.comwww.upiicsa.ipn.mx
Recommended reading
Introduction to Control System Technology (7th Edition),
Robert N., PE Bateson, Robert N. Bateson, Prentice Hall; 7th edition, 706 Pp.
Conclusions
The sensors allow the robot to react autonomously to the presence of faults, before an eventual general blockage of the system or the execution of inconsistent tasks according to the planning carried out. They also offer the possibility of using the robot manipulator to carry out tasks under partially predetermined conditions, in which its decision-making capacity replaces the rigorous programming of each of its movements.
Regardless of the type of sensor, the fundamental part for its selection is to carefully attend to the application, since its correct selection depends to a great extent. The environment is another important variable, since it can hinder the sensing medium in a certain range, in addition to operating problems. It is important to heed the manufacturer's recommendations for use and application, in particular due to the fact that some sensors are expensive and an error in their installation or handling can cause an additional investment when buying them again.
Undoubtedly, the use of sensors and transducers, allow us improvements in a process that is being carried out, translated into: accuracy, safety, reduction of times, few failures, etc.
Thus, in the present work the different types of sensors that exist were revealed, as well as their characteristics depending on each manufacturer.
In some sensors, the generation of a signal is determined by the type of material being handled and the distance, in the same way other factors may intervene, such as color or shape. For a capacitive sensor, the distances to detect a metallic material are usually very small, in the case of non-metallic materials, their detection is not possible.
On the other hand, for a capacitive sensor, the detection distances are larger than the inductive sensor, adding to this the ability to detect materials of all kinds.
With reference to an optical sensor, it has to be detected at much greater distances than the previous sensor and in the same way it detects various types of metallic and non-metallic materials.
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