Robotics Sensors Lecture 3 PDF

Robotics Mahmoud Abdelaal Mahmoud Abdelaal Robotics_@_AI-BU_2022 1  Lecture # 3 Sensors for Robotic Systems I Mahmoud Abdelaal Robotics_@_AI-BU_2022 2 Sensors for Robotic Systems Definition The term sensor is used for an input device that provides a usable output in respo...

Robotics Mahmoud Abdelaal Mahmoud Abdelaal Robotics_@_AI-BU_2022 1  Lecture # 3 Sensors for Robotic Systems I Mahmoud Abdelaal Robotics_@_AI-BU_2022 2 Sensors for Robotic Systems Definition The term sensor is used for an input device that provides a usable output in response to a specified physical input. For example, a thermocouple is a sensor which converts a temperature difference into an electrical output. The General Question Given the sensory reading what was the world like? Example: Skin Sensors Simple ➔ complex Mahmoud Abdelaal Robotics_@_AI-BU_2022 3 Sensors for Robotic Systems Types of Sensors Contact: switch Distance: Ultrasound, radar, infra red Light level: photo cells, cameras Sound level: microphone Strain: strain gauge Rotation: encoders Smell: chemical Temperature: thermal, infra red Inclination: inclinometers, gyroscopes Pressure: pressure gauges Altitude: altimeters Mahmoud Abdelaal Robotics_@_AI-BU_2022 4 Sensors for Robotic Systems Levels of Processing A switch: open = 0 volts Closed = 5 volts A digital scale: Microphone: Camera: Mahmoud Abdelaal Robotics_@_AI-BU_2022 5 Sensors for Robotic Systems Ohm’s Law V= IR V =voltage (volts) I =current (Amps) R = resistance (Ohms) Mahmoud Abdelaal Robotics_@_AI-BU_2022 6  Mechanical switches / Limit Switches Mahmoud Abdelaal Robotics_@_AI-BU_2022 7 Mechanical switches / Limit Switches Mechanical switches A mechanical switch generates an on-off signal or signals as a result of some mechanical input causing the switch to open or close. Ex. the presence of a workpiece on a machining table The absence of the workpiece is indicated by the switch being open and its presence by it being closed. The 1 level might correspond to a 24 V DC input, the 0 to a 0 V input. Mahmoud Abdelaal Robotics_@_AI-BU_2022 8 Mechanical switches / Limit Switches Mechanical switches The term limit switch is used for a switch which is used to detect the presence or passage of a moving part. It can be actuated by a cam, roller or lever. Figure: Limit switches actuated by: (a) lever, (b) roller, (c) cam Mahmoud Abdelaal Robotics_@_AI-BU_2022 9 Mechanical switches / Limit Switches Limit Switches Figure: Typical form of a limit switch with (a) an interior view of switch body, and (b) a switch body equipped with an actuator head. Mahmoud Abdelaal Robotics_@_AI-BU_2022 10 Mechanical switches / Limit Switches Limit Switches The only difference being the type of sensor head. Figure: Various types of actuator head for transmitting the external force to the changeover mechanism suitably and thereby engaging the movable contact’s (a) push plunger, (b) roller plunger, (c) roller lever plunger (motion from the front or side), (d) roller lever plunger (motion from the back or side), (e) fork type, (f) adjustable roller arm, (g) adjustable rod, and (h) spring flexible rod. Mahmoud Abdelaal Robotics_@_AI-BU_2022 11 Mechanical switches / Limit Switches Limit Switches Advantages Limit switches are cheaper in comparison to proximity or photoelectric switches There is also a wide range of limit switches regarding their size. Mahmoud Abdelaal Robotics_@_AI-BU_2022 12  Proximity Switches Mahmoud Abdelaal Robotics_@_AI-BU_2022 13 Proximity Switches Proximity Switches There are three basic types of proximity switches with respect to their operating principles: inductive, capacitive, and magnetic. The operation of inductive proximity switches is based on the variation of the magnetic inductance. The capacitive proximity switches base their operation on the variation of capacitance and the magnetic proximity switches on the variation of the magnetic flow. The most widely-used proximity switches in industrial applications are the inductive and the capacitive ones. Figure: Typical external view of a proximity sensor. Mahmoud Abdelaal Robotics_@_AI-BU_2022 14 Proximity Switches Proximity Switches Proximity switches are used to detect the presence of an item without making contact with it. There are a number of forms of such switches, some being only suitable for metallic objects. The voltage amplitude is thus a measure of the proximity of metallic objects. The voltage can be used to activate an electronic switch circuit, basically a transistor which has its output switched from low to high by the voltage change, and so give an on-off device. The range over which such objects can be detected is typically about 0.5 to 20 mm. Mahmoud Abdelaal Robotics_@_AI-BU_2022 15  Proximity Switches Capacitive Proximity Switch Mahmoud Abdelaal Robotics_@_AI-BU_2022 16 Proximity Switches Capacitive Proximity Switch A proximity switch that can be used with metallic and non-metallic objects is the capacitive proximity switch. The capacitance of a pair of plates separated by some distance depends on the separation, the smaller the separation the higher the capacitance. The sensor of the capacitive proximity switch is just one of the plates of the capacitor, the other plate being the metal object whose proximity is to be detected. Mahmoud Abdelaal Robotics_@_AI-BU_2022 17 Proximity Switches Capacitive Proximity Switch The sensor can also be used to detect non-metallic objects since the capacitance of a capacitor depends on the dielectric between its plates. In this case the plates are the sensor and the earth and the non-metallic object is the dielectric. The change in capacitance can be used to activate an electronic switch circuit and so give an on-off device. Capacitive proximity switches can be used to detect objects when they are typically between 4 and 60 mm from the sensor head. Mahmoud Abdelaal Robotics_@_AI-BU_2022 18 Proximity Switches Proximity Switches Figure: Internal elements of a capacitive proximity sensor and object- affecting dielectric constant. Mahmoud Abdelaal Robotics_@_AI-BU_2022 19  Proximity Switches Capacitive Proximity Switch Mahmoud Abdelaal Robotics_@_AI-BU_2022 20 Proximity Switches Inductive Proximity Switch Another type, the inductive proximity switch, consists of a coil wound round a ferrous metallic core. When one end of this core is placed near to a ferrous metal object there is effectively a change in the amount of metallic core associated with the coil and so a change in its inductance. This change in inductance can be monitored using a resonant circuit, the presence of the ferrous metal object thus changing the current in that circuit. The current can be used to activate an electronic switch circuit and so give an on-off device. The range over which such objects can be detected is typically about 2 to 15 mm. Mahmoud Abdelaal Robotics_@_AI-BU_2022 21 Proximity Switches Proximity Switches Figure: Internal elements of an inductive proximity sensor and object- electromagnetic field interaction. Mahmoud Abdelaal Robotics_@_AI-BU_2022 22 Proximity Switches Inductive Proximity Switch Figure: Object movement in front of the sensing area of a proximity sensor perpendicular to the sensor’s longitudinal axis (a), and parallel to the sensor’s longitudinal axis (b). Mahmoud Abdelaal Robotics_@_AI-BU_2022 23 Proximity Switches Proximity Switches The main types of proximity switches, with respect to their external form and shape, are the cylindrical, rectangular, and the slot ones. Figure: Basic exterior forms of proximity sensors for various applications. Mahmoud Abdelaal Robotics_@_AI-BU_2022 24 Proximity Switches Proximity Switches Electric connection of proximity sensors: Inductive and capacitive proximity switches can be connected to an AC or DC power supply source in a series or separately with a load, the so-called two-wire or three-wire connections. The connected load may be the coil of a power relay, the electronic circuit of a control unit, or the digital input of a programmable logic controller. Figure: Electric circuit of a proximity switch with (a) a two-wire connection, and (b) a three-wire connection. Mahmoud Abdelaal Robotics_@_AI-BU_2022 25 Proximity Switches Proximity Switches The advantages of using proximity switches over other detectors (e.g., limit switches) are as follows: Long electrical life. Durability (able to last a long time without being damaged) to vibrations, accelerated motion, and toxic environments. Operation under DC or AC voltage. Effective response in objects’ high speed approaching and high frequency excitation. Absence of any kind of movable elements. Mahmoud Abdelaal Robotics_@_AI-BU_2022 26 Proximity Switches ▪ Proximity Switches Figure: Typical applications of proximity switch usage. Mahmoud Abdelaal Robotics_@_AI-BU_2022 27  Photoelectric sensors and switches Mahmoud Abdelaal Robotics_@_AI-BU_2022 28 Photoelectric sensors and switches Photoelectric sensors and switches Photoelectric switches, called also photocells, are solid-state sensors that are able to detect the presence of an object at long ranges. The non-contact detection is performed by a beam of light, visible or infrared, which can be interrupted or ignored by the detected object. In general, there are three basic types of photoelectric switches, regarding the utilized reflection type of the beam, which can be categorized as the “through-beam”, “retro-reflective”, and “diffuse”. Figure: Typical exterior view of a photoelectric sensor. Mahmoud Abdelaal Robotics_@_AI-BU_2022 29 Photoelectric sensors and switches Through-beam photoelectric switches: Any photoelectric switch consists of an emitter and a receiver of the light beam. Figure: Through-beam photoelectric switch. Mahmoud Abdelaal Robotics_@_AI-BU_2022 30 Photoelectric sensors and switches Through-beam photoelectric switches: In a through-beam type of photoelectric switch, the emitter and the receiver are separate units, contained in different housings and positioned opposite each other. The sensing distance is the maximum operating distance between the emitter and receiver, and is achieved when the two units have perfect alignment and hence the maximum amount of the emitted energy reaches the receiver. This type of photoelectric switch has the advantage of a longer sensing distance, in comparison to other types of photoelectric switches. Mahmoud Abdelaal Robotics_@_AI-BU_2022 31 Photoelectric sensors and switches Or Break Beam Sensors ▪ Any pair of compatible emitter-detector devices can be used to make a break- beam sensor Examples: ▪ Where have you seen these? Mahmoud Abdelaal Robotics_@_AI-BU_2022 32 Photoelectric sensors and switches Retro-reflective photoelectric switches: In this type of photoelectric switch, the emitter and receiver are embedded in the same housing unit, and a reflector is used for the reflection of the light beam. Reflectors are usually made up of many small, corner-cube prisms that reflect the beam back to the receiver, almost in parallel to the entering beam. A simple mirror can also be used as reflector. Mahmoud Abdelaal Robotics_@_AI-BU_2022 33 Photoelectric sensors and switches Retro-reflective photoelectric switches: Figure: Retro-reflective photoelectric switch. Mahmoud Abdelaal Robotics_@_AI-BU_2022 34 Photoelectric sensors and switches Diffuse-reflective photoelectric switches, which base their operation on the reflection of the light beam directly on the surface of the detected object, are also called “direct reflection” photoelectric switches for this reason. Obviously, in this case the emitter and the receiver are embedded together in one housing unit, as in the retro-reflective type. The emitter emits a beam of light that is not returned by reflection to the receiver. When the target object is inserted in the light beam trajectory, the beam is diffused in many directions, one of which is reflected back to the receiver. Mahmoud Abdelaal Robotics_@_AI-BU_2022 35 Photoelectric sensors and switches Diffuse-reflective photoelectric switches Figure: Diffuse-reflective photoelectric switch. Mahmoud Abdelaal Robotics_@_AI-BU_2022 36 Photoelectric sensors and switches Photoelectric sensors and switches The detection distance varies with the type, size, and model of the photoelectric switch and ranges usually from less than 10 cm up to 1500 cm. A new term, used only in photoelectric switches, refers to their “Dark On” or “Light On” operations. The “Dark On” operation means that the photoelectric switch is energized when the beam of light is interrupted, or simply when the switch is “in the dark”. Instead, the “Light On” operation means that the switch is energized when the beam of light reaches the receiver or simply when the switch is “in the light”. Mahmoud Abdelaal Robotics_@_AI-BU_2022 37 Photoelectric sensors and switches Photoelectric sensors and switches The Dark-on and Light-on Operation of a Photoelectric Switch Mahmoud Abdelaal Robotics_@_AI-BU_2022 38 Photoelectric sensors and switches Reflective Opto-sensors ▪ Emitter and detector ▪ Emitter: ▪ LED ▪ Detector: ▪ Photodiode ▪ Phototransistor Mahmoud Abdelaal Robotics_@_AI-BU_2022 39 Photoelectric sensors and switches Applications ▪ object presence detection ▪ object distance detection ▪ surface feature detection (finding/following markers/tape) ▪ wall/boundary tracking ▪ rotational shaft encoding Mahmoud Abdelaal Robotics_@_AI-BU_2022 40 Photoelectric sensors and switches Sensor limitations ▪ Light reflectivity: ▪ Surface color ▪ Texture ▪ Ambient light ▪ Sensor calibration Mahmoud Abdelaal Robotics_@_AI-BU_2022 41  Light Sensors Mahmoud Abdelaal Robotics_@_AI-BU_2022 42 Light Sensors Light Sensors A variable resistor that changes based on the light. ▪ Brighter light ➔ low resistance ▪ Darker light ➔ Higher resistance ▪ Note: Shielding, position, and directionality of the photocells are important. Mahmoud Abdelaal Robotics_@_AI-BU_2022 43  Potentiometers Mahmoud Abdelaal Robotics_@_AI-BU_2022 44 Potentiometers Potentiometers ▪ Volume control in your stereo ▪ Typically called pots Mahmoud Abdelaal Robotics_@_AI-BU_2022 45 GuideCane Application GuideCane Mahmoud Abdelaal Robotics_@_AI-BU_2022 46 GuideCane Application GuideCane Mahmoud Abdelaal Robotics_@_AI-BU_2022 47 Thank you for your attention ! Good Luck ! Mahmoud Abdelaal Robotics_@_AI-BU_2022 48