Robotics Sensors PDF

Chapter 5 Robotics Sensors Sensors of Wheeled Robots A sensor: is a window for a robot to the environment. Sensors allow robots to understand and measure the geometric and physical properties of objects in their surrounding environment, such as position, orientation, velocity, acceleration...

Chapter 5 Robotics Sensors Sensors of Wheeled Robots A sensor: is a window for a robot to the environment. Sensors allow robots to understand and measure the geometric and physical properties of objects in their surrounding environment, such as position, orientation, velocity, acceleration, distance, size, force, moment, temperature, luminance, weight, etc. They can be : 1. Active Sensors which transmits a signal into the environment and then measures the response that comes back. 2. Passive sensor is one which just ‘listens’ to what is happening. Model of Sensing  Attributes of a sensor 1. Field of view and range: every sensor has a region of space that it is intended to cover. The width of that region are specified by the sensor’s field of view, often abbreviated as FOV. The other aspect is the range, or how far the sensor can make reliable measurements. Field of view and range are obviously critical in matching a sensor to an application. If the robot needs to be able to detect an obstacle when it’s 8 feet away in order to safely avoid it, then a sensor with a range of 5 feet will not be acceptable.  Attributes of a sensor 2. Accuracy, repeatability, and resolution: Accuracy refers to the difference between a measured distance and the actual distance. For example, you can think of measuring the distance of a truck backing up towards a loading bay. To prevent a collision it is importance to know whether the sensor will indicate 1 meter remaining when there is 50 cm to go in reality. When the sensor has an absolute accuracy of ±10 cm the real value will be somewhere in between 40 and 60 cm, in relation to the previous example with a 50 cm measurement. The repeatability of accuracy is the difference between two measurements. When the first measurement indicates a distance of 101 mm and a second measurement - under exactly the same circumstances - indicates 102 mm we can say that the repeatability of the sensor is ±1 mm. Attributes of a sensor The resolution: is the smallest possible change that a sensor can perceive. A sensor with a low resolution will only detect or report displacements in whole centimeters, for example. When a sensor with a higher resolution is used, it is possible to do this down to millimeters. Of course, this is only of use when the application requires it. 3. Power consumption: power consumption is always a concern for robots. Since most robots operate off of batteries, the less power they consume, the longer they run. 4. Hardware reliability: Sensors often have physical limitations on how well they work. For example, Polaroid sonars will produce incorrect range reading when the voltage drops below 12V. Other sensors have temperature and moisture constraints which must be considered. Classifications of Sensors Sensors are generally classified into two groups: Internal sensors and external sensors. Internal sensors: obtain the information about the robot itself. such as its position sensor, velocity sensor, acceleration sensors, motor torque sensor, etc. External sensors: gather the information in the surrounding environment. such as cameras, range sensors (IR sensor, laser range finder, and ultrasonic sensor) contact and proximity sensors (photodiode, IR detector, RFID, touch, etc.) and force sensors. Types of Robot Sensors 1. Light Sensor 2. Sound Sensor 3. Temperature Sensor 4. Contact Sensors 5. Proximity Sensor 6. Distance Sensors 7. Navigation and Positioning Sensors 8. Acceleration Sensor  Light Sensor A Light sensor is used to detect light and create a voltage difference. Photoresistor is a type of resistor whose resistanc varies with change in light intensity; more light lead to less resistance and less light leads to more resistance. These inexpensive sensors can be easily implemented in most light dependent robots. Photovoltaic cells convert solar radiation into electrical energy. This is especially helpful if you are planning to build a solar robot. Sound Sensor This sensor detects sound and returns a voltage proportional to the sound level. simple robot can be designed to navigate based on the sound it receives. Imagine a robot which turns right for one clap and turns left for two claps. Implementing sound sensors is not as easy as light sensors because sound sensors generate a very small voltage difference which should be amplified to generate measurable voltage change.  Temperature Sensor What if your robot has to work in a desert and transmit ambient temperature? Simple solution is to use a temperature sensor. Tiny temperature sensor ICs provide voltage difference for a change in temperature. Few generally used temperature sensor IC’s are LM34, LM35, TMP35, TMP36, and TMP37.  Contact Sensors Contact sensors: are those which require physical contact against other objects to trigger. These sensors are mostly used for obstacle avoidance robots. When these switches hit an obstacle, it triggers the robot to do a task, which can be reversing, turning, switching on a LED, Stopping etc. A good example is a bumper sensor. Contact sensors can be easily implemented, but the drawback is that they require physical contact. In other words, your robot will not turn until it hits an object. A better alternative is to use a proximity sensor.  Bumper Sensors  A sensor that can detect physical contact. It is used to inform a Robot when it has made contact with an object or wall.  It is a switch that reports if it is pressed or released.  The Bumper Sensor will report a sensor value of TRUE when the Bumper Sensor is pressed. The Bumper Sensor will report a sensor value of FALSE when the Bumper Sensor is released. Proximity Sensor This is a type of sensor which can detect the presence of a nearby object within a given distance, without any physical contact. The working principle of a Proximity sensor is simple. A transmitter transmits an electromagnetic radiation or creates an electrostatic field and a receiver receives and analyzes the return signal for interruptions. 1. Triangulation means distance measurement by angle calculation. In measurement technology, a sensor projects a laser spot onto the Measurement object. The reflected light falls incident onto a receiving element at a certain angle depending on the distance. 2. Time-of-Flight principle (ToF) is a method for measuring the distance between a sensor and an object, based on the time difference between the emission of a signal and its return to the sensor There are different types of Proximity sensors: 1. Infrared (IR) Transceivers 2. Ultrasonic Sensor Ultrasonic Sensor Distance (meters) = (time elapsed [seconds] * 343 [meters/second]) / 2’]  Polaroid 6500 Ranging It is commonly used on mobile robots for obstacle avoidance The operation of the 6500 is quite simple. It sends out an acoustic pulse at 49.4 kHz and indicates when it has received the echo that has reflected from an object in the transducers path. The microcontroller can measure the time between when the pulse was sent and when the echo was received. Sources of Error 1. Cross-talk: two ultrasound sensors cannot be used side by side because if they use the same frequency it is not possible to distinguish which one has emitted a wave. 2. Obstacle Shape: The shape of obstacles plays an essential role, because it may cause the robot to incorrectly visualize its environment. 3. Specular reflection 4. Opening Angle Specular reflection Specular reflection: is when the wave form hits a surface at an acute angle and the wave bounces away from the transducer. Ideally all objects would have a flat surface perpendicular to the transducer, but of course, this rarely happens. To make matters worse, the reflected signal may bounce off of a second object, and so on, until by coincidence return some energy back to the transducer. In that case, the time of flight will not correspond to the true relative range. Opening Angle When an ultrasonic sensor gets a range response of R meters, the response simply represents a cone within which the object may be present. There is no way to pin- point exactly where the position of the object is. The figure below conveys the idea. The opening angle of the ultrasonic sensor is 2α and the object can be anywhere in the shaded region for the response R. Infrared (IR) Transceivers Can be classified into two: 1. Active infrared sensors (IR)work with radar technology and they both emit and receive infrared radiation. This radiation hits the objects nearby and bounces back to the receiver of the device. Through this technology, the sensor can not only detect movement in an environment but also how far the object is from the device. This is especially useful in robotics to detect proximity. 2. For the physical security use case, however, passive infrared sensors (PIR sensors) are definitely more widespread. The PIR sensors do not emit radiation but simply receive the one that the objects nearby are naturally emitting. The basic functioning is that the passive infrared sensors trigger the alarm when there is an anomaly in the infrared waves measured in the room. This happens if, for example, a warm object (like an intruder) crosses ways to the signal of the device. Distance Sensors 1. Ultrasonic Sensors 2. Infrared sensor 3. Laser range Sensor: Laser light is transmitted and the reflected light is captured and analyzed. Distance is measured by calculating the speed of light and time taken for the light to reflect back to the receiver. These sensors are very useful for longer distances. (triangulation) 4. Encoder Sensor: It is a device that can provide precise information on the speed, direction and positioning of a piece of mechanical equipment.It convert angular position of a wheel into a digital code. There are two different types of encoders: linear and rotary. A linear encoder responds to motion along a path, while a rotary encoder responds to rotational motion. 360 Degree Omnidirectional Laser Range Scanning The core of RPLIDAR A1 runs clockwise to perform a 360 degree omnidirectional laser range scanning for its surrounding environment and then generate an outline map for the environment. Robots Equipped with Laser Scanners-Example Range Sensor (Kinect)