Sensors For Robotics

What makes a machine a robot?

Solar Cell Digital Infrared Ranging Compass Touch Switch Pressure Switch Limit Switch Magnetic Reed Switch Magnetic Sensor Miniature Polaroid Sensor Polaroid Sensor Board Piezo Ultrasonic Transducers Pyroelectric Detector Thyristor Gas Sensor Gieger-Muller Radiation Sensor Piezo Bend Sensor Resistive Bend Sensors Mechanical Tilt Sensors Pendulum Resistive Tilt Sensors CDS Cell Resistive Light Sensor Hall Effect Magnetic Field Sensors Compass IRDA Transceiver IR Amplifier Sensor IR Modulator Receiver Lite-On IR Remote Receiver Radio Shack Remote Receiver IR Sensor w/lens Gyro Accelerometer IR Reflection Sensor IR Pin Diode UV Detector Metal Detector

Why do robots need sensors?

What Is a Sensor? Anything that detects the state of the environment. Collect information about the world Sensor - an electrical/mechanical/chemical device that maps an environmental attribute to a quantitative measurement Each sensor is based on a transduction principle - conversion of energy from one form to another

What you (and the robot) can do without sensors? Close your eyes. Plug your ears. Hold your nose. Tie your hands behind your back. –Shut your mouth. Tie your shoelaces together. Spin yourself around a few times. Now walk. How does it feel? That's exactly what your robot feels: nothing - without sensors. You have been given many types of sensors that can be used in a variety of ways to give your robot information about the world around it. We will explain each of the sensors you can find in the lab, how it works, what it's good for, and how to build it.

The simplest possible use of sensors The diagram serves to illustrate the general case of sensing a specific phenomenon. –In this case it is the presence or absence of light. The sensor in this case is a photo-resistor. –When sufficient light strikes it, its internal resistance is reduced to several hundred Ohms. –When no light strikes it its resistance is typically several million Ohms. light

Simple and Complex Sensors Sensors range from simple to complex in the amount of information they provide:  a switch is a simple on/off sensor  a human retina is a complex sensor consisting of more than a hundred million photosensitive elements (rods and cones) Sensors provide raw information, which can be treaded in various ways, For example, we can simply react to the sensor output:

How to detect people? For example, how would you detect people? Some options include:  temperature: pyroelectric sensors detect special temperature ranges  movement: if everything else is static  shape: now you need to do complex vision processing  color: if people are unique colored in your environment

How to detect people? Let's think about something even more simple: how would you measure distance:  ultrasound sensors give you distance directly (time of flight)  infra red through return signal intensity  two cameras (i.e., stereo) can give you distance/depth  a camera can compute it from perspective  use a laser and a fixed camera, triangulate  structured light; overlying grid patterns on the world  frequency and phase modulation interferometry

Biological Analogs All of the sensors we describe in this lecture exist in biological systems  Touch/contact sensors with much more precision and complexity in all species (spiders?)  Polarized light sensors in insects and birds  Bend/resistance receptors in muscles  and many more...

You have to understand sensors we need to make one point very clear: –Sensors are not magical boxes. –All information you get from sensors must be decoded by you, the human builder and programmer. Sensors convert information about the environment into a form that can be used by the computer. –The sensors that are on the robot can be related to sensors found in humans.

You have to understand sensors These sensors convert information about the environment into neural code that your brain can understand: –Touch sensors embedded in your skin, –visual sensors in your retina, –and hair cells in your ears Your brain needs to understand the neural code before you can react. –Since you will be programming the robot, you will need to understand the output of the sensors before you can program your robot to react to different stimuli.

Types of Sensors Active –send signal into environment and measure interaction of signal w/ environment –e.g. radar, sonar Passive –record signals already present in environment –e.g. video cameras

Types of Sensors Classification by medium used –based on electromagnetic radiation of various wavelengths –vibrations in a medium –concentration of chemicals in environment –by physical contact

Types of Sensors Exteroceptive: deal w/ external world –where is something ? –how does is look ? (camera, laser rangefinder) Proprioceptive: deal w/ self –where are my hands ? (encoders, stretch receptors) –am I balanced ? (gyroscopes, INS)

Types of Sensors Interoceptive –what is my thirst level ? (biochemical) –what is my battery charge ? (voltmeter) For the most part we’ll ignore these in this class

Analog versus Digital Sensors In all our robotics kits the sensors are digital or analog. For instance, in HandyBoard, analog sensors can be plugged into the analog sensor ports, which return values between 0 and 255. Digital sensors can be plugged into either the digital ports or the analog ports, but will always return either 0 or 1. ANALOG 0 =< x =< 255 DIGITAL 0 or 1 DIGITAL 0 or 1

Analog Sensors and Thresholding Analog sensors, such as photo-resistors, can tell you: – how far the sensor has bent, – or how much light is hitting the sensor. They answer questions with more detail. –Analog sensors, however can be converted to digital sensors using thresholding. Instead of asking the question “How much is the sensor bent?” you can ask the question: “Is the sensor bent more than half way?” The threshold can be determined by playing around with the specific sensor.

Touch sensors

Resistive Position Sensors: bending We said earlier that a photocell is a resistive device, i.e., it senses resistance in response to the light. We can also sense resistance in response to other physical properties, such as bending. These bend sensors were originally developed for video game control They are generally quite useful: –Video game accessories are in general useful for robotics and virtual reality and very cheap.

Bend Sensors Useful for contact sensing and wall-tracking The bend sensor is a simple resistance – As the plastic strip is bent (with the silver rectangles facing outward), the resistance increases You can remove it from Nintendo gloves

Bend sensor

Sensor zMeasure bend of a joint zWall Following/Collision Detection zWeight Sensor Sensors Sensor Applications of Resistive Analog Sensors

micro + - Single Pin Resistance Measurement Binary Threshold micro Analog to Digital (pull down) Inputs for Resistive Sensors Voltage divider: You have two resisters, one is fixed and the other varies, as well as a constant voltage V1 – V2 * (R2/R1+R2) = V R1 R2 measureKnown unknown V V1 V2 Comparator: if voltage at + is greater than at -, high value out

Potentiometer

Potentiometer: the main ideas Potentiometers are very common for manual tuning; you know them from some controls (such as volume and tone on stereos). Typically called pots, they allow the user to manually adjust the resistance. The general idea is that the device consists of a movable tap along two fixed ends. As the tap is moved, the resistance changes. As you can imagine, the resistance between the two ends is fixed, but the resistance between the movable part and either end varies as the part is moved. In robotics, pots are commonly used to sense and tune position for sliding and rotating mechanisms.

Potentiometers Mechanical varieties:Mechanical varieties: – Linear and rotational styles - make position sensors for both sliding mechanisms and rotating shafts – Resistance between the end taps is fixed, but the resistance between either end tap and the center swipe varies based on the position of the swipe Electrical varieties: – Linear taper - linear relationship between position and resistance. Turn the pot 1/4 way, the resistance between the nearer end and the center is 1/4 of end-to-end resistance – Audio taper - logarithmic relationship between position and resistance. At one end, 1/4 turn would swipe over a small bit of total resistance range, while at the other end, 1/4 turn would be most of the range

Figure 5.5: Potentiometer Assemblies Kits contain several sizes of potentiometers, also known as variable resistors. Potentiometers should be wired with Vcc and ground on the two outside pins, and the signal wire on the center tap. – This will, in effect, place the resistance of the potentiometer in parallel with the 47K pull-up on the expansion board and is more stable than just using one side and the center tab to make a plain variable resistor

Potentiometers have a variety of uses: – In the past, they have been used for menuing programs – For angle measurement for various rotating limbs – For scanning beacons. They can be used with a motor to mimic servos, but that's a difficult task. not designed to turn more than about 270 degrees. –It is important to notice that the pots are not designed to turn more than about 270 degrees. –Forcing them farther is likely to break them. Various uses of Potentiometers

Linear Potentiometers and their use in HandyBoard A linear potentiometer can be used to measure precise linear motion, –such as a gate closing, – or a cocking mechanism for ring balls or blocks. Frob-knobFrob-knob –The frob knob is the small white dial on the lower left corner of the Expansion Board. It returns values between 0 and 255 and provides a handy user input for adjusting parameters on the y or for menuing routines to select different programs.

Encoders oEncoders measure rotational motion. oThey can be used to measure the rotation of a wheel. oServo motors: Used in conjunction with an electric motor to measure the motor’s position and, in turn, control its position.

Encoders

Encoder Incremental encoder –usually requires 2 sensors to determine speed and direction Technology –magnet + hall sensors (incremental) –optical sensors with black/white segments (incremental)

Encoder Encoder signal (2 lines) are connected to microcontroller like 2 binary sensors (digital input lines) Microcontrollers usually have special internal registers for pulse counting z ⇒ This is done in parallel to normal calculations zDoes not slow down the cpu

Sensors Based on Sound SONAR: Sound Navigation and Ranging –bounce sound off of something –measure time for reflection to be heard - gives a range measurement –measure change in frequency - gives the relative speed of the object (Doppler effect) –bats and dolphins use it with amazing results –robots use it w/ less than amazing results

Sonar and IR Proxmity

Odor Sensors Detection of chemical compounds and their density in an area –spectroscopy - mostly lab restricted –fibre-optic techniques - recently developed –chemical detection - sniffers aand electronic noses via “wet chemistry on a chip” No major penetration in robotics yet applications are vast (e.g. mine detection)