Sensors and Actuators

Transcription

1 Marcello Restelli Dipartimento di Elettronica e Informazione Politecnico di Milano restelli@elet.polimi.it tel: Sensors and Actuators Robotics for Computer Engineering students A.A. 2014/2015

2 Effectors and Actuators Effector Any device robot that has an impact on the environment Effectors must match a robot s task Controllers command the effectors to achieve the desired task Actuator A robot mechanism that enables the effector to execute an action Robot effectors are very different than biological ones Robots: wheels, tracks, legs, grippers Robot actuators: Motors of various types Passive actuation 2

3 Types of Actuators Electric motors Hydraulics Pneumatics Photo-reactive materials Chemically reactive materials Thermally reactive materials Piezoelectric materials 3

4 Most Popular Actuators First robots used pneumatic and hydraulic actuators hydraulic actuators are expansive, weighing, and their maintenance is hard used only in big robots pneumatic actuators are used for application requiring stop-to-stop trajectories, such as pick-and-place Nowadays the most common actuators are electrical motors both DC and AC since these motors reach high speeds they are typically reduced by gearing that make the dynamics more complex typically each joint has its own motor, but it may happen that the same motor may actuate several joints through transmissions for stepper motors internal sensors are not required, but when an error occurs their position is unknown 4

5 DC Motors DC (direct current) motors Convert electrical energy into mechanical energy Small, cheap, reasonably efficient, easy to use How do they work? Electrical current through loops of wires mounted on a rotating shaft When current is flowing, loops of wire generate a magnetic field, which reacts against the magnetic fields of permanent magnets positioned around the wire loops These magnetic fields push against one another and the armature turns 5

6 DC Motors 6

7 DC Motors 7

8 DC Motors 8

9 DC Motors 9

10 DC Motors 10

11 DC Motors: Brushed and Brushless Motors Brushes are used to change the magnetic polarity of the electromagnet Brushed motors are cheap but have many drawbacks Brushes eventually wear out Brushes make noise Limit the maximum speed Hard to cool Limit the number of poles Brushless DC motors overcome these problems but they are more expensive Brushes are replaced by computer Permanent magnets on the rotor Electromagnets on the stator 11

12 Operating/Stall Current When provided with constant voltage, a DC motor draws current proportional to how much work it is doing Work = Force * Distance When there is no resistance to its motion, the motor draws the least amount of current When the robot pushes against an obstacle motors drain more current If the resistance becomes very high the motor stalls and draws the maximum amount of current (stall current) at its specified voltage 12

13 Torque Torque: rotational force that a motor can deliver at a certain distance from the shaft Strength of magnetic field generated in loops of wire is directly proportional to amount of current flowing through them and thus the torque produced on motor's shaft The more current through a motor, the more torque at the motor's shaft Stall torque: the amount of rotational force produced when the motor is stalled at its recommended operating voltage, drawing the maximal stall current at this voltage Torque units: ounces*inches or N*m 9.8 N*m torque means motor can pull a weight of 1kg through a pulley 1m away from shaft 13

14 Power of a Motor Power: product of the output shaft's rotational velocity and torque If there is no load on the shaft then P=0 rotational velocity is maximum, but the torque is 0 the motor is spinning freely If the motor stalled then P=0 it is producing its maximal torque rotational velocity is zero A motor produces the most power in the middle of its performance range ωm τ m= τ s 1 ω max τ ωm =ωmax 1 τm s ( ( ) ) 14

15 Efficiency of a Motor Motor efficiency is power out divided by power in Poutput η= Pinput Power out is mechanical energy Poutput = τ ω Power in is electrical energy Pinput =V I I= V s V R e V e = k e ω 15

16 Motor Efficiency and Operating Voltage DC motors are not perfectly efficient Due to friction some energy is wasted as heat Industrial-grade motors (good quality): 90% Toy motors (cheap): 50% Micro-motors for miniature robots < 50% To make the motor run, electrical power must be provided in the right voltage range if the voltage is lower than the motor runs fine even if it is less powerful if the voltage is higher the life of the motor becomes shorter 16

17 How Fast do Motor Turn? Free spinning speeds (most motors) RPM ( Hz) High speed, low torque drive light things that rotate very fast What happens with heavy robots or manipulators? it is required more torque and less speed The solution consists of using gearing Trade-off high speed for more torque 17

18 Gearing Torque: T = F x r rotational force generated at the center of a gear is equal to the gear's radius times the force applied tangential at the circumference Meshing gears: by combining gears with different ratios we can control the amount of force and torque generated Example: r2 = 3r1 Gear 1 turns 3 times while gear 2 turns only once T1*360 = T2*1080 T2 = 3*T1 = T1*r2/r1 18

19 Gearing Effect on Speed Combining gears has a corresponding effect on speed A gear with a small radius has to run faster to keep up with a larger gear Increasing the gear radius reduces the speed Decreasing the gear radius increases the speed Torque Speed tradeoff when a small gear drives a large one, torque is increased and speed is decreased analogously, when a large gear drives a small one, torque is decreased and speed is increased 19

20 Designing Gear Teeth Reduced backlash the looseness between mashing gear teeth Tight meshing between gears increases friction Proportionally sized gears a 24-tooth gear must have a radius three times the size of an 8-tooth gear Example Input (driving) gear: 8 teeth Output (driven) gear: 24 teeth Effect at the 24 teeth gear 1/3 reduction in speed 3 times increase in torque 20

21 Gear Reduction in Series By putting two 3:1 gear reductions in series ( ganging ) a 9:1 gear reduction is created the effect of each pair of reductions is multiplied key to achieve useful power from a DC motor With such reductions, high speeds and low torques are transformed into usable speeds and powerful torques 21

22 Motor Control: PWM Motors can be controlled by modulating the input voltage (or current) Use of linear amplifier Power inefficient and impractical Alternative: Pulse Width Modulation (PWM) switch voltage ON/OFF frequency from 2 to 20 khz (against a 100Hz bandwith) higher frequencies are preferred (non audible), but... over-heat voltage spikes interference become prominent 22

23 Motor Control: PWM 23

24 Motor Control: PWM 24

25 Servo Motors Specialized motors that can move their shaft to a specific position For DC motors is only possible to specify one direction Servo capability to self-regulate its behavior, i.e. to measure its own position and compensate for external loads when corresponding to a control signal often used in hobby radio control applications Servo motors are built from DC motors by adding Gear reduction Position sensor for the motor shaft Electronics that tell the motor how much to turn and in what direction Movement Limitations shaft travel is restricted to 180 degrees sufficient for most applications 25

26 Sensors Sensors allow a robot to accomplish more complex tasks autonomously Two main categories Internal sensors External sensors sensors with contact sensors without contact Other classification Passive sensors (measure a physical property) Active sensors (emitter + detector) 26

27 Encoders An encoder is a sensor for converting rotary motion or position to a series of electronic pulses Linear architecture Consist of a long linear read track, together with a compact read head Rotary architecture Serve as measuring sensors for rotary motion and for linear motion when used in conjunction with mechanical measuring standards such as leadscrews, and convert rotary motion (incremental or absolute) into electrical signals They are both effective and low cost feedback devices. 27

28 Incremental Encoders It is based on the photoelectric principle It consists of a disk with two traces where transparent and opaque zones are alternated The presence of two traces allows to identify the rotation direction N: number of steps (number of light/dark zones per turn) Since the two signals are ¼ step shifted, resolution is 360 /4N Notch to define an absolute mechanical zero 28

29 Absolute Encoders It is a disk with transparent and opaque areas, placed on concentric rings For an N-bit word there are N rings Resolution: 360 /2N To avoid reading ambiguities binary codes with single variations (Gray code) are used In robotic applications at least 12 rings are used (360 /4096) 29

30 What is perceived? Sensor may be classified according to what they measure distance proximity contact force and torque vision position 30

31 Distance Perception Measure the distance between a reference point and object placed in front of the sensor Human beings use stereo-vision, while other animals (like bats, dolphin, and whales) use echolocation Knowing the distance of the surrounding objects is useful for obstacle avoidance and for more complex planning activity 31

32 Distance Perception: Reflective Optosensors An ease way to compute distance is to use triangulation Reflective optosensors are active sensors emitter: a source of light (LED, light emitting diodes) detector: a light detector (photodiode or phototransistor) The emitter scans the surface with a beam of light The detector measure the angle corresponding to the maximum intensity of light Calling s the distance between the emitter and the detector, the distance from the object is computed as d= s tan αi 32

33 Distance Perception: Kinect Kinect is a motion sensing input device built by Microsoft for Xbox 360 A cheap device that provides several sensing information Used in many robotic research studies Provides 30Hz 8-bit RGB camera (640x480) 3D scanner Infrared projector Infrared camera (11-bit 640x480) Range m (up to m) Angular field of view: 57 h, 43 v Multi-array microphone 33

34 Distance Perception: Phase Shift Telemeter The light emitted is split into two parts against the object against a mirror placed inside the sensor The beam follows different optical paths and the two reflected waves have different phases The distance of the object must lead to phase displacement within [0 ;360 ] Laser wavelength is around 1e-6m Acoustic waves are not directional The solution is to modulate the laser light with a wave characterized by a long wave length 34

35 Distance Perception: Time-of-Flight Telemeter It measures the time between the instant the emitter produces the signal and the instant the detector receives its reflection The distance covered by the signal is 2d The time is ΔT = 2d/c The speed of light is too high for robotic applications Acoustic waves are better (v=340 m/s) are characterized by low directionality (20 40 ) the reflection is dumped and the signal is largely affected by noise Polaroid ultrasonic sensors range m accuracy 0.025m cone opening 30 Frequency 50 KHz 35

36 Proximity Perception Proximity sensors measure the presence of objects within a specified distance range They are used to grasp objects and avoid obstacles Sensors ultrasonic (low cost) inductive (perceive only ferromagnetic materials under the distance of 1mm) Hall effect (perceive only ferromagnetic materials, may be small, robust, and cheap) Capacitive (perceive any object, binary output, high accuracy only when calibrated for a particular object) Optical (infrared light, binary output) 36

37 Tactile Sensors These sensors are used for manipulation purposes Two main categories binary are realized by switchers typically they are placed on the fingers of a manipulator they may be arranged in arrays may be placed also on the external side of the hand to avoid obstacles analogical soft devices that produce a signal proportional to the local force typically realized with a spring coupled with a shaft otherwise soft conductive material that change its resistance according to its compression there are sensors that measure also movements tangential to the sensor surface 37

38 Force and Torque Sensors Typically these sensors are used at joint level and in the wrist For joints driven by DC motors the force is measured by the current The measure of the strain is based on elasticity 6 parameters in the Cartesian space 3 forces along axes 3 torques around axes Very expensive 38

39 Position Sensor The main sensor to determine the absolute position of a robot is the Global Positioning System (GPS) 21 satellites it is based on the flying time of a radio signal At least 4 sensors must be perceived Measuring rate is 2Hz Accuracy is about 1.5m with DGPS accuracy arrives at about 2cm Unfortunately GPS sensors may not be used in indoor environments, underground, underwater, or in urban situations with skyscrapers 39

40 Inertial Sensor Gyroscopes Angular velocities Accelerometers Gravitational vector Magnetometers/compass Magnetic field vector Used in many mobile and console devices 40

41 Sensor Fusion A man with one watch knows what time it is a man with two watches isn't so sure To have a better representation of the world we need to combine measurements from multiple sensors that present also redundancy Sensor fusion is a complex problem different sensor accuracy different sensor complexity contradictory information asynchronous perception Cleverness is needed to put this information together 41