Automation Building Blocks Sensors Analyzers Actuators Drives Vision

Automation Building Blocks Sensors Analyzers Actuators Drives Vision systems ME 486 - Automation By Ed Red

Objectives • To review basic building blocks for implementing automation • To consider application conditions • To introduce assessment criteria • To test understanding of the material presented ME 486 - Automation

Building Blocks • Sensors Inductive proximity sensors • Analyzers • Actuators • Drives • Vision system (integrated sensor/analyzer) ME 486 - Automation

Building Blocks – sensor features • Accuracy and repeatability • Precision • Range • Response time • Calibration methods • Minimum drift • Costs and reliability • Sensitivity ME 486 - Automation

Building Blocks – sensors & moving objects We can characterize a sensor’s capability by it’s operating frequency or by its response time. Both determine how well the sensor might measure the desired property (proximity, length…) of a moving object. Using a sensor’s specification, how might we determine how fast a moving object might move past the sensor and the sensor still read the object parameter correctly? ME 486 - Automation

Building Blocks – sensor devices See text for in depth description! • Photoelectric sensors Inductive proximity Linear sensors encoder • Proximity switches (inductive and capacitive) Absolute rotary encoder • Range sensors (ultrasonic/acoustic, laser reflectors…) • Transducers (encoders) ME 486 - Automation

Building Blocks – analyzers Encoder example – An absolute optical encoder has 8 rings, 8 LED sensors, and 8 bit resolution. If the output pattern is 10010110, what is the shaft’s angular position? Ring 1 2 3 4 5 6 7 8 Angle (deg) 180 90 45 22. 5 11. 25 5. 625 2. 8125 1. 40625 Pattern Value (deg) 1 180 0 0 1 22. 5 0 1 5. 625 1 2. 8125 0 Total 210. 94 ME 486 - Automation

Building Blocks – drives • Stepper Motors (index by open-loop control) • AC/DC servomotors (PID feedback control, holds torque when at rest) • Kinematic devices (intermittent operation, e. g. , geneva mechanism) • Digital drives ME 486 - Automation

Building Blocks – present drives Controller ME 486 - Automation Servo-loops Amplifiers Application Motion Planning & Control Set Points Servocard

Building Blocks – digital drives • Microprocessors and Digital Signal Processors (DSP’s) are replacing analog components with digital components (i. e. , digital drives). • EIA RS-431, the outdated ± 10 V standard, no longer need constrain control resolution. • Revolutions in computer operating systems, applications, and networking. • Networking standards, such as IEC 61491 and IEEE 1394, are changing motion control architectures and hardware configurations. • Need for A/D and D/A interfaces is rapidly declining, being replaced by a highspeed network between the master host (a PC) and the distributed digital slave devices. ME 486 - Automation

Building Blocks – digital drives DMAC Ormec’s servowire implementation of IEEE 1394 ME 486 - Automation

PWM and digital drives (binary control!) PWM – Pulse Width Modulation - a constant frequency, two-valued signal (e. g. , voltage) in which the proportion of the period for which the signal is on and the period for which it is off can be varied. • Percentage of time on is called the duty cycle. • Voltage value will depend on the application If direction is to be changed, requires another PWM signal. • PWM frequency must be high enough so that motor cannot respond to a single PWM signal On 25% duty cycle Off On 50% duty cycle Off T 2 T 3 T 4 T ME 486 - Automation T 2 T 3 T 4 T

A/D Signal Conversion Resolution of A/D is represented by number of conversion bits n: Nq = number of quantitization levels = 2 n R = conversion resolution = Voltage range/(Nq – 1) Variable (or Voltage) ±R Time ME 486 - Automation (± 10 V)

A/D Signal Conversion Successive approximation method is similar to the method we used to extract the encoder value from the binary output backwards. Here is simple example: Range (± 10 V) 6. 8 V 1. 8 0. 55 0. 2375 0. 08125 = error ME 486 - Automation Quantitizations Bit (on or off) 5 1 2. 5 0 1. 25 1 0. 625 0 0. 3125 1 0. 15625 1 Value 5 1. 25 0. 3125 0. 15625 6. 719 V

D/A Signal Conversion The decoding equation is: Eo = Eref [0. 5 B 1 + 0. 25 B 2 + 0. 125 B 3+…+(2 n)-1 Bn] where Eo = output analog signal value Eref = ref voltage For example: 10010 means B 1 = 1, B 2 = 0, B 3 = 0, B 4 = 1, B 5 = 0 ME 486 - Automation

Electromagnetism B F=Ilx. B I F l I B Current flow produces magnetic field and associated flux. Changing field (flux) through a coil induces a reactive electromotive force (emf) e: e = -N d. F/dt (Faraday’s Law) N = # turns in coil; F is flux in webers This in turn generates an induced current in opposite direction and a resulting opposing flux as described by: e = -L di/dt ME 486 - Automation L = inductance in henrys

AC motors Stator structure is composed of steel laminations shaped to form poles around which are wound copper wire coils. These primary windings connect to, and are energized by, the voltage source to produce a rotating magnetic field. Three-phase windings spaced 120 electrical degrees apart are popular in industry. Rotor (or rotating secondary) is another assembly of laminations over a steel shaft core. Radial slots around the laminations’ periphery house rotor bars—cast-aluminum or copper conductors shorted at one end and positioned parallel to the shaft (see photo). The motor’s name comes from the alternating current (ac) “induced” into the rotor by the rotating magnetic flux produced in the stator. Motor torque is developed from interaction of currents flowing in the rotor bars and the stator’s rotating magnetic field. ME 486 - Automation

(new tech) Linear motors Two basic classes: 1) permanent magnet (PM) brushless, and 2) asynchronous linear induction motors (LIMs). PM brushless motors abound in various subclasses, such as the moving coil and moving magnet types. Ironless refers to a core containing only copper coils (and epoxy encapsulation). Smooth "cog-free" motion is produced since no attractive force exists between coil and magnet-but at the cost of lower force output. Tubular linear motor Slot-less refers to a special design of steel laminations where the windings go through holes in the stator rather than slots. The result is a smoother surface facing the magnet. This design also reduces cogging by eliminating variation in attractive force. Tubular linear motors roll up the unit about an axis parallel to its length. In one style, an outer thrust block carrying the motor coils envelops and moves along a stationary thrust rod that houses magnets. Another style has a central rod with magnets that moves relative to an outer stator member. Travel is limited since thrust rod must be supported at both ends (or at one end for the moving-rod version). ME 486 - Automation

(new tech) Switched reluctance motor Reluctance - opposition of a material to magnetic lines of force Both stator and rotor of the switched reluctance motor have projecting poles. In the image, poles 1 and 1' are energized. These are wired in series. The rotor has no permanent magnets or windings. Thus when one of the four phases of the stator is energized, the closest set of poles of the rotor (made up of reluctance magnets) are pulled into alignment. By turning off phase 1 and energizing phase 2, you can visualize how the rotor will rotate 15' CCW to align the rotor poles closest to phase 2. A four phase converter capable of accepting feedback is used to energize the coils in order to control the switched reluctance motor. The feedback is necessary to run the motor in selfsynchronous mode, which enables a continuous smooth speed operation. By energizing the phases in reverse sequence, the motor can also run CW. The switched reluctance motor along with the four phase converter are meant to be used as a precise speed control device, and they are approximately 2% more efficient than the other AC speed control systems. ME 486 - Automation

Control Servo DCI Set Points Motion Planning & Control CPU 2 RTOS Controls Application CPU 1 IEEE 1394, USB 2, Fiber Optic, etc. Windows Application PC DMAC ME 486 - Automation Digital Drive Network

Building Blocks Assessment 1. Who are major vendors of proximity switches, servomotors? 2. What are the limits to sensor proximity distances? 3. What types of proximity accuracies might you expect from proximity sensors? 4. Which sensors work on which materials? 5. Are sensors affected by speed by which materials move past them? 6. What are weight to torque ratios for common servomotors? ME 486 - Automation

Building Blocks Assessment 7. What does torque speed curve look like for the motors typically used to control robots? 8. What is difference between absolute encoder and relative encoder? How do encoders measure directional changes? 9. What is difference between a resolver and digital encoder? 10. Costs of sensors, motors, etc. ? 11. How do the new linear drives work, and what are their response characteristics? ME 486 - Automation

Building Blocks – machine vision Definition – “Machine vision is the capturing of an image (a snapshot in time), the conversion of the image to digital information, and the application of processing algorithms to extract useful information about the image for the purposes of Algorithm pattern recognition, part inspection, or part positioning and PC orientation”…. Ed Red ME 486 - Automation

Building Blocks – machine vision Equipment: • Computer • Frame grabber • Camera (CCD array) • Lenses • Lighting • Calibration templates • Algorithms ME 486 - Automation Types: Front Back Side Structured Strobe

Machine Vision – structured lighting Structured Lighting is used in a front lighting mode for applications requiring surface feature extraction. Structured lighting is defined as the projection of a crisp line of light onto an object. The patterned light is then used to determine the 3 -D characteristics of an object from the resulting deflections observed. Note the non-typical approach of projecting a grid array of light on an object to detect features ME 486 - Automation

Machine Vision – image processing Segmentation – Define and separate regions of interest Thresholding – Convert each pixel into binary (B or W) value by comparing bit intensities Edge detection – Locate boundaries between objects Feature extraction – Determine features based on area and boundary characteristics of image Pattern recognition – Identify objects in midst of other objects by comparing to predefined models or standard values (of area, etc. ) ME 486 - Automation

Machine Vision – applications Dimensional measurement Object verification Proper position/orientation Flaws and defects Counting Guidance and control (offsets, tracking) ME 486 - Automation

Machine Vision – example • 8 -bit image of metallic iron as it appears in iron ore (lighter objects in the image represent the metallic iron) • Histogram displays pixel intensity distribution …background appears at gray level 40, ore shows up at gray level 70, and high-intensity iron turns up at gray levels above 150. Image clearly differentiates components. • Blob analysis - set threshold to gray level 148…all the pixels with gray levels of 148 or lower get set to zero. Pixels with gray levels of 149 or higher get set to one • Morphology functions slightly change or eliminate the shapes of objects so imaging software can easily count them. ME 486 - Automation

Machine Vision – example Suppose we wish to calculate the area and centroid of the selected binary region in the last figure, how would you do it? Assume that you have a camera such that the pixels are square and you have a matrix of pixel values as depicted in the figure shown. Y What equations would you apply? ME 486 - Automation X

Machine Vision Assessment 1. Who are major vendors of vision systems and the various components? 2. What are typical camera resolutions? 3. What are typical camera calibration techniques? 4. What is camera distortion? 5. Is color vision imaging used? In what applications? 6. How long does it take to process images? As a function of image processing function? 7. What are typical costs for imaging systems? For frame grabbers, cameras, lenses, lighting? ME 486 - Automation

Building Blocks What have we learned? ME 486 - Automation