Survey of Electronics

Transcription

1 Survey of Electronics Testing Boe Bot s Speed Control Controlling other servo motors Slot Reel Stepper Motors DC Motors AC Motors

2 Testing Boe Bot s Speed Control Servo Motor Speed verses Pulse Width Pulse width is a common way to describe how long a pulse lasts. The reason it is called pulse "width" is because the amount of time a pulse lasts is related to how wide it is on a timing diagram. Pulses that last longer are wider on timing diagrams 2

3 Testing Boe Bot s Speed Control Servo Motor Speed verses Pulse Width Pulse width: a way to describe how long a pulse lasts. Width" is the amount of time a pulse lasts verses its period Pulses that last for short periods of time are narrow 3

4 Testing Boe Bot s Speed Control Servo Motor Speed verses Pulse Width Pulse width: a way to describe how long a pulse lasts. Width" is the amount of time a pulse lasts verses its period Pulse generation using the BASIC Stamp on the Boe Bot What commands would be needed to make a pulse signal with a pulse width of 500 ms and a period of 1 second? What command would be required to generate the bottom of the two waveforms on the previous slide 4

5 Testing Boe Bot s Speed Control Servo Motor Speed verses Pulse Width New PBASIC commands to simplify these tests DEBUGIN Syntax: DEBUGIN modifier, variable USAGE: pulsewidth VAR WORD DEBUG Enter pulse width: DEBUGIN DEC, pulsewidth Now, the DEBUGIN command can be used to capture a decimal value that you enter into The Terminal s Transmit windowpane and store it in pulse Width: 5

6 Testing Boe Bot s Speed Control Servo Motor Speed verses Pulse Width New PBASIC commands to simplify these tests DEBUGIN Uses the Transmit pane of the DEBUG Terminal Sends the content of the pane to the Boe Bot The DEBUG command displays messages from the Boe Bot in the Receive pane Test Program Enter the program starting at the bottom of page 113 Special NOTE: Variable pulsewidthcomp How its generated How it is used 6

7 Testing Boe Bot s Speed Control Servo Motor Speed verses Pulse Width Test Program How the Program works Three variables are declared counter for the FOR NEXT loop counter VAR Word pulsewidth for the DEBUGIN and PULSOUT commands pulsewidth VAR Word pulsewidthcomp which stores a value that is used in a second PULSOUT command pulsewidthcomp VAR Word The FREQOUT command is used to signal that the program has started. FREQOUT 4,2000,3000 7

8 Testing Boe Bot s Speed Control Servo Motor Speed verses Pulse Width Test Program How the Program works The duration of the two pulses in each loop of the counter always adds to 3 ms Pin 12 pulse equals 2 μs x pulsewidth entry Pin 13 pulse equals 2 μs x ( pulsewidth entry) The FOR.. TO.. NEXT instructions take s about 24.6 ms to run Thus a count of 244 => 6 seconds FOR counter = 1 TO 244 PULSOUT 12, pulsewidth PULSOUT 13, pulsewidthcomp PAUSE 20 NEXT 8

9 Testing Boe Bot s Speed Control Servo Motor Speed verses Pulse Width Use the program to plot the rotational speed of a servo against the width of the pulses sent to the motor Rotational velocity should be on the vertical axis Zero should be in the middle Pulse width should be on the horizontal axis in milliseconds Use the provided chart to record your data Plot the data on the provided graph paper 9

10 Other Servo Motors and Controlled Motors Overview Types A stepper motor such as on a slot machine Is an device Which converts electrical signals (such as from a CPU) into discrete mechanical movements. Directional/Speed controlled Direct Current Motors The polarity of the applied voltage controls rotational direction The speed of rotation is controlled by the voltage across the motor Both items can be controlled by switches or by microcontroller outputs AC Motors The phase shifts of the applied starting voltages can determine rotational direction The speed of rotation is controlled by the voltage across the motor Both items can be controlled by switches or by microcontroller outputs

11 Overview Reel Stepper Motors A stepper motor is an device Which converts electrical signals (such as from a CPU) into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. Direction of rotation Is directly related the sequence of the applied pulses The speed of the motor shafts rotation Is directly related to the frequency of the input pulses The length of rotation Is directly related to the number of input pulses applied.

12 Reel Stepper Motors Overview Advantages of using Stepper Motors The rotation angle of the motor is proportional to the input pulse. The motor has full torque at stand still (if the windings are energized) Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3-5% of a step and this error is non cumulative from one step to the next. Excellent response to starting/ stopping/reversing. Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing. The motors response to digital input pulses provides open loop control, making the motor simpler and less costly to control.

13 Reel Stepper Motors Overview Advantages of using Stepper Motors It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses. Disadvantages of using Stepper Motors Resonance movements can occur if not properly controlled. Not easy to operate at extremely high speeds. Types of Stepper Motors Variable reluctance (VR) This type of stepper motor has been around for a long time.

14 Reel Stepper Motors Types of Stepper Motors Variable reluctance (VR) It is probably the easiest to understand from a structural point of view. Figure 1 shows a cross section of a typical V.R. stepper motor. This type of motor consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC current the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles.

15 Reel Stepper Motors Types of Stepper Motors Permanent Magnet (PM) Often referred to as a "tin can" or "canstock" motor Is a low cost and low resolution type motor Typical step angles of 7.5 degrees to 15 degrees. (48-24 steps/revolution) PM motors Has permanent magnets added to the motor structure. The rotor no longer has teeth as with the VR motor. Instead the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft.

16 Reel Stepper Motors Types of Stepper Motors Permanent Magnet (PM) Motor structure (continued) These magnetized rotor poles provide an increased magnetic flux intensity Thus the PM motor has improved torque characteristics when compared with the VR type. Hybrid (HR) More expensive than the PM stepper motor but provides better performance with respect to step resolution, torque and speed Typical step angles for the HR stepper motor range from 3.60 to 0.90 ( steps per revolution). Combines the best features of both the PM and VR type stepper motors.

17 Reel Stepper Motors Types of Stepper Motors Hybrid (HR) Price/Performance (continued) The rotor is multi-toothed like the VR motor Contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations in the air gap. This further increases the detent, holding and dynamic torque characteristics of the motor when compared with both the VR and PM types. Most commonly used types Permanent magnet Normally several times less expensive Hybrid types

18 Reel Stepper Motors Calculation of the number of Steps Example # 1 Given: For Reel A the Fanfare is two rotations, The stepper motor rotates per pulse sent to the motor Starting position is 18 0 Ending position is 99 0 Find number or pulses to send to the reel stepper motor = 81 0 => 81 0 /1.8 0 = 45 2 rotations = x 2 = => /1.8 0 =400 Thus 445 pulses are needed Example # 2 Given: 1 rotation Fanfare, Start: 36 0, End: Find number or pulses to send to the reel stepper motor

19 Directional/Speed Controlled Direct Current Motors Types of control Overview Rotational Direction The polarity of the applied voltage controls rotational direction Some uses: Stair Lifts / Small Elevators Electric Wheel Chairs Automotive starters Etc Rotational Speed The speed of rotation is controlled by the voltage across the motor Some uses: Stair Lifts / Small Elevators Electric Wheel Chairs Any other situations that require variable speed motor action

20 Directional/Speed Controlled Direct Current Motors Types of control Overview Controls Both rotational direction and rotational speed can be controlled by various methods. Such as: Switches Microcontroller outputs Etc Simple Speed Control Circuit LED will substitute for a DC motor First circuit will use a potentiometer to control a transistor which will supply varying voltage levels across the LED substitute for a DC Motor

21 Directional/Speed Controlled Direct Current Motors Types of control Simple Speed Control Circuit LED will substitute for a DC motor First circuit will use a potentiometer to control a transistor which will supply varying voltage levels across the LED substitute for a DC Motor Build the Circuit shown Resisters 100k Red- Black-Yellow Transistor 2N3904

22 Directional/Speed Controlled Direct Current Motors Types of control Simple Speed Control Circuit LED will substitute for a DC motor Turn the knob on the potentiometer and verify that the LED changes brightness in response to a change in the position of the potentiometer s wiper terminal Micro-Controller as a motor On/Off Controller Build the Circuit Load a program that turns the LED on for 500 ms of ever second Run it Does it Work

23 Directional/Speed Controlled Direct Current Motors Types of control Simple Speed Control Circuit Micro-Controller as a motor On/Off Controller What s the difference between this and connecting an LED circuit to an I/O pin? BASIC Stamp I/O pins have limitations on how much current they can deliver. Transistors have limitations too, but they are much higher. Such a circuit can be used to drive small motors, solenoids, relays, etc. Through the use of a three-phase contactor a small microcontroller can control a large three-phase industrial motor

24 Directional/Speed Controlled Direct Current Motors Types of control Simple Speed Control Circuit Micro-Controller as a motor On/Off and Speed Controller Introducing the Digital Potentiometer The figure shows a pin map of the digital potentiometer you will use in this activity. Chip has 8 pins, four on each side, spaced for a breadboard The manufacturer places a reference notch on the plastic case so that you can tell the difference between pin 1 and pin 5. The reference notch can be used as a reference for the pins on the chip. The pin numbers on the chip count upwards, counterclockwise from the reference notch.

25 Directional/Speed Controlled Direct Current Motors Types of control Simple Speed Control Circuit Micro-Controller as a motor On/Off and Speed Controller Introducing the Digital Potentiometer Here is a summary of each of the AD5220 s pins and functions: 1. CLK The pin that receives clock pulses (low-high-low signals) to move the 1. CLK The pin that receives clock pulses (low-high-low signals) to move the wiper terminal. 2. U/D The pin that receives a high signal to make the wiper (W1) terminal move towards A1, and a low signal to make it move towards B1. This pin just sets the direction, the wiper terminal doesn t actually move until a pulse (a low high low signal) is sent to the CLK pin. 3. A1 The potentiometer s A terminal. 4. GND The ground connection. The ground on the Board of Education and BASIC Stamp HomeWork Board is the Vss terminal. 5. W1 The potentiometer s wiper (W) terminal. 6. B1 The potentiometer s B terminal. 7. CS The chip select pin. Apply a high signal to this pin, and the chip ignores all control signals sent to CLK and U/D. Apply a low signal to this pin, and it acts on any control signals it receives. 8. Vdd Connect to +5 V, which is Vdd on the Board of Education

26 Directional/Speed Controlled Direct Current Motors Types of control Simple Speed Control Circuit Micro-Controller as a motor On/Off and Speed Controller Introducing the Digital Potentiometer The AD5220 Part Datasheet Build the circuit We don t have enough AD5220 s - You will all use my circuit

27 Directional/Speed Controlled Direct Current Motors Types of control Simple Speed Control Circuit Micro-Controller as a motor On/Off and Speed Controller Programming Digital Potentiometer Control Imagine that the knob on the manual potentiometer from the previous exercise has 128 positions. Imagine also that the potentiometer is in the middle of its range of motion. That means you could rotate the knob one direction by 63 steps and the other direction by 64 steps. Let s say you turn the potentiometer s knob one step clockwise. The LED will get only slightly brighter. This would be the same as sending a high signal to the AD5220 s U/D pin and sending one pulse (high-low-high) to the CLK pin. HIGH 5 PULSOUT 6, 1

28 Directional/Speed Controlled Direct Current Motors Types of control Simple Speed Control Circuit Micro-Controller as a motor On/Off and Speed Controller Programming Digital Potentiometer Control Imagine next that you turn your manual potentiometer 3 steps counterclockwise. The LED will get a little bit dimmer. This would be the same as sending a low signal to the U/D pin on the AD5220 and sending three pulses to the CLK pin. LOW 5 FOR counter = 1 TO 3 PULSOUT 6, 1 PAUSE 1 NEXT

29 Directional/Speed Controlled Direct Types of control Simple Speed Control Circuit Micro-Controller as a Motor On/Off and Speed Controller Current Motors Programming Digital Potentiometer Control Enter the following program and take your turn to run it on the provided BASIC Stamp board What's a Microcontroller - DigitalPotUpDown.bs2 ' Sweep digital pot through values. ' {$STAMP BS2} ' {$PBASIC 2.5} DEBUG "Program Running!" counter VAR Byte DO LOW 5 FOR counter = 0 TO 127 PULSOUT 6, 1 PAUSE 10 NEXT HIGH 5 FOR counter = 0 TO 127 PULSOUT 6, 1 PAUSE 10 NEXT LOOP

30 Alternating Current Motors Characteristics Motors are usually much larger that DC motors They come with dedicated starter circuits in the devices that use them For larger commercial grades of motors there are third party controls for starting them i.e., Allen Bradley s SMC-Flex motor controller Some my be on a control network to simplify overall management of motors See the next slide

31 Alternating Current Motors