Lec4 Motor Control

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    Spike Cancellation contd Active Braking

    What happens if both direction bits are the same state, andthe enable bit is turned on? Effectively, both terminals ofmotor are connected together

    Motor acts as a generator, creating electricity out of kinetic

    energy already stored in the load connected to the shaft If none of the transistors is ON, then the motor is allowed to

    spin freely (Coasting) till it stops eventually

    Servo Motors Servo Motors

    The servo motor is actually an assembly

    of four things:

    a normal DC motor

    a gear reduction unit

    a position sensing device

    a feedback control circuit

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    Three wires of a servo: power, ground, control. The power source must be constantly applied

    Servo shaft typically does not rotate 3600 like a DC motor, but can only

    turn 2000 degrees or so back and forth.

    servo has its own power electronics, so very little power flows over thecontrol signal.

    Servo Motors cntd..

    Servo motors require a 5~6 V DC power supply. This can be taken fromthe control board power if the servo's aren't doing too much work.Otherwise, a separate power supply is recommended

    If available DC Voltage is higher, 1N4001 diodes can be used to dropthe voltage down to around 6V.

    Control Signal is pulse width modulated (PWM), here the positive-goingpulse duration tells the desired position of the servo shaft

    1.520 ms pulse width commands the center position of Futaba S148servo. A longer pulse commands a clockwise position from the centerposition, and a shorter pulse commands a counter-clockwise positionfrom the center position.

    The servo control pulse modulated frequency is 50Hz (20 ms period),which means that you can command the servo in every 20ms go here

    Servo Motor Control

    Positioncommands

    Servo Motor Control cntd.. Servo PWM method is different from the speed control PWM

    Speed control PWM: overall duty cycle (% on-time) determines the

    power/speed of the motor

    Servo PWM: length of the pulse is the desired position of the shaft

    920ms (full counterclockwise), 1520ms (center), 2120ms (full clockwise)

    control

    servo 1servo 25V

    Servo Motors on MIT Handy Board

    control

    9.6V

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    Winch Servo

    Winch servo rotates continuously

    Can be used for robots main drive motors

    Conversion

    Potentiometer is replaced by a pair of fixed resistors, which fixes thefeedback signal to center position irrespective of the shaft motion.

    Therefore, motor spins continuously for any given position command.

    This methods allows both speed and direction control. the farther thecontrol signal is away from the center position, the faster the motorturns

    Stepper Motors

    Stepper MotorsCoil identification

    Unipolar motors (easy to drive type) have four coils and are likely tohave 5 or 6 wires attached.

    Use an ohmmeter to identify the connections A-E1, B-E1,C-E2,D-E2R

    A-B, C-D

    2R A-C, A-D, B-C, B-D, E1-E2

    ,sequence. there are two sequences that will work. You can use eitherdrive sequence, but find the most reliable one for the application.

    E1 E2unipolar stepper motorcoil configurations

    Sequence 1: Energizes just one coil at atime (4step/cycle). As the current isswitched from coil to coil, the motor movesby one step (say 1.80). Reverse thesequence to reverse the direction of rotation

    Sequence 2: Energizes two coils at a time.There are still four steps to this cycle. Twice

    Stepper Motor Control

    ,and about 1.4 times more torque than insequence 1.

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    Stepper Motor Control cntd..

    Half stepping

    Batteries

    Important parameters to look for:

    Pack / cell voltage

    Capacity

    Discharge rate Weight

    Charging time

    Char in e ui ment and characteristics

    Battery types

    Lead-Acid (normal and sealed)

    Nickel Cadmium

    Nickel-metal-hydride

    Lithium-ion