Post on 08-Apr-2018
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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