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

Documents

Lec4 Motor Control