Actuator

Introduction:

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As per the TAMUBot version 3.0 design the Parker01 robot uses 4 geared DC carbon-brush motors as actuators.
Each actuator consists of an assembly of a single motor and a planetary gearbox.

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Parts List:

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The parts list specified here is specific to the TAMUBot (Parker01) robot.

    1. IG32GM DC carbon motor
            Specifications
                    24 volt operating voltage
                   1/27 reduction ratio

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Data Sheets and Ordering Information:

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The parts are listed on the website for the manufacturer Shayang Ye Industrial Co. Ltd.
The page listing the planetary geared motors can be found at this link: Planetary Geared Motors

The data sheets for the Planetary Gear and the DC motor used are given in the image below.
The PDF can be found here and here respectively (From Shayang Ye Industral Co. Ltd.)

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    Planetary Gear:





    Pre-assembled motor and gear.


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Features of Component:

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The motor is 04 type (24 volts)

As listed above its features are:

Rated Torque = 130 g-cm (gramforce*centimeter) = 0.012748645 N.m
    Rated Torque = Torque that can be produced by the motor

Rated Speed = 6160 rpm = 645.0752 radians/second
    Rated Speed = Speed of motor when it operates at the given rated torque

Rated Current <= 500mA
    Rated Current = Current drawn while operating at rated torque

Rated Output = 8.5W
    Rated Output = Power output the motor can produce at rated torque


No Load Speed = 7300 rpm = 764.456 radians/second
No Load Current <= 80 mA

Weight = 110 g

With the 1/27 reduction due to the planetary gear we have a marked increase in rated torque and decrease in rotational speed.
This is expected as a decrease in the rotational speed with increase the amount of torque produced. (This is explained in the section "Working of Component")

This gives us the final attributes of the actuator.

Rated Speed = 229 rpm = 23.98088 radians/second

Rated Torque = 2.4Kg.cm = 2400g.cm = 0.2353596 Nm

Dimensions are given in the schematic in the above figures.

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Working of Component

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        Relationship between voltage, current, resistance, torque, speed and power

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As a motor operates, the spinning motion of the motor causes a voltage known as Counter-EMF (Electromotive Force).
The Counter-EMF causes a reduction in voltage that can be used by the motor i.e. it opposes the applied voltage
             Vutilized = Vapplied - Vc-emf   (1)
            
The resistance of the circuit is the resistance of the current carrying windings of the motor which is called Armature Resistance (Rarmature)
Thus using Ohm's Law we can find out the total current that runs through the motor
                I = Vtotal/Rarmature                (2)

The mechanical power produced by the motor can thus be given as:
                P = I*Vc-emf                         (3)


Power (Watts)  = Torque(Nm)  x Angular Speed (radians/second) (4)


Hence for the above Rated Torque and Rated Speed without the planetary gear attached we have a Power Output of 8.224 W
With the planetary gear attached we have a Power Output of  5.644 W




The above image is a motor characteristic graph:
   
The graph above shows the operational capabilities of the motor used.
The X axis shows torque measured in gf.cm. (Note: Motor characteristic graphs also commonly show current as the Y axis. This is fine as the motor current is directly proportional to the
torque
{1}, this giving us similar curves)

Now let us look at the motor characteristic graph in order of it Y axes:

     1.    Po = Power Output in Watts   
            We can see that the power output peaks at the middle of the graph, approximately at the intersection of the curve indicating current[I] in Amperes
            and the curve of the  angular speed[N] in rotations per minute.

This is synonymous to the above equation (4)

               Power (Watts)  = Torque(Nm)  x Angular Speed (radians/second)          (4)
since       Torque = Current*constant                                                            (from {1})
thus         Power= Current*constant  x Angular Speed

Hence we have a peak in Power output when the two curves intersect.

    2.    EF is the efficiency curve. This curve shows the relationship between the mechanical output and the electrical input.
           Note the peak of the EF curve is not the peak of the power output curve. The efficiency peak value in the graph corresponds to the peak 130g.cm as  given in the data sheet

    3.    I is the current curve. It shows the relationship of the current versus the torque. Notice that the start current without a load is 80mA and is shown on the Y axis of the
           graph
 
    4.   N is the rotations per minute curve.

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        Speed Control

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          This sections will go through the Pulse-Width-Modulation technique to control the speed of the above DC motor.

          In general the speed of a DC motor increases and decreases proportionally to the voltage and/or current supplied to it. (3)

         Pulse width modulation consists of square waves whose width is controlled. These pulses can be thought of as switching the power supply on and off while the how long the switch stays on or off can be thought of as the width of the each pulse.

        Intuitively we can think of it as increasing or decreasing the widths of the pulses results in a higher or lower average voltage supplied to the motor hence a faster or slower rotation speed.

Speed control and other control issues are handled in depth in the Motor Controller part of this guide.
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