Build Your Own Combat Robot phần 3 docx

UILDING a robot requires that you make many decisions—from the

type of sensors you’ll use to the color you’ll paint it. Some of these decisions are

trivial, while others will make or break your robot. One decision in the

make-or-break category is motors—not just deciding which ones you’ll use, but

determining how you’ll optimize their performance.

Most robots use the same class of motor—the permanent magnet direct current

(PMDC) motor. These commonly used motors are fairly low in cost and relatively

easy to control. Other types of electric motors are available, such as series-wound

field DC motors, stepper motors, and alternating current (AC) motors, but this

book will discuss only PMDC -type motors. If you want to learn more about other

types of motors, consult your local library or the Internet for that information.

Some combat robots use internal combustion motors, but they are more com￾monly used to power weapons than to drive the robots, largely because the inter￾nal combustion engine rotates only in one direction. If you are using an internal

combustion engine to drive the robot, your robot will require a transmission that

can switch into reverse or use a hydraulic motor drive system. With electric mo￾tors, however, the direction of the robot can be reversed without a transmission.

Many combat robots combine the two, using electric motors for driving the robot

system and internal combustion motors for driving the weapons. Another use for

internal combustion engines is to drive a hydraulic pump that drives the robot

and/or operates the weapons.

Since most robots use PMDC motors, most of the discussion in this chapter

will be focused on electric motors. At the end of this chapter is a short discussion

of internal combustion engines.

Electric Motor Basics

Because the robot’s speed, pushing capability, and power requirements are di￾rectly related to the motor performance, one of the most important things to un￾derstand as you design your new robot is how the motors will perform. In most

robot designs, the motors place the greatest constraints on the design.

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Direct current (DC) motors have two unique characteristics: the motor speed is

proportional to the voltage applied to the motor, and the output torque (that is,

the force producing rotation) from the motor is proportional to the amount of

current the motor is drawing from the batteries. In other words, the more voltage

you supply to the motor, the faster it will go; and the more torque you apply to the

motor, the more current it will draw.

Equations 1 and 2 show these simple relationships:

The units of Kv are RPM per volt and Kt are oz.-in. per amp (or in.-lb. per amp).

Torque is in oz.-in. and RPM is revolutions per minute. Kv is known as the motor￾speed constant, and Kt is known as the motor-torque constant.

These equations apply to the “ideal” motor. In reality, certain inefficiencies exist

in all motors that alter these relationships. Equation 1 shows that the motor speed

is not affected by the applied torque on the motor. But we all know through expe￾rience that the motor speed is affected by the applied motor torque—that is, they

slow down. All motors have a unique amount of internal resistance that results in

a voltage loss inside the motor. Thus, the net voltage the motor sees from the bat￾teries is proportionally reduced by the current flowing through the motor.

Equation 3 shows the effective voltage that the motor actually uses. Equation 4

shows the effective motor speed.

Where Vin is the battery voltage in volts, Iin is the current draw from the motor in

amps, R is the internal resistance of the motor in ohms, and Vmotor is the effective mo￾tor voltage in volts. It can easily be seen in Equation 4 that as the current increases

(by increasing the applied torque), the net voltage decreases, thus decreasing the

motor speed. But speed is still proportional to the applied voltage to the motor.

With all motors, a minimum amount of energy is needed just to get the motor to

start turning. This energy has to overcome several internal “frictional” losses. A

minimum amount of current is required to start the motor turning. Once this

threshold is reached, the motor starts spinning and it will rapidly jump up to

the maximum speed based on the applied voltage. When nothing is attached to the

output shaft, this condition is known as the no-load speed and this current is

known as the no-load current. Equation 5 shows the actual torque as a function of

the current draw, where I0 is the no-load current in amps. Note that the motor de￾livers no torque at the no-load condition. Another interesting thing to note here is

Chapter 4: Motor Selection and Performance 63

4.1

4.2

4.3

rpm K V K (V I R) = = v v motor in in − 4.4

64 Build Your Own Combat Robot

that by looking at Equation 4, the voltage must also exceed the no-load current

multiplied by the internal resistance for the motor to start turning.

Some motors advertise their no-load speed and not their no-load current. If the

motor’s specifications list the internal resistance of the motor, the no-load current

can be determined from equation 4.

With these equations, as well as the gear ratio, wheel size, and coefficient of

friction between wheels and floor, you can determine how fast the robot will move

and how much pushing force the robot will have. (How you actually determine

this will be explained in Chapter 6.) If you want the robot to go faster, you can ei￾ther run the motors at a higher voltage or choose a lower gear reduction in the

drive system.

Equation 5 is an important equation to know and understand, because it will

have a direct effect on the type and size of the batteries that you will need. By rear￾ranging this equation, the current draw requirements from your batteries can be

determined. Equation 6 shows this new relationship.

For any given torque or pushing force, the battery current requirements can be

calculated. For worst-case situations, stalling the motors will draw the maximum

current from the batteries. Equation 7 shows how to calculate the stall current,

where Istall is the stall current in amps. The batteries should be sized to be able to de￾liver this amount of current. Batteries that deliver less current will still work, but

you won’t get the full performance potential of the motors. Some builders pur￾posely undersize the battery to limit the current and help the motors and electron￾ics survive, and others do this simply because they have run out of weight

allowance. For some motors, the stall current can be several hundreds of amps.

Another set of relationships that needs to be considered is the overall power being

supplied by the batteries and generated by the motor. The input power, Pin, to the

motor is shown in equation 8. Note that it is highly dependent on the current draw

from the motor. The output power, Pout, is shown in mechanical form in equation 9

and in electrical from in equation 10. Motor efficiency is shown in equation 11.

The standard unit of power is watts.

4.5

4.6

4.7

4.8

4.9

4.10

The output power is always less than the input power. The difference between

the two is the amount of heat that will be generated due to electrical and frictional

losses. It is best to design and operate your robot in the highest efficiency range to

minimize the motor heating. If the motor is able to handle the heat build-up, it

might be best to design the robot (or weapon) to be operated at a higher percent￾age of the motor’s maximum power (to keep the motor as light as possible). For

example, a motor that is used to recharge a spring-type weapon might be fine if

operated at near-stall load for just a few seconds at a time. The maximum amount

of heat is generated when the motor is stalled. A motor can tolerate this kind of

heat for short periods of time only, and it will become permanently damaged if it’s

stalled for too long a period of time. This heat is generated in the armature wind￾ings and the brushes, components that are hard to cool by conduction.

Figure 4-1 shows a typical motor performance chart. These charts are usually

obtained from the motor manufacturer, or a similar chart can be created if you

know the motor constants. The motor shown in Figure 4-1 is an 18-volt Johnson

Electric motor model HC785LP-C07/8, which can be found in some cordless

drills. The constants for this motor are shown in Table 4-1. This motor is dis￾cussed here as an example motor to describe how all of the motor constants relate

to each other and how they affect the motor performance.

Figure 4-1 graphically displays how the motor speed decreases as the motor

torque increases and how the motor current increases as the applied torque on the

motor increases. For this particular motor, maximum efficiency is approximately

Chapter 4: Motor Selection and Performance 65

4.11

FIGURE 4-1

Typical motor

performance

curves.