Build Your Own Combat Robot phần 5 pdf

are called the Field Effect Transistor (FET) and the Metal Oxide Semiconductor

Field Effect Transistor (MOSFET). For the following discussions, FET will be

used as a generic term to represent both MOSFETs and FETs.

Field Effect Transistor

An FET works something like a semiconductor implementation of a relay. An FET

has two leads, known as the source and the drain, connected to a channel of semi￾conductor material. The composition of the material is such that current cannot

normally flow through it. A third lead, called the gate, is connected to a conductive

electrode that lies on top of the semiconductor junction but is insulated from it by a

thin non-conducting layer. When voltage is applied to the third electrode, it creates

an electric field that rearranges the electrons in the semiconductor junction. With

the field present, current is able to flow between the source and drain pins. When

the gate is driven to a low voltage, the electric field reverses and current is unable to

flow. The FET acts as a voltage-controlled switch, where an applied voltage to the

gate will control the current flow between the drain and source.

The layer of insulation between the gate and the source/drain channel must be

very thin for sufficient field strength to reach from the gate into the semiconductor

channel. This thinness makes the FET vulnerable to being damaged by too high a

voltage. If the voltage between either the drain or source and the gate exceeds the

breakdown voltage of the insulation layer, it will punch a hole through the layer

and short the gate to the motor or battery circuit. This can be caused by connect￾ing the FET up to too high a voltage, or simply by zapping the FET circuit with

static electricity. You should be careful when handling FETs and attached elec￾tronics to avoid accidentally discharging static electricity into them. It is also good

practice to use FETs with a voltage rating of twice the battery voltage you wish to

run your motors on to avoid the possibility of inductive spikes momentarily ex￾ceeding the FET breakdown rating.

When using an FET as a high-current PWM switch, it is important that you

switch the gate from the off voltage to the on voltage as quickly as possible. When at

an intermediary state, the FET will act as a resistor, conducting current inefficiently

and generating heat. Commercial PWM FET-based controllers use specialized

high-current driver chips to slam the FET gates from low to high voltage and back

as quickly as possible, minimizing the time spent in the lousy intermediary state.

The power that can be switched by an FET is fundamentally limited by heat

buildup. Even when fully in the on state, an FET has a slight resistance. Heat buildup

in the FET is proportional to the resistance of the semiconductor channel times the

square of the current flowing through it. The resistance of the semiconductor

channel increases with its temperature—so once an FET begins to overheat, its ef￾ficiency will drop; and if the heat cannot be sufficiently carried away by the envi￾ronment, it will generate more and more heat until it self-destructs. This is known as

thermal runaway. A FET’s power-switching capacity can be improved by removing

142 Build Your Own Combat Robot

the heat from it more quickly, either by providing airflow with cooling fans or by

attaching the FET to a large heat sink, or both.

The current capacity of an FET switching system can also be increased by wiring

multiple FETs together in parallel. Unlike relays, FETs can be switched on and off

in microseconds, so there is little possibility of one FET switching on before the

others and having to carry the entire current load by itself. FETs also automati￾cally load-share—because the resistance of an FET increases with temperature,

any FET that is carrying more current than the others will heat up and increase its

resistance, which will decrease its current share. Most high-powered commercial

electronic speed controllers use banks of multiple FETs wired in parallel to handle

high currents.

Bi-directional and variable-speed control of a motor can be accomplished with

a single bank of PWM-control FETs and a relay H-bridge for direction switching,

or with four banks of FETs arranged in an H-bridge. A purely solid-state control

with no relays is preferable but electronically more difficult to implement. Building

a reliable electronic controller is a surprisingly difficult task that often takes longer

to get to work than it did to put the rest of the robot together. The design and con￾struction of a radio controlled electronic speed controller is an involved project

that could warrant an entire book of its own.

Commercial Electronic Speed Controllers

Fortunately, several commercial off-the-shelf speed controller solutions are

readily available for the combat robot builder. Several companies make

FET-based motor controllers designed to interface directly to hobby R/C gear;

and many brands of commercial motor drivers and servo amps, with some engi￾neering work, can be adapted to run in combat robots. Building a motor control￾ler from scratch will usually end up costing you more money and more time than

buying an off-the-shelf model, so there is little reason for a robot builder to use

anything other than a pre-made motor control system.

Hobby Electronic Speed Controllers

Hobby ESCs were originally designed to control model race cars and boats. Early

R/C cars often had gas-powered engines, but refinements in electric motors and

the use of nickel-cadmium rechargeable batteries saw a switchover to electric

drive cars. The first systems used a standard R/C servo to turn a rheostat (a

high-power version of a potentiometer) in series with the drive motor to control

the speed of a race car. This system had a bad feature, in that the rheostat literally

“burned away” excess power in all settings except for full speed. Needless to say,

this did not help the racing life of the batteries.

There had to be a better way to conserve battery life and allow better control of

the motors. The result was the hobby electronic speed controller. All of the major

R/C system manufacturers are now producing various styles and capacities of

Chapter 7: Controlling Your Motors 143

ESCs. These controllers typically have only one or two FETs per leg of the

H-bridge, and most use a small extruded aluminum heat sink to dissipate the heat

from the FETs.

These controllers are intended for use in single-motor models. The initial units had

only forward speed as model boats and cars rarely ever had to reverse. Their technical

specifications were geared for the model racing hobby using NiCad batteries and

were written accordingly for non-technical people. To this day, most of the manufac￾turers still specify the “number of cells,” rather than the minimum and maximum

voltage requirements of a particular ESC, and use the term “number of windings” (on

the motor’s armature) as a measurement of current capacity. This can be confusing to

those who feel comfortable with the terms “volts” and “amps.”

Figure 7-12 shows a block diagram of a hobby electronic speed controller.

The number of cells designation literally means you can multiply that number

by 1.2 volts to get the actual minimum and maximum voltage requirements of the

particular ESC. You must remember that many of the cars used stacks of AA or

sub-C cells packaged in a shrink-wrapped plastic cover and were rated at about

9.6 volts (eight cells) maximum. Few cars used 10 cells to arrive at 12 volts, the basic

starting point for robot systems.

Many model boats use motors that draw relatively high currents, as do most

competition race cars. Most of the specifications for standard ESC’s speak of

“16-turn” windings for the DC permanent magnet motors as being the norm. This

144 Build Your Own Combat Robot

FIGURE 7-12

Block diagram of a

hobby electronic

speed controller.

Chapter 7: Controlling Your Motors 145

means that each of the poles of the motor’s armature has 16 turns of wire wrapped

around the pole. As the number of turns decreases, the diameter of the wires in￾creases, which results in a higher torque motor that has a higher current draw.

Current Capacity in Hobby ESCs True current capacity of a hobby ESC can be

difficult to determine; and the ratings given by the manufacturer are generally mis￾represented, since they reflect the instantaneous peak current capacity of the semi￾conductor material in the FETs rather than a realistic measure of the current the

controller can handle. Real current capacity of a hobby motor controller will be

determined largely by the builder’s ability to ensure that the little heat sink on the

speed controller stays cool enough to keep the electronics inside from cooking.

Since most hobby controllers are designed for low-average currents and with a

high airflow in mind, continuous high-current operation will likely cook a hobby

controller even with cooling fans installed.

Many of the cheaper hobby controllers are non-reversible, which means that

they’re designed for running the motor in one direction only. These controllers

should not be used in a combat robot. Hobby controllers that are reversible usu￾ally have a lower current rating in reverse than in forward—the FETs used in the

reverse-going side of the H-bridge have a lower current capacity than the for￾ward-going FETs. Many hobby controllers designed for R/C car or truck use have

a built-in reverse delay, so that, when the throttle goes from forward to reverse

quickly, the controller will brake the motor for a preset interval before starting to

reverse. In an R/C car, this helps controllability and lengthens the life of the motor

and geartrain; but in a combat robot, it can make smoothly controlled driving dif￾ficult—if not impossible.

Many hobby-type controllers have what is known as a battery eliminator cir￾cuit (BEC). The speed controller contains an internal 5-voltregulatorthat generates

the power for the electronics inside the speed controller. This power is then fed out

through the ESC with the intention being the ability to power the R/C receiver

from the main drive batteries. While this is a great help in an R/C car, where the

extra weight of a radio battery can make a real performance difference, the more

powerful drive motors of a competition robot create a lot more electrical noise

that can cause radio interference in the receiver. A robot builder can defeat the

BEC by popping the power pin out of the ESC’s servo connector and then use a

separate battery pack to supply power to the receiver.

Hobby ESCs in Combat Robotics Hobby ESCs have been proven to be usable in

small combat robots. These are usually seen in weight classes of 30 pounds and

under, but rarely in larger robots. Determining the appropriate hobby controller

can be a challenge. If you enter a larger hobby shop that specializes in model boat

and car racing, or check out catalogs or Web pages of some of the main suppliers,

you will find literally hundreds of models to choose from. Your first instinct may

be to talk with an employee for advice, but keep in mind this person might know a

lot about cars and/or boats but absolutely nothing about the use of ESCs in robots.