3pi智能小车：入门指南与示例项目

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本文档详细介绍了3pi国外的一款智能小车，它以其快速轻便和较低的自制难度而受到好评。3piRobot是一款基于Pololu平台设计的机器人，旨在提供一个易于上手且功能丰富的开发平台。以下将深入探讨3pi的主要组成部分、使用方法、编程以及两个示例项目。 1. **简介**: 3piRobotUser'sGuide介绍了3pi的基本概念，包括其设计目标，旨在帮助用户快速了解这款小车及其潜在用途。文档强调了其作为一款智能设备，具备电池管理、电机控制和传感器集成的能力。 2. **安全与操作指南**: 在章节3中，用户需要阅读重要的安全警告和操作注意事项，确保在使用过程中遵循正确的方法，防止任何可能的机械或电子伤害。 3. **快速入门**: - **所需设备**：章节4列出所需的硬件组件，如电源、配套软件等。 - **启动与预加载程序**：如何给3pi供电，并运行预装的演示程序，以便熟悉基本操作。 - **配件清单**：详细列出了附带的配件，如电池盒、电线等。 4. **工作原理**: - **电池与电源管理**：解释了电池的使用和3pi的节能模式。 - **电机与齿轮箱**：介绍了驱动3pi移动的电机和它们的配合装置，确保动力传输效率。 - **数字输入与传感器**：讨论了3pi的输入接口，包括如何连接外部传感器进行环境感知。 - **简化电路图**：提供了3pi的简化电路图，有助于理解内部结构。 5. **编程与控制**: - **C/C++库下载与安装**：指导用户如何获取并安装用于控制3pi的编程库，为后续开发打下基础。 - **编写简单程序**：通过实例展示如何编写和编译基础代码，使新手也能上手编程。 6. **示例项目**: - **线跟随** (Example Project #1): 包括算法介绍（如基本的线跟踪算法和高级PID控制），以及项目的具体实现步骤。 - **迷宫解决** (Example Project #2): 介绍如何设计机器人在迷宫中找到路径，涉及多文件编程、左右边墙检测以及改进代码性能的方法。 7. **引脚分配表**: 提供了3pi各个引脚的功能分配，方便用户根据需求连接外部设备或扩展功能。 8. **扩展信息**: - 可能的扩展选项，如增加额外传感器、电机或其他附件，以及如何在现有基础上进行定制。总结来说，3pi智能小车是一套完整的硬件和软件开发平台，适合初学者入门，也适合有一定经验的用户进行创新实验。通过学习和实践文档中的内容，用户能够掌握从基础设置到高级应用的技能，充分利用3pi进行各种有趣的机器人项目。

efficient than linear regulators, especially for high-current applications and large changes in voltage. Also,

switching regulators can convert low voltages into higher voltages! A key component of a switching regulator

is the inductor, which stores energy and smooths out current; on the 3pi, the inductor is the gray block near the

ball caster labeled “100”. A desktop computer power supply also uses switching regulators: peek through the

vent in the back of your computer and look for a donut-shaped piece with a coil of thick copper wire wrapped

around it – that’s the inductor.

The power management subsystem built into the 3pi is shown in this block diagram:

The voltage of 4 x AAA cells can vary between 3.5 – 5.5 V (and even to 6 V if alkalines are used). This means it’s not

possible simply to regulate the voltage up or down to get 5 V. Instead, in the 3pi, a switching regulator first boosts the

battery voltage up to 9.25 V (Vboost), and a linear regulator regulates Vboost back down to 5 V (VCC). Vboost

powers the motors and the IR LEDs in the line sensors, while VCC is used for the microcontroller and all digital

signals.

Using Vboost for the motors and sensors gives the 3pi three unique performance advantages over typical robots,

which use battery power directly:

• First, a higher voltage means more power for the motors, without requiring more current and a larger motor

driver.

• Second, since the voltage is regulated, the motors will run the same speed as the batteries drop from 5.5 down

to 3.5 V. You can take advantage of this when programming your 3pi, for example by calibrating a 90° turn

based on the amount of time that it takes.

• Third, at 9.25 V, all five of the IR LEDs can be powered in series so that they consume the lowest possible

amount of power. (Note that you can switch the LEDs on and off to save even more power.)

One other interesting thing about this power system is that instead of gradually running out of power like most robots,

the 3pi will operate at maximum performance until it suddenly shuts off. This can take you by surprise, so you might

want your 3pi to monitor its battery voltage.

A simple circuit for monitoring battery voltage is built in to the 3pi. Three resistors, shown in

the circuit at right, comprise a voltage divider that outputs a voltage equal to two-thirds of the

battery voltage, which will always be safely below the main microcontroller’s maximum

analog input voltage of 5 V. For example, at a battery voltage of 4.8 V, the battery voltage

monitor port ADC6 will be at a level of 3.2 V. Using 10-bit analog-to-digital conversion,

where 5 V is read as a value of 1023, 3.2 V is read as a value of 655. To convert it back to the

actual battery voltage, multiply this number by 5000 mV×3/2 and divide by 1023. This is

handled conveniently by the read_battery_millivolts() function (provided in the Pololu

AVR Library; see Section 6.a for more information), which averages ten samples and returns

the battery voltage in mV:

5. How Your 3pi Works Page 11 of 66

A typical small brushed DC motor, with no gearbox.

The 30:1 gearmotor used on the 3pi.

unsigned int read_battery_millivolts()

{

return readAverage(6,10)*5000L*3/2/1023;

}

5.c. Motors and Gearboxes

A motor is a machine that converts electrical energy to motion. There

are many different kinds of motors, but the most important for low-cost

robotics is the brushed DC motor, which is the type used on the 3pi. A

brushed DC motor typically has permanent magnets on the outside and

several electromagnetic coils mounted on the motor shaft (armature).

The “brushes” are sliding pieces of metal that switch the power from

one coil to the next as the shaft turns so that magnetic attraction

between the coil and the magnets continuously pulls the motor in the

same direction.

The primary values that describe a running motor are its speed, measured in rpm, and its torque, measured in kg·cm or

oz·in (pronounced “ounce-inches”). The units for torque show the dependence on both force and distance; for

example, a motor that produces 6 oz·in of torque can product a force of 6 oz. with a 1-inch lever arm, 3 oz. with a

2-inch lever, and so on. Multiplying the torque and speed (measured at the same time) give us the power delivered by

a motor. We see, therefore, that a motor with twice the speed and half the torque as another has the same power

output.

Every motor has a maximum speed (when no force is applied) and a maximum torque (when the motor is completely

stopped). We call these the free-running speed and the stall torque. Naturally, a motor uses the least current when no

force is applied to it, and the current drawn from the batteries goes up until it stalls, so the free-running current and

stall current are also important parameters characterizing the motor. The stall current is usually much higher than the

free-running current, as shown in the graph below:

Motor operation: current and speed vs. torque.

The free-running speed of a small DC motor is usually many thousands of rotations

per minute (rpm), much high than the speed we want the wheels of a robot to turn. A

gearbox is a system of gears that converts the high-speed, low-torque output of the

motor into a lower-speed, higher-torque output that is a much better suited for

driving a robot. The gear ratio used on the 3pi is 30:1, which means that for every 30

turns of the motor shaft, the output shaft turns once. This reduces the speed by a

factor of 30, and (ideally) increases the torque by a factor of 30. The resulting

parameters of the 3pi motors are summarized in this table:

5. How Your 3pi Works Page 12 of 66

Gear ratio: 30:1

Free-running speed: 700 rpm

Free-running current: 60 mA

Stall torque: 6 oz·in

Stall current: 540 mA

The two wheels of the 3pi each have a radius of 0.67 in, which means that the maximum force it can produce with two

motors when driving forward is 2×6/0.67 = 18 oz. The 3pi weighs about 7 oz with batteries, so the motors are strong

enough to lift the 3pi up a vertical slope or accelerate it at 2 g (twice the acceleration of gravity). The actual

performance is limited by the friction of the tires: on a steep enough slope, the wheels will slip before they stall – in

practice, this happens when the slope is around 30-40°.

Driving a motor with speed and direction control

One nice thing about a DC motor is that you can change the direction of rotation by switching the polarity of the

applied voltage. If you have a loose battery and motor, you can see this for yourself by making connections one way

and then turning the battery around to make the motor spin in reverse. Of course, you don’t want take the batteries out

of your 3pi and reverse them every time it needs to back up – instead, a special arrangement of four switches, called

an H-bridge, allows the motor to spin either backwards or forwards. Here is a diagram that shows how the H-bridge

works:

If switches 1 and 4 are closed (the center picture), current flows through the motor from left to right, and the motor

spins forward. Closing switches 2 and 3 causes the current to reverse direction and the motor to spin backward. An H-

bridge can be constructed with mechanical switches, but most robots, including the 3pi, use transistors to switch the

current electronically. The H-bridges for both motors on the 3pi are all built into a single motor driver chip, the

TB6612FNG, and output ports of the main microcontroller operate the switches through this chip. Here is a table

showing how output ports PD5 and PD6 on the microcontroller control the transistors of motor M1:

PD5 PD6 1 2 3 4 M1

0 0 off off off off off (coast)

0 1 off on on off forward

1 0 on off off on reverse

1 1 off off on on off (brake)

Motor M2 is controlled through the same logic by ports PD3 and PB3:

5. How Your 3pi Works Page 13 of 66

PWM speed control, showing gradual deceleration.

PD3 PB3 1 2 3 4 M2

0 0 off off off off off (coast)

0 1 off on on off forward

1 0 on off off on reverse

1 1 off off on on off (brake)

Speed control is achieved by rapidly switching the motor between two

states in the table. Suppose we keep PD6 high (at 5 V, also called a

logical “1”) and have PD5 alternate quickly between low (0 V or “0”)

and high. The motor driver will switch between the “forward” and

“brake” states, causing M1 to turn forward at a reduced speed. For

example, if PD6 is high two thirds of the time (a 67% duty cycle), then

M1 will turn at approximately 67% of its full speed. Since the motor

voltage is a series of pulses of varying width, this method of speed

control is called pulse-width modulation (PWM). An example series of

PWM pulses is shown in the graph at right: as the size of the pulses

decreases from 100% duty cycle down to 0%, the motor speed decreases

from full speed down to a stop.

In the 3pi, speed control is accomplished using special PWM outputs of the main microcontroller that are linked to the

internal timers Timer0 and Timer2. This means that you can set the PWM duty cycle of the two motors once, and the

hardware will continue to produce the PWM signal, in the background, without any further attention.

The set_motors() function in the Pololu AVR Library (see Section 6.a for more information) lets you set the duty

cycle, and it uses 8-bit precision: a value of 255 corresponds to 100% duty cycle. For example, to get 67% on M1 and

33% on M2, you would call

set_motors(171,84);

To get a slowly decreasing PWM sequence like the one shown in the graph, you would need to write a loop that

gradually decreases the motor speed over time.

Turning with a differential drive

The 3pi has an independent motor and wheel on each side, which enables a method of locomotion called differential

drive. It is also known as a “tank drive” since this is how a tank drives. It is completely unlike the steering system of

automobile, which uses a single drive motor and steerable front wheels. Turning with a differential drive is

accomplished by running the two motors at different speeds. In the previous set_motors() example, the left wheel

will spin faster than the right, driving the robot forward and to the right. The difference in speeds determines how

sharp the turn will be, and spinning in place can be accomplished by running one motor forward and one backward.

Spinning is an especially effective maneuver for a round robot, and you won’t have to worry about parallel parking!

5. How Your 3pi Works Page 14 of 66

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