Robot powered by solar cells. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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Childhood leaves a trace in each of us, regardless of age; it is most often associated with a love of toys.

Apparently, the love for such toys as robots took possession of us later under the influence of the general interest in space exploration, however, the more reasons that encourage us to be addicted to robots, the better. This chapter provides an opportunity to meet a charming little robot friend named Harvey. It's a lot of fun to play with it, but it's just as fun to make it yourself.

While most robots are highly capable, Harvey is less prominent in this regard. He is a straightforward individual with one goal: to follow the white line. In fact, he will tirelessly follow the intended path around the globe and return back. In addition, it "feeds" from the sun.

Robot control

Any robot must have mobility, i.e., move from place to place, as well as navigational abilities in the process of movement.

These two distinct but related requirements are met by two separate devices. The first controls the mechanical movement of the robot. Servo mechanisms are used for this.

A servo is a mechanical part of a robot, similar to human muscles. Harvey requires two servo systems: one to move forward (like a car engine), the other to control movement.

The joint operation of these two systems is not always easy to ensure. The problem is solved in two ways. In the first of them, both functions are combined into one. Let us turn to Fig. 1 for an explanation. XNUMX.

Fig.

Active Motion Control System

To move the trolley (Harvey's robot), the easiest way is to put the drive wheels on the axle and rotate it. Designed for this device invented a long time ago, they include chain, V-belt and gear drives, direct drive (from the motor). When both wheels rotate at the same speed, the robot will move forward in a straight line (naturally, if both wheels have the same diameter). The speed of movement of the robot is proportional to the speed of rotation of the wheels.

Consider the case when the speeds of rotation of the wheels are not the same. This can be achieved by splitting the axle in half and providing each wheel with a separate drive. As before, the robot moves in a straight line if both wheels rotate at the same speed.

If the speed of rotation of one wheel, for example, the left one, decreases, the trolley will turn to the left. Why? The whole reason lies in the fact that a wheel rotating at a lower speed actually forms a fulcrum (even if it is moving), around which another wheel with a higher rotation speed moves. In practice, if the left wheel is completely stopped, then the cart will describe a small circle in place with a radius equal to the distance between the wheels.

Similarly, the slow rotation of the right wheel relative to the left causes the robot to turn to the right. In fact, the functions of two mechanisms are combined here in one. Separate change in the speed of rotation of the wheels provides not only the movement of the trolley, but also the control of the direction of movement.

In many robots, a short-term shutdown of the rotation of one or the other wheel is most often used, and thereby the necessary motion control is achieved. This principle of movement is accompanied by a slight shaking, however, if the time during which the wheel does not rotate is short enough, the jerks are smoothed out and the movement becomes relatively smooth.

Passive motion control

In the second method, the movement and control functions are separated. There is one fixed axle to ensure straight-line movement, and a front swivel steering wheel (or pair of wheels) is used to change direction. Driving is based on this principle.

Ris.2

When the swivel wheel is parallel to the driving wheels, the robot moves exactly forward (Fig. 2). Turn the wheel to the left and it will turn to the left, turn to the right and the robot will turn to the right, just like a car.

The advantage of this method is the presence of smooth control. The robot can turn gradually or all at once, while the rear wheels must never stop.

For reasons that will become clear later, this method was chosen to control Harvey's robot. In this case, the steering wheel is driven by a small electric motor.

ектронное авление

We have come to the next stage of creating a robot - a tracking control system. Without a certain amount of intelligence, Harvey would simply randomly "prowl" from side to side. Most often, motor control is a matter of electronics.

To "see" Harvey's white line, "eyes" are needed. Harvey's eyes are a pair of phototransistors Q1 and Q2 shown in fig. 3. A phototransistor is an ordinary transistor with the top of the case removed and the base illuminated by light. Light is usually focused on the p-n junction using a lens, which also serves as a cover for the transistor case.

Fig.3 (click to enlarge)

When light hits the base region, a collector current flows through the transistor proportional to the intensity of the light. In other words, the signal that would normally go to the base pin is now generated by the incident light. In most cases, which includes ours, the phototransistor has only two outputs, and there is no base output.

The phototransistors are connected to operational amplifiers (op-amps) according to the current-to-voltage converter circuit. As you know from the basics of electronics, an operational amplifier is a current amplifier.

The output voltage of the amplifier depends on the current flowing through the inverting input. In a conventional circuit, the output signal is fed back to the inverting input, where the signal is summed. When the feedback current and the input current are equal, the amplifier is in a state of equilibrium. If a resistor (R2 in Fig. 3) is included in the feedback circuit, the voltage drop across this resistor will be proportional to the current flowing through it. This voltage, moreover, is proportional to the input signal and is taken from the pin at the output of the op-amp

In addition, the op-amp has another interesting feature that we took advantage of. This refers to the presence of differential inputs. Their peculiarity is that the signal applied to the non-inverting differential input will actually be subtracted from the signal at the inverting differential input. There is a sort of balancing act going on.

When the input currents on pins 2 and 3 are equal, they cancel each other out and no feedback current is needed to balance the circuit. Therefore, the voltage drop across resistor R2 is zero even in the presence of a signal.

The input currents are determined by the collector currents of the phototransistors Q1 and Q2. With equal irradiance of transistors with light, equal currents flow. Since it is impossible to find a pair of transistors with perfectly matching characteristics, a variable resistor VR1 is used in the circuit to eliminate the small difference between both "eyes" of Harvey.

The phototransistors are placed on a small panel like the one shown in Fig. 4, and are separated by a partition, on which a powerful infrared LED SD1 is located. Since the phototransistors are fenced off from this light source, its radiation does not fall directly on them.

If you bring the device closer to a reflective surface, everything changes. Light is reflected from the surface and detected by phototransistors. The amount of light reaching the phototransistors depends on the optical properties of the reflecting surface. A similar principle underlies Harvey's vision.

More light will be reflected from a mirror-like light surface than from a dark one. The white surface has the highest reflectivity, the reflectivity of all other colors decreases depending on their absorption coefficient. A black surface reflects the least amount of light.

Ris.4

You can analyze the principle of Harvey's action using a white line on a dark background.

First, let's place the robot exactly above the white line, so that the photosensors respond equally to IR radiation. Then there will be no voltage at the output of IC1. If you move the robot to the left or right, the corresponding phototransistor will move off the white line and therefore receive less light than the other. A voltage of one polarity or another will appear at the output of the operational amplifier.

Now we have a signal corresponding to the position of the robot relative to the white line when it moves along this "highway".

The output voltage of the op-amp is fed to two comparators, IC2 and IC3, connected in a two-threshold circuit. With this connection, both outputs are low potential if the input voltage lies within certain limits set by the divider on resistors R4, R5 and R6.

If the output voltage of the op-amp falls below the lower limit of the set range, the comparator on IC3 is triggered and its output is set to a high potential. The base current opens transistor Q4 and connects the steering motor to the negative terminal (-3 V) of the power supply. The motor, in turn, by changing the angle of rotation of the steering wheel, eliminates the displacement of the light-receiving surface of the phototransistors relative to the white line.

The same thing happens when the voltage at the output of the op-amp exceeds the upper limit. The comparator on IC2 fires and turns on transistor Q3. Now the steering motor is connected to the positive terminal (+3V) of the power supply and rotates in the opposite direction, once again compensating for the yaw. If the op-amp output voltage is zero, both transistors Q3 and Q4 are closed.

Making Robot Harvey

Now that we've completed our introduction to the basic systems of the robot, we've come to the long awaited step in building your very own Harvey robot. Building a robot will take a little more effort than most of the crafts in this book, especially if you use a variety of materials at hand. I must confess that I greatly simplified the matter.

I went to the nearest radio parts store on the first day of the New Year and bought (better than making it myself) a toy remote-controlled car with all the mechanical components already ready. I chose a broken car returned to the store after the holidays that was going to be thrown away. The toy did not have a transmitter unit, but all motors and motion control mechanism were serviceable and in working order. First of all, the purchase saved a lot of time and money.

Now that my conscience is clear and I confessed how I managed to accelerate the work on the creation of the robot, let's continue. First, take everything out of the car. It is necessary to leave only the chassis with wheels, the drive wheel motor and the steering device with its own motor. The car usually has a battery compartment. If the car is remote controlled, save the receiver and transmitter for your future homemade devices.

First of all, install the panel with phototransistors and LED at the bottom and in front of the car chassis. From a piece of thick dark plastic, I cut out a panel, the shape of which is shown in fig. 4.

If desired, you can install phototransistors and LED directly on the chassis of the car, while there must be sufficient clearance between the lowest point of the chassis and obstacles that may be encountered on the way. Also, keep in mind that the more you move the photodetector forward, the more sensitive it will be to small changes in road conditions (offset from the white line). If you want to compromise between the speed of the robot and the smoothness of its movement, install phototransistors closer to the drive wheels.

Ris.5

Ris.6

Don't forget to shield the phototransistors from the LED. You can use a small piece of opaque plastic or paper as a damper.

The next step is to assemble the control circuit. As in the case of most of the devices described in this book, it is assembled using a printed circuit board, the schematic of which is shown in fig. 5, and the placement of parts - in fig. 6.

Check the connection of all power sources. Do not take the time for this, otherwise the robot will work unstably. The drive wheel and steering motors are protected by RC chains (R9, C8 and R10, C9 respectively).

After mounting the radio components on the board, insert it into place of the radio control receiver board. For final assembly, secure the phototransistor leads as far as possible from the connection leads to the motors. IC1 has a very high gain and can easily amplify interference signals. If you need to deal with noise, use shielded wire to connect the phototransistors.

There is enough space in the battery compartment for the batteries that power Harvey, but their inclusion must be changed in accordance with the diagram, by making a tap from the connection point of the two batteries. Use the switch provided in the electrical circuit of the toy car.

The robot requires a set of batteries with a total voltage of 9 V. Therefore, the free space in the battery compartment can be used to place other circuit components, some of which are discussed below.

Harvey health check

After making sure that the installation is correct, you can proceed to the first test of the robot's performance. With the power switch turned off, place four nickel-cadmium rechargeable batteries in the battery compartment. After turning on the toggle switch, the robot should move forward and turn.

Carefully check the nature of the movement of the robot. Steering can be tested by shining a flashlight first on one phototransistor, then on the other. If the direction of rotation of any motor is incorrect, reverse the polarity of the connection of its outputs.

Now check the work of Harvey's robot on a circle described by a white stripe, drawn best on a black background. The radius of the circle must not be less than the turning radius of the steering wheel.

After placing Harvey on the track strip, turn on the power and follow the movement of the robot.

Electricity source

In the design of the Harvey robot, essentially two optoelectronic systems are used, which differ in the principle of operation. We have already dealt with one of them ("vision" of the robot); its operation is provided by photosensitive elements (phototransistors) that control the current of the steering motor.

Ris.7

Another optoelectronic system of the robot is a solar battery that keeps the batteries charged. It's hard to believe, but Harvey "eats" a very small amount of electricity. In fact, a fully charged set of rechargeable batteries will last for approximately 1 hour of autonomous operation. After that, the robot must be illuminated to resume operation. If Harvey is in the sun, he will recharge while moving.

Ris.8

Ris.9

To meet his needs, only 12 solar cells are needed. While any cell that generates 80mA or more can be used, I have found two sizes to be the most suitable.

A battery of elements of the first standard size, shown in fig. 8 is made of three round elements divided into four parts; these parts are connected in series with the arrangement shown in the figure. The result is a battery of elements arranged in the form of three circles, similar to a "ladybug".

Harvey's more solid appearance is obtained by using 12 crescent-shaped elements arranged in a line as shown in fig. 9. The robot becomes like an insect (centipede or worm) and glides as it moves.

Of course, you can make a battery of any other configuration. You can even make interchangeable lids for the robot, allowing it to express itself in more ways.

It must be remembered: the lower the output current of the solar cell, the longer the batteries will be charged. If you use good enough cells, be careful not to overcharge the batteries. See chap. 10, which discusses nickel-cadmium batteries and their characteristics.

Additional robot equipment

There are many ways to further modify the robot. For example, a robot can look spectacular when equipped with a pair of blinking glowing "eyes" (not to be confused with true photosensitive "eyes"). A robot can be "taught" to make sounds. A number of microcircuits are commercially available that generate sounds in a wide range.

Now that Harvey (or Harrietta) is ready, it's time for the fun. And familiarity with robotics!

Author: Byers T.

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