Make: Electronics
Page 38
Figure 5-100. When the on/off switch at bottom-right opens, the relay connects its upper contacts. This causes the motor to run clockwise until its arm opens the lower limit switch. Limit switches avoid the overheating and possible damage that are likely when power is delivered to a motor that is prevented from turning.
Fundamentals
All about motors
Brushed DC motor
This is the oldest, simplest design for an electric motor, shown in very simplified form in Figure 5-101. Coils are attached to a shaft where they can interact with stationary magnets around them. The magnetic attraction turns the shaft a little, at which point the next coil on the shaft is energized to turn the shaft a little more, and then the next coil—and so on. To make this happen, electricity has to be fed into the coils by “brushes,” often consisting of soft carbon pads that conduct power to a hub, known as a commutator, divided into sections, each of which is connected to a separate coil.
This basic design has several advantages if we want to build a small motorized gadget, such as a miniature robot or even a model airplane:
Widely available
Low cost
Simple
Reliable
Will run in reverse when voltage reverses
In addition, brushed motors are often sold with reduction gearing built in. Such units are known as gearhead motors or gear motors. They free you from the need to use your own gears or belts to adjust the output speed yourself. You simply choose the motor that fits your specification.
DC stepper motor
This requires a controller, consisting of some electronics to tell the motor to rotate its shaft in small, discrete steps. The advantages of a stepper motor are:
Precise positioning of the shaft
Precise speed adjustment
Stepper motors are ideal for devices such as computer printers, where the paper has to roll up by a precise distance and the print head has to move laterally by an equally precise distance, but they are also useful in robots. If the motor is small enough to draw less than 200mA and will run on 12 volts or less, you can control it with pulses from a 555 timer. I’ll describe stepper motors in more detail in Experiment 33.
Servo motor
This is generally used in conjunction with a programmable microcontroller, which sends instructors to rotate the motor shaft to a specific position and then hold it there. I’ll mention servo motors when I introduce you to microcontrollers, but we won’t be dealing with them in detail.
Other types of motors exist, including brushless DC motors (which require a different type of controller and are found in computer disk drives and CD players), and AC motors (including synchronous motors, which synchronize their rotation with the frequency of AC voltage, and were used extensively in clocks, before clocks mostly became digital).
In this book, I’ll be talking mostly about brushed DC motors and DC stepper motors.
Figure 5-101. The basic principle of a simple DC motor. The commutator passes electricity through a coil, creating a magnetic field that interacts with a magnet around the motor. The coil turns, and the commutator turns with it, until the electric field through the coil is reversed. This causes the process to repeat. In reality, a motor is likely to have a commutator formed from multiple segments, connected with multiple coils. The principle, however, remains the same.
Take-home messages from this experiment include the following:
You can buy simple DC motors with reduction gearing built in, providing your choice of RPM. Literally hundreds of websites will sell you small motors for robotics projects.
When you reverse the voltage to a DC motor, the motor runs in reverse.
A DPDT relay can be wired so that when it closes its contacts, it reverses a power supply to a motor.
You can use two limit switches and a pair of diodes to stop a motor at two positions. In each of its stopping positions, the motor consumes no power and you won’t have the risk of it burning out.
What other projects can you imagine using this simple set of techniques?
Mechanical Power
In the United States, the turning force, or torque, of a motor is usually measured in pound-feet or ounce-inches. In Europe, the metric system is used to measure torque in dynes.
A pound-foot is easy to understand. Imagine a lever pivoted at one end, as shown in Figure 5-102. If the lever is one foot long, and you hang a one-pound weight at the end of it, the turning force is one pound-foot.
Figure 5-102. The rotational force created by a motor is known as “torque,” and in the United States it is measured in pound-feet (or ounce-inches, for small motors). In the metric system, torque is measured in dynes. Note that the torque created by a motor will vary according to the speed at which the motor is running.
Fundamentals
Wire gauges
If you’re going to power larger motors, or other components that take more current than LEDs or small relays, you really need to know about wire gauges. In particular, what’s the relationship between wire thickness and AWG (American Wire Gauge)? And what gauge of wire should you use for any given current?
You can find numerous charts and tables if you go online, but many of these sources contradict each other, especially on the topic of how much current is safe to run through each gauge of wire.
After making several comparisons (and testing some wire samples myself), I’ve compiled the table in Figure 5-103, which I recommend as a compromise. Note the following:
This table applies to solid-core copper wire.
For stranded wire, or copper that has been tinned (giving it a silver appearance), the number of ohms per foot will increase, the number of feet per ohm will decrease, and the maximum amperage will decrease, probably by around 20%.
Figure 5-103. American wire gauges (AWG) and their properties.
The maximum amperage assumes that the wire is insulated, preventing it from radiating heat as effectively as a bare conductor. I am also assuming that the wire is likely to be at least partially enclosed, inside a box or cabinet. At the amperages listed for each gauge of wire, you should expect the wire to become noticeably warm, and personally I would tend to use thicker wire instead of the maximums indicated in the table.
Most tables of this type only tell you the resistance of each gauge of wire in ohms per 1,000 feet. I have included that number but have also expressed the function the other way around, as the number of feet per ohm, as this doesn’t require you to do so much arithmetic with decimals.
Theory
Calculating voltage drop
Another fact that you often need to know is how much of a voltage drop a particular length of wire will introduce in a circuit. If you want to get maximum power from a motor, you don’t want to lose too much voltage in the wires that go to and from the motor.
Voltage drop is tricky, because it depends not only on the wire, but also on how heavily the circuit is loaded. Suppose that you are using 100 feet of 22-gauge wire, which has a resistance of about 1.5 ohms. If you attach it to a 12-volt battery and drive an LED and a series resistor offering a total effective resistance of about 1,200 ohms, the resistance of the wire is trivial by comparison. According to Ohm’s Law:
amps = volts / ohms
so the current through the circuit is only about 10mA.
Again, by Ohm’s Law:
volts = ohms × amps
so the wire with resistance of 1.5 ohms imposes a voltage drop of 1.5 × 0.01 = 0.015 volts.
Now suppose you’re running a motor. The coils in the motor create impedance, rather than resistance, but still if we measure how much current is going through the circuit, we can establish its effecti
ve resistance. Suppose the current is 1 amp. Repeating the second calculation:
volts = ohms × amps
So the voltage drop in the wire is now 1.5 × 1 = 1.5 volts! This is illustrated in Figure 5-104.
Bearing these factors in mind, I have compiled a table for you. I’ve rounded the numbers to just two digits, as variations in the wire that you use make any pretense of greater accuracy unrealistic.
To use this table, you need to know how much current is passing through your circuit. You can calculate it (by adding up all the resistances and dividing it into the voltage that you are applying) or you can simply measure the current with a meter. Just make sure that your units are consistent (all in ohms, amps, and volts, or milliohms, milliamps, and millivolts).
In the table, I have arbitrarily assumed a length of 10 feet of wire. Naturally you will have to make allowances for the actual length of wire in your circuit. The shorter the wire, the less the loss will be. A circuit with only 5 feet of wire, and the same amperage and voltage, will suffer half of the percentage loss shown in the table. A circuit with 15 feet of wire, and the same amperage and voltage, will suffer 1.5 times the percentage loss. So, to use the table:
1. Divide your length of wire by 10. (Make sure that you measure the length in feet.)
2. Use the result to multiply the number in the table.
The table also arbitrarily assumes that you have a 12-volt supply. Again, you will have to make allowances if you are using a different voltage. So, to use the table:
1. Divide 12 by the actual voltage of your power supply.
2. Use the result to multiply the number in the table.
I can summarize those two steps like this:
Percent voltage lost = P × (12 / V) × (L / 10)
where P is the number from the table, V is your power-supply voltage, and L is the length of your wire.
Figure 5-104. The voltage drop imposed by wiring will depend on the current and the resistance in the circuit. The drop will be greatest when the resistance of the circuit is low and the amperage is high.
Theory
Calculating voltage drop (continued)
This table shows the percent voltage lost in a circuit with 10-foot wire at 12 volts.
Wire Gauge
Amperes
1
2
3
4
5
6
7
8
9
10
10
0.08
0.17
0.25
0.33
0.42
0.50
0.58
0.67
0.75
0.83
12
0.13
0.27
0.40
0.53
0.66
0.80
0.93
1.1
1.2
1.3
14
0.21
0.42
0.63
0.84
1.1
1.3
1.4
1.5
1.9
2.1
16
0.33
0.67
1.0
1.3
1.7
2.0
2.3
2.7
3.0
3.4
18
0.53
1.1
1.6
2.1
2.7
3.2
3.7
4.3
4.8
5.3
20
0.85
1.7
2.6
3.4
4.3
5.1
6.0
6.8
7.7
8.5
22
1.3
2.7
4.0
5.4
6.7
8.1
9.4
11
12
13
24
2.1
4.3
6.4
8.6
11
13
15
17
19
21
26
3.4
6.8
10
14
17
20
24
27
31
34
28
5.4
11
16
22
27
32
38
43
49
54
30
8.6
17
26
34
43
52
60
69
77
86
Remember, though, that the wire resistance will be higher if you are using stranded copper wire or tinned copper wire, and this will increase the percentage of voltage lost.
Experiment 33: Moving in Steps
Time now to build something more sophisticated: a cart that orients itself toward a light source. I’m going to tell you all you need to get started on this project, but this time I won’t go all the way to the end in exhaustive detail. I want you to get into the habit of figuring out the details, improving on plans, and eventually inventing things for yourself.
You will need:
555 timers. Quantity: 8.
Trimmer potentiometer, 2K linear. Quantity: 2.
LEDs. Quantity: 4. If you get tired of using series resistors to protect LEDs in a 12-volt circuit, consider buying 12-volt LEDs such as Chicago Miniature 606-4302H1-12V, which contain their own resistors built in. However, the schematic in Figure 5-108 assumes that you will use regular 2V or 2.5V LEDs.
Stepper motor: Unipolar, four-phase, 12-volt. Parallax 27964 or similar, consuming 100mA maximum. Quantity: 2.
Photoresistors, ideally 500 to 3,000Ω range. Quantity: 2.
ULN2001A or ULN2003A Darlington arrays by STMicroelectronics. Quantity: 2.
CMOS octal or decade counter. Quantity: 2.
Various resistors and capacitors.
Exploring Your Motor
I’ve specified a unipolar, four-phase, 12-volt motor because this is a very common type. A typical sample is shown in Figure 5-105. If you can’t easily find the one that I’ve listed, you should feel safe in buying any other that has the same generic description. “Unipolar” means that you don’t have to switch the power supply from positive to negative and back to positive again, to run the motor. Four-phase means that the pulses that run the motor must be applied in sequence to four separate wires. Because you will be running your motor directly from 555 timers, the lower its power consumption, the better.