ACTUATORS - DC MOTORS TUTORIAL

ACTUATORS - DC MOTORS TUTORIAL
From the start, DC motors seem quite simple. Apply a voltage to both terminals, and weeeeeeee it spins. But what if you want to control which direction the motor spins? Correct, you reverse the wires. Now what if you want the motor to spin at half that speed? You would use less voltage. But how would you get a robot to do those things autonomously? How would you know what voltage a motor should get? Why not 50V instead of 12V? What about motor overheating? Operating motors can be much more complicated than you think.
Voltage
You probably know that DC motors are non-polarized - meaning that you can reverse voltage without any bad things happening. Typical DC motors are rated from about 6V-12V. The larger ones are often 24V or more. But for the purposes of a robot, you probably will stay in the 6V-12V range. So why do motors operate at different voltages? As we all know (or should), voltage is directly related to motor torque. More voltage, higher the torque. But don't go running your motor at 100V cause thats just not nice. A DC motor is rated at the voltage it is most efficient at running. If you apply too few volts, it just wont work. If you apply too much, it will overheat and the coils will melt. So the general rule is, try to apply as close to the rated voltage of the motor as you can. Also, although a 24V motor might be stronger, do you really want your robot to carry a 24V battery (which is heavier and bigger) around? My recommendation is do not surpass 12V motors unless you really really need the torque.

Current
As with all circuitry, you must pay attention to current. Too little, and it just won't work. Too much, and you have meltdown. When buying a motor, there are two current ratings you should pay attention to. The first is operating current. This is the average amount of current the motor is expected to draw under a typical torque. Multiply this number by the rated voltage and you will get the average power draw required to run the motor. The other current rating which you need to pay attention to is the stall current. This is when you power up the motor, but you put enough torque on it to force it to stop rotating. This is the maximum amount of current the motor will ever draw, and hence the maximum amount of power too. So you must design all control circuitry capable of handling this stall current. Also, if you plan to constantly run your motor, or run it higher than the rated voltage, it is wise to heat sink your motor to keep the coils from melting.
Power Rating
How high of a voltage can you over apply to a motor? Well, all motors are (or at least should be) rated at a certain wattage. Wattage is energy. Innefficieny of energy conversion directly relates to heat output. Too much heat, the motor coils melt. So the manufacturers of [higher quality] motors know how much wattage will cause motor failure, and post this on the motor spec sheets. Do experimental tests to see how much current your motor will draw at a desired voltage.
The equation is:

Power (watts) = Voltage * Current

Power Spikes
There is a special case for DC motors that change directions. To reverse the direction of the motor, you must also reverse the voltage. However the motor has a built up inductance and momentum which resists this voltage change. So for the short period of time it takes for the motor to reverse direction, there is a large power spike. The voltage will spike double the operating voltage. The current will go to around stall current. The moral of this is design your robot power regulation circuitry properly to handle any voltage spikes.

Torque
When buying a DC motor, there are two torque value ratings which you must pay attention to. The first is operating torque. This is the torque the motor was designed to give. Usually it is the listed torque value. The other rated value is stall torque. This is the torque required to stop the motor from rotating. You normally would want to design using only the operating torque value, but there are occasions when you want to know how far you can push your motor. If you are designing a wheeled robot, good torque means good acceleration. My personal rule is if you have 2 motors on your robot, make sure the stall torque on each is enough to lift the weight of your entire robot times your wheel radius. Always favor torque over velocity. Remember, as stated above, your torque ratings can change depending on the voltage applied. So if you need a little more torque to crush that cute kitten, going 20% above the rated motor voltage value is fairly safe (for you, not the kitten). Just remember that this is less efficient, and that you should heat sink your motor.
Velocity
Velocity is very complex when it comes to DC motors. The general rule is, motors run the most efficient when run at the highest possible speeds. Obviously however this is not possible. There are times we want our robot to run slowly. So first you want gearing - this way the motor can run fast, yet you can still get good torque out of it. Unfortunately gearing automatically reduces efficiency no higher than about 90%. So include a 90% speed and torque reduction for every gear meshing when you calculate gearing. For example, if you have 3 spur gears, therefore meshing together twice, you will get a 90% x 90% = 81% efficiency. The voltage and applied torque resistance obviously also affects speed.

Control Methods
The most important of DC motor control techniques is the H-Bridge.Refer previous post for making a motor driver circuit. After you have your H-Bridge hooked up to your motor, to determine your wheel velocity/position you must use an encoder. And lastly, you should read up on good DC Motor Braking methods.
Other Information
Place small microfarad capacitors across motor leads to extend motor life. This works really well with noisy and other el-cheapo motors, almost doubling motor life. However there is much less improvement using this technique with the more expensive higher end motors.

Step by step instructions for building Motor Driver circuit...................

Dual H-bridge Motor Driver - L293D IC

Motor Driver and H-bridge basics

Generally, even the simplest robot requires a motor to rotate a wheel or performs particular action. Since motors require more current then the microcontroller pin can typically generate, you need some type of a switch (Transistors, MOSFET, Relay etc.,) which can accept a small current, amplify it and generate a larger current, which further drives a motor. This entire process is done by what is known as a motor driver.
Motor driver is basically a current amplifier which takes a low-current signal from the microcontroller and gives out a proportionally higher current signal which can control and drive a motor. In most cases, a transistor can act as a switch and perform this task which drives the motor in a single direction.
Turning a motor ON and OFF requires only one switch to control a single motor in a single direction. What if you want your motor to reverse its direction? The simple answer is to reverse its polarity. This can be achieved by using four switches that are arranged in an intelligent manner such that the circuit not only drives the motor, but also controls its direction. Out of many, one of the most common and clever design is a H-bridge circuit where transistors are arranged in a shape that resembles the English alphabet "H".
As you can see in the image, the circuit has four switches A, B, C and D. Turning these switches ON and OFF can drive a motor in different ways.

1. Turning on Switches A and D makes the motor rotate clockwise
2. Turning on Switches B and C makes the motor rotate anti-clockwise
3. Turning on Switches A and B will stop the motor (Brakes)
4. Turning off all the switches gives the motor a free wheel drive
5. Lastly turning on A & C at the same time or B & D at the same time shorts your entire circuit. So, do not attempt this.

H-bridges can be built from scratch using relays, mosfets, field effect transistors (FET), bi-polar junction transistors (BJT), etc. But if your current requirement is not too high and all you need is a single package which does the job of driving a small DC motor in two directions, then all you need is a L293D IC. This single inexpensive package can interface not one, but two DC motors. L293, L293B and few other versions also does the same job, but pick the L293D version as this one has an inbuilt flyback diode which protects the driving transistors from voltage spikes that occur when the motor coil is turned off.

Introduction to L293D IC

L293D IC generally comes as a standard 16-pin DIP (dual-in line package). This motor driver IC can simultaneously control two small motors in either direction; forward and reverse with just 4 microcontroller pins (if you do not use enable pins). Some of the features (and drawbacks) of this IC are:

1. Output current capability is limited to 600mA per channel with peak output current limited to 1.2A (non-repetitive). This means you cannot drive bigger motors with this IC. However, most small motors used in hobby robotics should work. If you are unsure whether the IC can handle a particular motor, connect the IC to its circuit and run the motor with your finger on the IC. If it gets really hot, then beware... Also note the words "non-repetitive"; if the current output repeatedly reaches 1.2A, it might destroy the drive transistors.
2. Supply voltage can be as large as 36 Volts. This means you do not have to worry much about voltage regulation.
3. L293D has an enable facility which helps you enable the IC output pins. If an enable pin is set to logic high, then state of the inputs match the state of the outputs. If you pull this low, then the outputs will be turned off regardless of the input states
4. The datasheet also mentions an "over temperature protection" built into the IC. This means an internal sensor senses its internal temperature and stops driving the motors if the temperature crosses a set point
5. Another major feature of L293D is its internal clamp diodes. This flyback diode helps protect the driver IC from voltage spikes that occur when the motor coil is turned on and off (mostly when turned off)
6. The logical low in the IC is set to 1.5V. This means the pin is set high only if the voltage across the pin crosses 1.5V which makes it suitable for use in high frequency applications like switching applications (upto 5KHz)
7. Lastly, this integrated circuit not only drives DC motors, but can also be used to drive relay solenoids, stepper motors etc.

L293D Connections

The circuit shown to the right is the most basic implementation of L293D IC. There are 16 pins sticking out of this IC and we have to understand the functionality of each pin before implementing this in a circuit

1. Pin1 and Pin9 are "Enable" pins. They should be connected to +5V for the drivers to function. If they pulled low (GND), then the outputs will be turned off regardless of the input states, stopping the motors. If you have two spare pins in your microcontroller, connect these pins to the microcontroller, or just connect them to regulated positive 5 Volts.
2. Pin4, Pin5, Pin12 and Pin13 are ground pins which should ideally be connected to microcontroller's ground.
3. Pin2, Pin7, Pin10 and Pin15 are logic input pins. These are control pins which should be connected to microcontroller pins. Pin2 and Pin7 control the first motor (left); Pin10 and Pin15 control the second motor(right).
4. Pin3, Pin6, Pin11, and Pin14 are output pins. Tie Pin3 and Pin6 to the first motor, Pin11 and Pin14 to second motor
5. Pin16 powers the IC and it should be connected to regulated +5Volts
6. Pin8 powers the two motors and should be connected to positive lead of a secondary battery. As per the datasheet, supply voltage can be as high as 36 Volts.

Truth table

I have shown you where to connect the motors, battery and the microcontroller. But how do we control the direction of these motors? Let us take an example:
Suppose you need to control the left motor which is connected to Pin3 (O1) and Pin6 (O2). As mentioned above, we require three pins to control this motor - Pin1 (E1), Pin2 (I1) and Pin7 (I2). Here is the truth table representing the functionality of this motor driver.

Pin 1	Pin 2	Pin 7	Function
High	High	Low	Turn Anti-clockwise (Reverse)
High	Low	High	Turn clockwise (Forward)
High	High	High	Stop
High	Low	Low	Stop
Low	X	X	Stop

• High ~+5V, Low ~0V, 

• X=Either high or low (don't care)

In the above truth table you can observe that if Pin1 (E1) is low then the motor stops, irrespective of the states on Pin2 and Pin7. Hence it is essential to hold E1 high for the driver to function, or simply connect enable pins to positive 5 volts.
With Pin1 high, if Pin2 is set high and Pin7 is pulled low, then current flows from Pin2 to Pin7 driving the motor in anti-clockwise direction. If the states of Pin2 and Pin7 are flipped, then current flows from Pin7 to Pin2 driving the motor in clockwise direction.
The above concept holds true for other side of the IC too. Connect your motor to Pin11 and Pin14; Pin10 and Pin15 are input pins, and Pin9 (E2) enables the driver.

Parts required

Parts	Details	Quantity
L293D IC	Motor Driver IC	1
16 Pin Socket	8x2 DIP Socket	1
Perforated board	With 20x12 or more holes	1
Connecting wires	Any color of your choice	10-12 small pieces
7805	Linear Voltage Regulator	1
Male breakaway headers	40 pin single row headers	1
Solder	Lead / Lead free	50-100 gms
Solder Iron	Preferably temperature controlled	1

Building the circuit - Part I

Let's begin building the circuit. First, add 16 pin socket to the board as shown in the image. As usual, click on any image for an enlarged view with higher resolution. If the step requires soldering, then the soldered part is highlighted to help you with soldering.

Solder first and 16th pin of the socket to copper side of the board.

Add two 2-pin headers (two 2 pin headers) to the left of socket. Make sure there is at least a gap of two holes between socket and header.

Solder horizontal pins together.
The vertical two pins will be later used to connect the first motor. The other two pins are there in case we need to use this output for some other reason. If you do not need this, you can use a single 2-pin header and plug it vertically and individually solder the two pins to the board.

Add two more 2-pin headers to the right side of the socket with a minimum gap of two holes between socket and header. The gap makes it easier to plug other female headers once the board is completed.

Solder these two headers similar to the left headers

Now solder the bottom pins of two headers on either side to Pin6 and Pin11 of the IC, as shown in the image.

Add two more 2-pin headers to either side of the board. These two headers are input pins which are later connected to microcontroller.

Solder each pin of the header to the board, but take care not to solder them together. As shown in the image, solder the top two header pins on either side to Pin2 and Pin15 respectively.

We need two more 2-pin headers, one header for enable pins and other for +5V. Push the two 2-pin headers as pictured below. Let the first two vertical pins be in the same line as the socket pins.

Observe carefully how the soldering happens from here on if you are not sure what is happening. I can assure that you would have a complete working circuit if you follow the tutorial exactly.
The top two header pins are soldered together. One of the pin in bottom header is soldered to Pin1 of the socket. Click on the image to understand what I mean.

We will add two more pins for powering the circuit (we will call these two pins as Power pins to make soldering easier).

Solder each pin of the header to the board, but do not short them.

Add a 7805 voltage regulator such that the center pin is in line with power pins. I have bent the regulator to make it look compact (or I just like to keep it like that). If you do not want to bend it, keep it upright.
The top lead of regulator is input (Vin), middle lead is ground (Gnd) and the bottom lead is output (Vout) which gives a regulated +5volts.

Bend the regulator leads as shown. The middle pin almost touches one of the power pins. If the regulator lead (the middle one) is too long such that it touches both the power pins, cut it short. Vin should also be bent in the same fashion. Vout is bent such that it touches the headers above enable pins.




Add a red wire (or any color) to connect Pin9 to enable pin. Push one side of the wire next to Pin9 and the other side to a hole above the socket (as shown below). I have used a marker to mark the pins to avoid confusion.

Solder the pins and wires carefully as mentioned below:

1. Solder one side of the wire to Pin9 (Click to enlarge the image. The bottom highlighted part)
2. The other side of the wire will be soldered to second pin of enable header
3. Vout from regulator (bottom lead) is soldered to header above enable pins
4. Gnd from regulator (middle lead) is connected to left power pin (left pin if the copper side of board is on top)
5. Vin from regulator (top lead) is connected to right power pin

I have highlighted Vin in RED just to make sure you do not solder ground and power pins together. If you do connect, you will end up shorting the circuit and frying it.

Add a red wire running from positive of power pin to Pin8 of the socket.

Solder one end of red wire to positive of power pin and the other end to Pin8

Add another black wire to connect ground. Push one end below the ground of power pins and the other end next to Pin4 of the socket

Solder the black wire as shown here. One side connects ground of power pins and other side connects Pin4.

Cut and trim a small wire and place it in two holes next to Pin3 and Pin5 of the socket.

This wire connects output1 of L293D IC to header pin. Solder top side of the wire to Pin3 and bottom side of the wire to first output header pin on the left. (marked O1)

Add a slightly larger black wire next to previously added black wire such that the top end of the wire is in the same row of the black small wire, but the bottom end is two holes next to Pin7 of the socket.

Solder the top side of this wire to second pin of input header (marked I2) and solder the bottom end to Pin7 of the socket.

Add a similar wire to right side of the board as shown in the image. The top end of the wire should be pushed next to Pin14 and bottom end next to Pin10 of the socket. This connects input pin of IC to right input pin (marked I3)

Solder top end of the wire to header pin (marked I3) and bottom end to Pin10.

You are almost done!! Push the top end of a wire into a hole next to Pin14 and the bottom end next to Pin12 of the socket.

Solder the top end to Pin14 of the socket and bottom end to right output header pin (marked O4)

Since we need to power the components inside the IC, add a red wire which connects +5V to Pin16 of the socket. The two pins above enable pins are already connected to +5V and we can push one end of the wire to a hole next to that and other end next to Pin16.

Solder Pin16 to bottom end of the wire while top end of the wire is soldered to header pins above enable pins, which is further connected to Vout of regulator.

Lastly, solder all four ground pins together. Pin4, Pin5, Pin12 and Pin13 are all ground pins; solder them together.

DONE!! You have a completed (and hopefully working) L293D motor driver board which can control two motors.
As you can see in the image, I have drilled four holes (actually five including one below the regulator) and pushed four screws into it.

Whenever you implement it, always remember that enable pins are not connected. Either connect enable pins to your microcontroller pins and programmatically set it high, or use a jumper and connect each enable pin to the header just above it (which is connected to +5V)
Here is the implementation on one of my robots. You can also see that there is another small board which is also a motor driver, but the board is built using PCB etching method.

Here is the schematic I had developed. If anybody is interested in building a board using PCB Etching method, or any other method, please request in the forum and I will share the complete board designs.