For beginning robot builders and those of the Jerry Sanders Design Competition
The drive train of the robot is most likely the most important system that needs to work and work well in order for your robot to compete and be successful in any type of competition. The drive train consists of the motors, gear reduction, and transmission of the robot. It is the first thing you want to design when you are going to build your robot.
A Bit Of Practical Theory
Robots are basically classified in three categories when it comes to their drive trains: two-wheeled, four-wheeled, and other.
Two-wheeled robots have a set of wheels, one on each side of their robot. Because only two points touch the ground, these robots have a tendency to tip back and forth. Also, they will rear up when they start because before the wheels will move, the motors will actually turn the frame of the robot until it hits the ground, at which point the wheels will begin to move the robot. To compensate for this, many people decide to add castors or ball-transfers to their robot to make them less prone to tipping and more stable. However, there are several important factors to consider when designing a two-wheeled robot. Firstly, you want your weight to be centered over your wheels left to right. If they are not, the robot will turn to the side when you are trying to go forward and it will be very difficult to drive. Second, you want the weight to be balanced over the wheels from front to back. If it is not, your robot will lose a great deal of power as it scrapes the ground, or will lose traction due to castors. It also will have a very hard time climbing ramps, because it will have less traction and thus less pushing/driving/climbing force. Furthermore, a four-wheeled robot of the same weight as a two-wheeled robot should be able to out push the two-wheeled robot, because the two-wheeled robot always loses traction due to scraping along the ground or castors taking away traction. If you do use castors, you want to make sure that it is not possible for casters on both the front and the back cannot touch the ground at the same time, else this will prop your robot off of the ground. You do not generally want to use castors on the sides of your robot, because they will generally cause your robot to have an extremely difficult time navigating any bumps or ramps. Finally, if you do use castors, regardless of how well you place them, it will significantly increase the difficulty of driving your robot. Two-wheeled robots will not drive straight because they are by-nature non-stable. They will generally overturn while moving and spin in place when starting, even if you are not trying to. Two-wheeled robots have an extremely difficult time climbing ramps because of this. They start off almost straight, but will slowly turn towards one side of the ramp. About halfway up the ramp (if it is about a 4 foot ramp) they will end up facing perpendicular to the direction they want to go. When the driver attempts to turn them, they will begin to slide back down the ramp.
Four-wheeled robots are the best type of robot to start with (even though most people always make a two-wheeled robot first before they learn their lesson). There are a few important factors that should be considered when making a four-wheeled robot. Firstly, when your robot is to turn, unless you have a fancy driving mechanism, all of your tires will be in a line. If you want to turn right, your tires on the right side of the robot will turn slower or backwards, while the tires on the left will turn forward faster than the right, hence turning the robot. This is called “skid-steering”, sometimes called “tank-steering”. When a four-wheeled robot skid-steers, it is fighting a lot of friction. Thus, the placement of the wheels affects greatly how the robot will turn. If the wheels are set really wide apart, but are close together the robot will turn more easily because the angle between straight and the turning radius is small. Robots with narrow wheels but a long wheelbase (left to right not far, but front to back far) will have more difficulty turning. In fact, if the wheels are too far forward, the robot will probably not turn at all. A general rule of thumb is that you want your wheels to be about as wide apart as they are front-to-back. If they are wider apart, this is better (to a point where the robot will become tipsy and behave like a two wheeled robot.)
Other robots include 6-wheeled, 8-wheeled robots, treaded robots, and walking robots. Generally, these robots are more complex than a beginner would want to start off with. A few important notes are: with 6, 8, etc wheeled robots, you want the wheels to be placed more or less in a circle. Thus the two middle wheels of a 6-wheeled robot will be further away from the centerline than the other 4. This makes turning easier (and possible.) Treaded robots involve driving much like a tank with two or more treads (one or more per side). Tracked or treaded robots are very vulnerable to having their tracks ripped off, fall off, cut or jammed. The designer would want the treads to be protected from the side. Walking robots are extremely cool to watch, but unfortunately are usually difficult to design, slow, weak, and vulnerable to being hit.
Motors
The motors that are most often used in robots are permanent-magnet direct-current motors (PMDC). What this means is that the motor consists of an armature that spins in a magnetic field created by magnets inside of your motor. The motor operates on direct current, rather than alternating current. Any battery is a direct current source. A PMDC motor usually has two terminals to which the wires from your speed controller will attach. When a voltage is applied so that one terminal is positive and the other is negative, the motor will spin in one direction. If the voltage is switched so that the positive and negative sides go to the opposite terminals, the motor will spin in the other direction. Some more powerful motors may have 4 terminals, one set per brush (in which case you have two pairs of terminals, 2 “positive” and 2 “negative”, you need to determine which ones are which so that the motor will spin when these pairs have an applied voltage to each.)
Motors can be obtained from anywhere: power tools, car junkyards, surplus stores, websites, etc. The ideal motor is light and can create a lot of torque at high RPMs. Do a little online research before you decide on which motor to use. Make sure you are aware of what the operating voltage is of your motor (which is usually 6 volts to 24 volts for most robots.)
Every motor has its own properties; the most important are the torque constant and the voltage constant. If you do not know any of these, you can determine them by testing. Once you know these parameters, you will be able to complete the calculations listed later in this document.
If you want to use your motor at maximum efficiency, you will have to operate your robot at that voltage or higher. Operating at less voltage is fine, you will just not get the power out of your robot that you would if you operate it at its specified voltage. Operating at higher voltage than operating voltage is possible depending on the type of motors you have. Small motors, such as drill motors, cannot be run above their operating voltage. Larger motors are more tolerant to overvolting, but this means within reason (usually no more than 50% over specified.) This also means that the motor has a far greater chance of burning up, breaking, and will reduce the lifespan of the motor dramatically.) However, you will get increased power out of your motor, for as long as it lasts. If you are familiar with the motors that you are using and their specifications, then overvolting is a possibility. If you are unsure of the motor, do not have the specifications, or are using smaller motors, it is recommended that you do not overvolt your motors (unless you have an extra motor that you feel comfortable testing to destruction.) It is important to note that you do not want to stall an overvolted motor at all cost (more on this later.)
There are several things you will want to do to the motors you have in order to use them in your robot. Before you begin, you need to check if either terminal is connected to the can of the motor, use a multimeter to do this. If there is a connection (resistance is not either infinity or a very large value), you have two choices: 1) open up your motor and disconnect whatever it is that connects the leads to the can, 2) when mounting the motor to your body, you must use a non-conductive material in between you motor and your frame (unless your frame is wood, in which case you are OK as long as there is not metal that connects one motor to the other.) The reason for this is that if you have one motor driving forward, and one driving reverse, there will be a dead short across you speed controllers, which will turn them into nice balls of flame and smelly smoke. Now that you have checked this, you will want to observe the following tips. First, you will want to attach capacitors across the leads of your motors. Make sure you use ceramic capacitors and not electrolytic (look like a tootsie roll) capacitors (these will explode!) Solder one capacitor across the leads of your motor, and one from each lead to the can of the motor (the metal cylinder that the motor is enclosed in.) The purpose of this is to reduce electrical interference. If this is not done, you will probably notice that your radio receiver will act strange, this is because the motors are creating noise, which your receiver is interpreting as a signal.) Second, you will want to make sure your motor is rigidly mounted whenever you run it, either testing it, or inside your robot. Most motors have holes so that you can bolt your motor to your frame. If you don’t have this, then some type of clamping device needs to be found or made so that you can clamp your motor to your frame. (Even small motors will twist out of your hands if you turn them on while holding them, larger motors that operate at high RPMs will twist out of your hand and most likely break a finger or worse if you try to hold them, so please be careful!)
Gear Reduction
The gear reduction of your robot is what alters your motors speed to get the desired value of your wheel RPM. Motors that operate at high speed do not produce much torque. Unless your motor has a built in gearbox, you will need to gear your motor down. What this means is that you will be trading rotation speed for torque. In you halve your motor rotation speed, you will double your torque. If you cut your RPMs in four, you get four times the torque. The way you do this is by using chains and sprockets, belts and pulleys, or gears. Each has their strengths and weaknesses. Chains are the easiest to get and work with, and are efficient, but can fall off. Gears are very efficient, but are costly and must be precisely aligned. Belts and sprockets can be inefficient (though toothed belts can be efficient.) Belts can break or tear.
To create your gear reduction, you will be using different sized gears, sprockets, or belts. I will use sprocket terminology, which refers to teeth on a sprocket, but the diameter of a belt, or teeth on a gear could be used in their place. You will attach a sprocket that has fewer teeth (S1) to your motor to a sprocket that has more teeth on your wheels (S2). The gear reduction you end up with is S2/S1. (Ex: 10 tooth sprocket on motor, 18 tooth sprocket on wheel, reduction =1.8)
You can do a multistage reduction using multiple sprockets. Going from wheel (S1) to jackshaft (S2) (a jackshaft is basically something that allows sprockets to turn that is not the axle of your wheel) Connected to the jackshaft also, connected to S2 is another sprocket that spins at the same speed (S3) that is attached to your wheel/axle (S4). You total gear reduction is the MULTIPLE of the ratios (S2/S1)*(S4/S3) (Ex: 10 tooth on motor which is connected by chain to a 20 tooth on jackshaft, also on the shaft connected to the first sprocket (not by chain) is a 10 tooth sprocket which is connected by chain to a 30 tooth sprocket on the wheel. Total reduction = 2*3=6.)
Remember that altering the size of your wheels doesn’t change the gear reduction value, but changes the torque you can apply to the ground. Increasing your wheel diameter decreases the torque with which you have to move. Decreasing your wheel diameter increases the torque with which you have to move. Smaller wheels are lighter, but larger wheels can allow your robot to be invertible (able to operate upside down.) Furthermore, the larger your wheels, the greater your top speed.
Transmission
I will refer to the transmission as everything in your drive train that is not a motor or sprocket, pulley, or gear, of which belts and chains are discussed above.
Your robot must have axles. The wheels can spin on these axles so that the axles spin and the wheels are firmly attached to the axles. These are called live-axles. If your axles do not spin and your wheels spin around your axles, you have fixed-axles. Jackshafts are also axles and can be either live or fixed.
Fixed axles: Basically a fixed axle is a cylindrical piece of (metal), which is clamped, welded, duct taped, or whatever, onto the robot. The wheel will spin around this axle. Important items you will need are bearings or bushings: These are smooth pieces of metal that reduce friction so that the wheels will spin, rather than get stuck on the shaft/axle. The bearings/bushings are inserted into the wheels (or are already there depending on where you get your wheels.) The final gear, sprocket, or pulley has to be attached to your wheel. This is either accomplished by bolting through your wheel (not recommended) or having some type of hub that your wheel bolts to and that your sprockets et al. bolt to. Fixed axle advantages: less likely to break, disadvantages: More likely to not be aligned correctly, it can be difficult to get your wheels on and off fixed axles.
Live axles: A live axle is an axle that spins. It is necessary that all live axles be supported at two points along its length (else they would fall off). It is further recommended that one support be on one side of the wheel and the other support is on the other side of the wheel. A wheel that is supported with both supports on one side is “overhung” as is more prone to damage, wheels falling off, and more vulnerable to other robots. A wheel supported on both sides is “double hung” and is less prone to damage. Advantages to an overhung: the wheels can stick outside your robot, changing wheels can be quicker (because you will probably be changing them when they get knocked off :) ). Disadvantages are: if you compete against a powerful robot he will either knock your wheels off or bend them. Advantages to double hung are: protection, unless your robot receives catastrophic damage, your wheels should be okay. Disadvantages are: you have to have a bigger frame that extends to the sides of your robot.
Live axles are usually supported by either bearings or bushings. Specifically, self-aligning bearings or something called a “self-aligning pillow-block”. These are wonderful devices that you stick your axles through and bolt to your frame. These allows your axles to spin, even if you don’t have the pillow blocks perfectly aligned in your robot (which is likely, because most bushings and non-aligning bearings must be accurate to 0.5 degrees or less, which unless you have extensive machining experience, is not going to happen.) Furthermore, your axles often have something called a “keyway”. This is a square groove in the axle that you can use to your advantage. You buy a component that is “keyed”; say a keyed sprocket, which also has a keyway in it. Thus, you have two halves of a square that face each other. If you stick in a “key” which is a small piece of metal that is square in one plane and a rectangle in another into this groove, the axle and sprocket will spin together. (If you don’t understand this, consider this.) Draw a circle, then a larger circle around it that has the same center. Consider the inner circle your axle and the outer circle your sprocket. If you spin the axle, there is nothing really making the sprocket turn with it. Now look at where they meet (the edge of the smaller circle.) Draw a square so that half of the square is in the inner circle and half is in the outer circle. Remove this material from the axle and the sprocket (erase anything in this square). Now stick in a square piece of metal (shade it in). This locks the two together, so that one spins, they both spin. Keys are made by buying the appropriate size of “key stock” and cutting it to the length you need. . Note: there are devices called “set screws” which are small screws that are designed to be tightened against the shaft, so that they push on the shaft and pull on the part, keeping them from spinning. This is the worst method of attachment, as it relies on the shear strength of the setscrew to keep the wheel spinning with the axle. This type of an arrangement WILL FAIL and is why “set screws suck”. A setscrew should never be used in this arrangement by itself. They do have uses, such as keeping the key from falling out (the setscrew is tightened against the key, but in this arrangement the key takes the shear stress of the arrangement.)
Finally, you need to attach your wheels to your axles. Since you already have a sprocket or pulley attached to the axle (probably via a key and keyway) you could just bolt the wheel sideways through the wheel and sprocket. You could also make a hub or buy one, which has a keyway and sandwiches your wheel between its two halves when tightened. There are many methods of attaching wheels, these are just a few ideas. Finding the wheels you want often determines how you will attach them. Obviously you cannot bolt through pneumatic (air-filled) wheels, so you would need a hub or some type of external attachment. Solid wheels may not be able to use a hub and may not be able to withstand bolting, so you may have to come up with some other ideas.)
Putting It All Together
Now that you have an idea about how you are going to build your drive train, you need to do a little math to make it all work.
I expect the weight of my robot to be _________=weight.
Most likely, my robot will end up weighing _________=actualweight
(actualweight)=weight*1.1.
The maximum allowed my robot will weigh is ________=maxweight.
My robot could possibly get another robot on top of me ____ =other(1 yes, 0 no)
Therefore, the maximum weight my drive train would have to support would be:
=Totalweight=actualweight+other*maxweight (in pounds)
Figuring out stall torque of motor.
If you have the stall torque of your motor write it below, else figure it out using the following formulas:
Stall current=stall amperage=(my operating voltage)/(armature resistance.)
Armature resistance=1 volts/(amps pulled when I put 1 volt into the motor and fix it so the motor or its shaft cannot spin.)
The torque constant is equal to torque (in whatever units) per amp.
It can be found (inaccurately) by clamping a very stiff device to your motor shaft.
Using a spring connected to the device, measure the pulling force at 6 volts.
Alternately, have the device press against a scale and measure the reading.
Multiply to force you found by the distance the point of contact of the spring or scale
against the device is from the center of the motor shaft.
Stall torque=torque constant*stall current
Convert stall torque into pound*inches.
My stall torque is: __________=stall_torque. (in pound*inches)
The number of drive motors in my robot is: ______num_motors (all motors assumed to be the same)
Reality check:
The diameter of my wheels is __________=wheeldiam (in inches)
The radius of my wheels is ____________=wheelrad=wheeldiam/2 (in inches)
The minimum gear reduction I need is ___________=mingear
=(wheelrad*totalweight) / (stall_torque*num_motors)
I am planning to use a gear reduction of ________=mygear
IF mygear<mingear YOU SHOULD REDESIGN.
If mingear*1.3>=mygear>=mingear : “ I like to live on the bleeding edge and I
like fires!”
If 2*mingear>=mygear>=1.3*mingear: “Better safe than sorry”
If 2*mingear<mygear: “I can afford super motors or am overreducing”
If your gear reduction is inadequate; your motors will not be able to move your robot in some conditions on the course. You don’t want to stall any motor for very long. This causes excessive heat and large currents that can make your motor catch fire, your speed controllers catch fire, or your wires catch fire. If your speed controllers CONTINUOUS current rating is higher than the sum of all your motors stall currents connected to that speed controller, than your speed controller should be able to last forever with those motors (until the motors melt from the heat of course.)
Speed calculations:
My no load motor speed is _________=noload
=voltage constant*my operating voltage.
Note: This can be found using a tachometer.
My max speed is _________=maxspeed (MPH)
=noload*wheel_diam*3.14159 *(1/12)*60/(5280*mygear)
If maxspeed<3MPH : “I am pokey, syrup runs faster than my robot, I should
consider redesigning”
If 3MPH<maxspeed<6MPH: “Slow but steady wins the race”
If 6MPH<maxspeed<12MPH: “Yeah baby”
If 12MPH<maxspeed<15MPH: “I am hyperactive and routinely eat boxes of
sugar for breakfast, but I can control my robot, especially with great bursts
of speed when needed.”
If maxspeed>15MPH: you are probably OK, but make sure you practice some, I
doubt you’ll be driving at full throttle (and if you do, expect to get
airborne.)
References
For more information, there is a great calculator online at:
http://home1.gte.net/sgjudd/torquecalc.htm
Note: This information is used to help beginning robot builders, feel free to reproduce it if you feel it’s worthy. Sample values presented are skewed for competition in the Jerry Sanders Design Competition, where control is desired over top speed and the course is not flat. Always be careful when working with motors and high voltages, especially with high currents. Double check everything and then check it again. There is no such thing as a stupid question, and even if there is, better stupid than dead. Please consult the partner guide to this that shows how to hook up your electrical system to your motors. Build strong and kick some bot! - Stephen Bernsee