Tutorial: Choosing Wheels and Motors for your Rover

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    Choosing Wheels and Motors for your Rover

    1 Introduction

    Independent movement is one of the identifying features of a robot, particularly the ability to move freely through an environment. Wheeled robots, or rovers, are the most common of mobile robots and are capable of doing anything from vacuuming your home to exploring other planets (figure 1). If you decide to build a rover of your own you will have to decide on the size and configuration of wheels and drive motors that meet your needs. This tutorial will guide you through your options and give you handy formulas you can use to make good decisions quickly.

    Figure 1: Mars Spirit Rover

    Although you could power your rover with gasoline or compressed air or other more exotic means, for this tutorial we’ll only consider electric motors. The vast majority of rovers employ electric propulsion, and electric motors have features that make them very attractive for both beginning and advanced robot builders.

    1.1 About Electric Motors

    Electric motors move using magnetism. Opposing magnetic poles attract and alike poles repel, and a motor is basically a pair of magnets. One magnet is stationary (the “stator”) and the other can turn (the “rotor”). One of the magnets is an electromagnet which uses the flow of electricity to generate its magnetic force. This means the electromagnet can change its magnetic polarity (which end is south and which end is north) by reversing its electric polarity (the direction of flow of electric current). An electric motor operates by assuring that the polarity of the stator always properly attracts the magnetic poles of the rotor.

    It’s a little like a donkey and a carrot. The donkey (the rotor) is attracted to the carrot and moves towards it, but the carrot (the stator) always moves further away so the donkey never reaches it. The strength of the motor is determined by how badly the donkey wants the carrot (the strength of the magnetic attraction), and the top speed is determined by how quickly the carrot can move away from the donkey (how long it takes for the magnetic poles to reverse).

    If you power up an electric motor and clamp the shaft to stop it turning, the motor is said to be “stalled.” This means that the turning force – or torque – cannot overcome the force holding it still. Just because a stalled motor isn’t moving doesn’t mean it’s passive. In fact a stalled motor is burning up its maximum electrical load trying to move, and it’s exerting its maximum torque. The donkey, if you will, is straining on his lead trying his hardest to get to the perfectly positioned carrot (figure 2a).

    If you let the motor run free it will quickly reach its top speed. Turning full speed the motor has no extra force to use to do anything except move the rotor. In our analogy, the donkey is moving so fast that he’s keeping pace with the carrot and they are moving exactly in sync. There’s no force because there’s no attraction – the donkey is already at the carrot, and the carrot can’t move away from the donkey any faster (figure 2b).

    Figure 2: The Donkey & Carrot Analogy. (a) Stalled motor with the donkey pulling hard for the carrot but not being able to move; (b) Free-running motor with the donkey and carrot moving at the same rate but no force of attraction.

    These two features – the stall torque and the top speed – are the motor specs we’ll be looking at in this tutorial. They are also the easiest to find in catalogs or online (figure 3). Speed is given in revolutions per minute, or RPM, and torque is given in units of force multiplied by distance, like oz-in (“ounce-inches”) or kg-cm (“kilogram-centimeters”).

    Figure 3: Motor Specs as found on the Lynxmotion website.

    1.2 Optimal Motor Operation

    Ironically, when the motor is stalled or running at full speed it’s not doing any work. By “work” I mean moving something that resists motion, like pulling a full bucket of water from the bottom of a well. The heavier the bucket or the deeper the well, the more work you have to do to get it to the top. Likewise you want your rover to be able to do both: to power itself through obstacles and get around its environment in a reasonable time.

    A stalled motor is exerting a lot of force, but it’s not moving so it’s not doing any work – it’s basically making no headway. If rover motors are too weak then they run the risk of stalling, which is clearly no good, but even if they manage to keep the rover moving it will move very slowly considering the amount of force being used. This is like struggling all day to drag a heavy bucket up from a well – you’re spending too much effort over too little result (figure 4a).

    Likewise a motor running near its top speed is not capable of exerting much force, and so even though it may be moving quickly it is not doing very much work. This is like hauling water from the well a thimbleful at a time – it’s fast but not much gets done in the end (figure 4b). Also a rover designed to operate near its motor’s top speed will not perform well when the load changes, like going over an uneven surface.

    Figure 4: Work as defined by lifting water from a well. (a) Slowly lifting a heavy bucket does little work; (b) Quickly lifting a nearly empty bucket does little work.

    Ideally you want to design your rover to operate in the middle range of both the motor RPM and torque. In fact we're going to to shoot for 50% for each. For torque this means the motor can exert its target force at half of its maximum torque, and for speed this means the motor can get to its target speed at half its maximum RPM. With these trade offs we get the maximum amount of work done – the most water from the well in the least amount of time. It also means we have the best buffer against changing conditions. We can get bursts of higher force or higher speed from our rover depending if we need them given the circumstances.

    1.3 About Units and Precision

    Before doing anything that involves using real-world numbers or measurements and plugging them into formulas it’s critically important to pick a unit system. This way all the values that you use are measured in a consistent way. For example, a motor might have 110 ounce-inches of torque, but it also has 8 kilogram-centimeters of torque. Those are the really the same measurement but using different units. So if you have a formula where you have to plug in a number, clearly putting in 110 or 8 will give very different results. Which one should you use?

    For the purpose of this tutorial I’ll do every example in what’s called the “MKS” unit system – meters, kilograms, seconds. It’s easy to convert – you just need an online app like the ones you can find here (http://www.onlineconversion.com) – and you need to know the names and types of the various quantities in MKS. For our purposes you have to convert any values you have to the following units:
    Distance: meters (m)
    Mass: kilograms (kg)
    Time: seconds (s)
    Force: Newtons (N)
    Speed: meters per second (m/s)
    Torque: Newton-meters (Nm)
    I will also point out that the formulas in this tutorial are very rough. In many cases I have done a lot of rounding to make the formulas simpler, so don’t be surprised if computations are off by +/- 5%. That’s OK. None of your measurements are all that accurate and we’re not trying to do anything really exact – we’re just interested in ballpark figures. Your mileage may vary.

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    Attached Files
    • mt-1
    • mars_rover-small1
    • Nwheel
    • donkeys analogy
    • pitimesd