LEGO Gears and Motors Wellesley College（乐高齿轮和马达韦尔斯利学院）.pdf

LEGO Gears and Motors Motors are devices that convert electrical energy into mechanical movement. For a motor of a given design and operating voltage (LEGO motors are designed to operate at 9 volts) there is a maximum rate at which electrical energy can enter the motor and, because energy is “conserved”, a maximum rate at which energy can be added to the mechanical system. Consider the example below where a motor is used to lift an elevator. The (potential) energy of the elevator is equal to mgh, where h is it’s height above the ground and m is its mass. Figure 1 If h changes at a rate that is too big, then motor stalls, because electrical energy is not flowing into the motor fast enough to keep up with the demands of the mechanical system. (You might say, why doesn’t the motor simply slow down, instead of coming to a complete stop? The answer lies in the fact that motors are designed to be most efficient when their output shaft spins at a certain, typically fairly high, speed. If they start slowing down, they become much less efficient, which leads to their slowing down even more. So things go from bad to worse quickly, and while a slightly overloaded engine will indeed spin slowly, stall conditions are quickly reached as the load increases.) The above example illustrates a very general principle that exists when using a motor to move something: There exists what we can call a: “Stall Avoidance” vs. speed tradeoff That is, to avoid stalling the motor, we must design our mechanical system so that the elevator is not required to rise “too quickly”. 1 The concept of tradeoffs is one of the “big ideas” of engineering. Understanding this particular tradeoff is the key to the Vehicle Race Challenge. A familiar example of this type of stall avoidance vs. speed tradeoff occurs when you cycle up a hill. If you try to climb too fast, t