Part 2 of 2: Electric Motors
How to choose the right electric motor for your prototyping needs
Brushed DC motors, servos and steppers all have different characteristics that make them useful for different types of prototypes. They all use DC power.
BY JEREMY LOSAW
In Part 1 of this series, I discussed the history and applications of electric motors. In this second and final part, I will discuss the different types common for prototyping and how to pick the right one.
The three common types of motors used in prototyping are brushed DC, servos and steppers. All have different characteristics that make them useful for different types of prototypes. All use DC power and are safe to use.
DC-brushed motors, found in a multitude of consumer products, are used when continuous motion is required. If you are driving a belt or a wheel or using a gearbox, this is the right motor to use.
DC-brushed motors run just by applying DC voltage to its terminals. Its speed can be varied by using different voltages in its rated range.
One drawback is that they draw too much power to be controlled directly from an Arduino or similar microcontroller, so they require either relays or a motor driver circuit to be used with a controller. Because they provide no inherent feedback about their position or speed, they have to be combined with other sensors if precise motion is desired.
These motors are used when reciprocating, non-continuous motion is required.
Inside a servo is a DC motor (brushed or brushless), a gearbox to increase the torque output, and a potentiometer (angle sensor). The potentiometer, a three-terminal resistor, compares the angle that the motor is controlled to and its actual position, and will move it until it gets to where it is commanded.
Most servos have a 180-degree range of motion for a maximum of 90 degrees from the neutral position in either direction. This must be taken into account when considering their use. They often have an arm or wheel attached to the drive spline to attach the linkage that is being controlled.
Servos are commonly used in RC cars to control the steering arms, and are great for driving similar types of linkages.
As their name suggests, stepper motors move in incremental steps. Typically, they have 200 steps per revolution, which gives them an accuracy of 1.8 degrees per step.
Steppers are actually a form of brushless motor, as they have the coils in the stator (non-rotating housing of the motor). The magnets on the rotor have small teeth that, when energized, align with corresponding teeth on the stator that allow for use in small steps.
With appropriate circuitry, steppers can be commanded to move with high accuracy. They are often used on the axes of 3D printers to drive the print head for high-accuracy prints.
A gearbox to a motor is like whipped cream on a pumpkin pie. You don’t always need it, but they tend to work better together.
Gearboxes magnify the torque output of an electric motor at the expense of speed. They are most commonly used with continuous rotation motors like brushed DC. (Servos already have them, and stepper motors are usually used for low-torque applications.)
The simplest form of a gearbox is a small gear attached to the shaft of a motor that drives a larger gear on the shaft of what is being driven. In this case, the torque increase is the number of teeth on the driven gear divided by the teeth on the motor gear.
How to choose
With so many different options, it can be difficult to choose the right motor.
The first step is to figure out the type of motion you need. If you need continuous rotation, a DC-brushed motor is probably best. If you need precision motion of a linkage or a reciprocal motion, a servos is the right choice. Steppers are used in situations where you need precision control of something that requires multiple rotations and relatively little torque.
The size of the motor can be a daunting decision. Fortunately, most motors available through electronics suppliers have associated data sheets that show the torque output and speed of each motor. Then you can calculate whether the specs will be right for your application based on the load and speed of what you need to move.
Electric motors usually make the most amount of torque at zero speed, so make sure they have enough torque capacity to drive what you need at the motor speed you require. A good data sheet will show a curve of torque vs. speed to help.
A gut check is to compare the size of the motor you are thinking of using with similar commercially available products. If you are prototyping an immersion blender, the motor you use will probably be roughly the same size as a blender you can find in the store.