subject: Stepper-motor In Modern Hobbyist Projects Considerations Of Size [print this page] A stepper-motor is often used in projects where robustness is required. The devices simplicity ensures a good chance of it working even in some pretty hostile arenas for example where dust or sand is present in quantity, or where there is a risk of water ingress into a project. Its also relatively cheap, which means the hobbyist is able to instil some movement and positioning control into his or her project without spending over budget.
There is, though, a problem with the use of the stepper-motor. In fact there are two. One is that the motor itself limits the designed in terms of size and the other, that theres a torque limitation as well.
Lets attend to the size issue first.
The stepper-motor is essentially a metal gear cog surrounded by electro magnetic coils. The cog is moved by switching different coils on and off in a sequence. Every time the cog moves, it has performed one step in its complete sequence hence the name of the motor.
The more complex the dance you want your cog to be able to perform, the more cogs you need on the wheel. This is also true for positioningaccuracy. If you want to be able to accurately position something, such as a laser pointer, using a stepper-motor cog, you need lots of small teeth rather than four or six big ones.
When you make a big cog tooth walk through a step, its size and weight causes the wheel to run through the optimum point of magnetism and out the other side. The activated magnet then drags the point of the cog back to the pole position, causing a perceptible wobble in the motion of the wheel.
As this is unsatisfactory for accurate positioning requirements, the hobbyist must then start to think about introducing more teeth and potentially more magnetic coils into the motor itself. A stepper motor with more coils and teeth is able to undergo a much more precise regime of steps, enabling the designer to incorporate a high level of accuracy into his or her device.
The problem here is one of scale. As the cog gains more teeth, the whole motor starts to creep up in size. Because, which you can have finer teeth and more magnets to create accuracy, you still need enough of a demarcation between each tooth to ensure that an active magnetic attracts and holds a tooth, rather than constantly pulling the next in a series of tiny teeth through an endless slipped step. Beyond a certain size, then, the stepper-motor becomes unwieldy, too heavy, and impractical for many hobbyist devices.
The stepper-motor loses torque at high speeds for similar reasons. The teeth slip through the magnetic fields too quickly and so all accuracy and governance is effectively lost. The motor performs at its best at slow speeds, drawing relatively low power for the torque it is able to exert.
The stepper-motor is an affordable and resilient positioning device but not one that should be used without question in every single project or hobbyists device.
by: Ewan Fisher
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