Adding Precise Speed Control to a Cheap Rotary Tool

Rotary tools, which are often referred to by the Dremel brand name, are versatile tools that can be used for a variety of jobs depending on the attachments you use. You can use a rotary tool to sand, polish, drill, cut, and much more. Instead of relying on torque, most rotary tools use extremely high speed motors to do the job. But cheap rotary tools don’t provide much control over the actual RPM of the motor. That can be a problem if you’re doing delicate work and don’t want the full speed. Fortunately, Hackaday.io user Vitaly has come up with a method for adding precise speed control to a typical rotary tool.

Vitaly performed this modification on a cheap Hilda brand rotary tool, but it should work with many other brands and models, as well. The principle that makes this modification possible is back EMF (ElectroMagnetic Force). Back EMF is generated naturally as a motor spins, and is proportional to the motor’s speed. By measuring the back EMF, you can determine how fast the motor is currently spinning. You can then use that in a feedback loop to control the speed of the motor with fairly high precision. In addition to providing speed control, this also keeps the RPM stable under load.

To do that, Vitaly replaced the Hilda rotary tool’s original control board with a custom PCB. That PCB has an STM32 microcontroller, as well as the many other discrete components necessary for monitoring the back EMF and adjusting power to the tool’s AC motor. This PCB is designed to fit the Hilda rotary tool specifically, so you may need to modify the dimensions if you have a different model. Luckily, Vitaly has provided all of his EasyEDA design files so you can do that easily. Then you can upload the provided code, reassemble the tool, and have far better performance than a typical rotary tool.