Low cost off axis encoders for moteus - a beginning

The moteus line of brushless controllers all have an integrated “on-axis” magnetic encoder. These encoders are designed to allow moteus to sense the position of a motor’s shaft directly, assuming that an appropriate diametrically magnetized sense magnet is attached to the rotating shaft and the moteus is mounted so that its sensor is positioned over the magnet.

This works great for many applications, but what about hollow shaft motors? moteus supports a few encoder types that will work for off axis encoders, most notably is the AksIM-2. This is a high performance off-axis encoder that gives great performance and is manufactured in configurations for a variety of hollow shaft diameters. However, it does have downsides. First, it comes with a commensurate price tag. In single quantities, the AksIM-2 and magnetic code disc are more expensive than an entire moteus brushless motor controller. Second, only the moteus-n1 has the necessary RS422 transceiver integrated into it. All other moteus boards need an external RS422 transceiver.

Many people have expressed interest in an alternate, lower cost, but lower performance solution. One options is to use existing on axis encoders in an alternate way. Products like the MA600, MA732 (and the AS5407P built in to the moteus), can be used in off axis configurations with a diametrically magnetized ring magnet and additional software tweaks.

When used in this configuration, the magnetic field will be highly non-linear and not directly related to the sensed rotation angle as it is for the on-axis method. Both the MA600 and MA732 encoder from Monolithic Power have explicit support for this mode of mounting by providing a means of trimming the sensed magnetic field in either the X or Y axis of the sensor.

Diametric ring magnets, even from RLS can be had for $20 USD with even cheaper options available, and the MA732 in breakout board form can be less than $20. So in theory, this could provide a very cost effective sensing mode for an off-axis configuration. Doubly so because the MA732 can be interfaced with moteus using logic level SPI over short distances, so it can be used with all moteus controllers with no additional hardware. Of course, SPI is not a panacea, the effective cable length can’t be more than 20cm or so, and EMI can render that even shorter, where the RS422 interface used by the AksIM-2 will work over meters.

This is the first in a series of posts that will look at what it takes to make a low cost encoder work in an off axis configuration with a diametrically magnetized ring magnet. I’m pretty sure I can make something work, but it is unclear how hard it will be and what the resulting performance will be, both in measured position non-linearity and measured position noise. Keep following for updates!