My team is working on making a small 6 DOF robotic arm and hoping to implement SimpleFOC for control. We will be using a hollow shaft BLDC outrunner with a harmonic drive with the goal of running wires throughout the shafts of each actuator. Each actuator will have an incremental encoder on the input side and an absolute encoder on the output side to avoid a homing sequence.
Input - incremental HEDS optical encoder → this seems straightforward to implement
Output - this is where our questions lie
We are hoping to use a magnetic encoder like the AS5147, but since we are running wires through the hollow shaft, it would have to be off-axis
Alternatively, if we use absolute optical encoder, we need to find a 50 mm size
Summary:
Has anyone had success mounting an absolute magnetic encoder to a hollow shaft output without interfering with the line of axis?
If so, how did that configuration look?
Anyone know where to find a 50 mm an optical encoder?
Any companies that you suggest we work with? (Celera, Avago, etc.)
For the off-axis magnetic encoder mounting, it’s possible to use the AS5147 with a hollow shaft by employing a few different approaches:
Side-mounted configuration:
Mount the encoder PCB to the side of the shaft
Use a diametrically magnetized disc mounted on an extension arm/wheel that rotates with the hollow shaft
The key is maintaining the correct 0.5-2mm air gap between magnet and sensor
This requires careful mechanical design to ensure the air gap remains consistent throughout rotation
For the 50mm optical absolute encoder options:
Renishaw’s RESA series offers ring-style absolute encoders that could work with your hollow shaft design
CUI Devices AMT33 series has some larger format options, though you may need to verify exact sizing
Consider the Broadcom (formerly Avago) AEAT-9000 series which offers through-hole mounting
For specific companies, I’d recommend:
Renishaw for high-precision optical solutions
RLS (a Renishaw associate) for magnetic options
CUI Devices for cost-effective solutions
Broadcom/Avago for established optical encoder expertise
Also, regarding SimpleFOC - have you already determined how you’ll handle the dual encoder inputs (incremental + absolute) in your control system? That could influence your encoder selection.
We are fairly new to this space so we appreciate your feedback and advice.
Magnetic Encoder:
Any suggestions for a good magnet supplier?
An idea we are exploring is possibly coupling the shaft motion with an off-axis disk via pulley so that the encoder would still be on-axis with the magnet. However, we are concerned about the stability of a pulley system that is that small and are not sure if there is any true benefit to this. What are your thoughts on this?
2 Encoders on Simple FOC:
This will be the first time the team is using SimpleFOC for motor control. With that being said, we are not familiar with the challenges that would arise from using an incremental and absolute encoder in SimpleFOC and how it could affect our hardware selection. Do you have any advice or guidance on this topic?