Motoman K6SB Retrofit

This is an interesting project. I've been looking to do something similar wirh a smaller robot but I don't have the space yet.

Few notes:

Steppers are a lot slower than servos. They also lose a lot of torque on the high end. Unlike ac servos. You get max torque around 800rpm and max rpm is around 1500. Compared to a servos 3000-3500. Keep that in mind when configuring it, you should also know the reduction ratios of the gearboxes so you can calculate the final drive ratios and speeds. It might be good to also take the reducers apart and clean them thoroughly.

You also need to build homing switches to the robot for homing. Either joint encoders, switches or use the drives builtin feature(not the best idea though). You can also manually set the home each time by hand but this isn't really repeatable unless you you something like a dial indicator etc.

I have also brought a copy of RoboDK to plan out my work cells in 3D. I will use that when I get to the stage of creating programs but like PlasmaC, I want it to be able to work on its own and not be tied to any one CAM program. This will be a pretty big project but one I hope will pay dividends in the future.

I have worked with robodk and the big issue with linuxcnc is that I couldn't find a way to turn a CIRC command(I'd, an arc) into either G2 or G3 command. You'd have to find a pythonic way(the post processors are in python) to evaluate the arc command determine if it's clockwise or counter-clockwise. Or somehow remap the linuxcnc interpreter so it understands singular arc command. Joint and linear commands are fairly easy to set as long as you configure inverse kinematics correctly.

The other issue I haven't really figured out is how to deal with multiple reference frames in linuxcnc. Robots use multiple and I don't know how this would really work and how you'd set up the additional reference frames used.

Depending if you want to program microcontrollers or not, check out these modules if you want to build custom applications for interacting with ethercat. www.bausano.net/en/hardware/ethercat-e-arduino/easycat.html I have an idea for an ethercat remote for linuxcnc

Thanks for you input Roiki,

I made notes of the arc issue you mentioned in my linear rail thread and I have been keeping that in mind.

With the steppers I am going to be powering them with 80Vac with is nearly equivalent to 100VDC. They should be good for more than 1500 RPM at those voltages. But even so with the large gear reductions in the harmonic drives I do not think ultra high speeds will be necessary for my welding application so they should be fine I hope. We will see.

I have looked in the AB&T modules and I have brought three to experiment with.




I have not had much time to play with them yet as I have a few other projects going on but I want to try to get the EasyCAT set up to control these drives, I have high hopes for the modules.

Does anyone have any advice on the type of grease I should use to replace the old?
I have done a bit of looking and cant find a definitive answer.

With the steppers I am going to be powering them with 80Vac with is nearly equivalent to 100VDC. They should be good for more than 1500 RPM at those voltages. But even so with the large gear reductions in the harmonic drives I do not think ultra high speeds will be necessary for my welding application so they should be fine I hope. We will see.

Steppers speed is not determined by the voltage but by the speed with which the driver can energize the coils. More voltage helps to overcome the inductance. Still I doubt you're going to get more than 1000rpm from nema 23 sized motors. Likely in the 800 range, also you have to Remember that at 1500 you'd have about 60% of the nominal torque. Higher speeds, even less. Driving the steppers faster will make the load of the robot go down, and at some point might reach a point where it can't overcome it's own mass. And because of the large reductions in the reducers the movements will we very slow at max torque speeds.

Cool to see you got those easycat modules. I've ordered one for tinkering myself.

Edit. I dont know specifically about cycloidals but I usually use just lithium grease for gearboxes. Has worked nicely so far.

The smaller motors are Nema 24's rated at 3N.m,


The larger motors are Nema 34's rated at 8N.m,


I understand that the torque decreases exponentially but given the starting torque is higher than the original motors torque, when it get to the working RPM it is still around the same torque if not just below.

The reason I brought these motors was not just because they are much cheaper than the DMM servos I was originally going to buy, but because I have used the non braked versions of the Nema 34's on my large industrial CNC plasma and I have not had a single issue performance wise with these motors. They FAR exceeded my expectations of what a stepper motor is capable of, and continue to everyday. I hope I have the same experience with these but only time will tell.

Nema 24 looks good, the 34 is on the high inductance side, but still at 100V should be plenty fast. Should be a smidge above 2Mn at 1500 RPM.
The main deal breaker for fast stepper motors is inductance, the lower the inductance = the faster they can spin at a given voltage.
And i do have Nema 23 small motors that can do way over 3000RPM at decent torque with 72V and Lam Tehnologies drives.