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   • Adding one DOF to match ARX kinematics is doable, with a price increase of $30–40.

You need at least six non redundant DOF to arbitrarily position the end effector in space, three for x-y-z translation and an additional three for roll-pitch-yaw. For research grade arms, I typically want at least a 7 DOF arm, which gives you a lot of cool abilities, most importantly the ability to work around kinematic singularities, and makes the inverse kinematics problem nontrivial in interesting ways. I understand you're hitting a price point, and each additional DOF costs money. I personally would pay for additional DOF. Maybe a modular design?

   • A tool changer is a great suggestion. A few of my friends are working on kinematic couplings, which would be ideal for this. I’ll need to give some thought to how to pass electrical signals and power to the tool, while also keeping it lightweight.

Yeah, typically with industrial tool changers there are spring loaded pins on the tool changer that hit pads or insert into sockets on the tool side. There will also typically be a ball detent for positive locking that is driven by a motor in the end effector. But even just a passive mounting plate and a documented connector interface would be huge.

   • Could you share what functionality you want in terms of encoders? The ST3215 uses 12-bit magnetic encoders, which can retain position after power loss. Are you looking for higher resolution? For torque sensing, if the order volume is large, I can add this for just a $20-30 price increase.

You take what you can get with encoders. Ideally, you want an encoder that uses grey code, so it always knows exactly where it is no matter what. But for cost reasons this is rarely done, and you get what is essentially a relative encoder and you have to count the steps. The reason the former is preferable is that it doesn't rely on the microcontroller keeping up with the encoder, so there's no issue if you miss counts. But, again, those are as far as I know a significant step up in cost.

You'd also ideally add torque sensing at the joints because it opens up a whole world of control techniques that you can't get with just joint position sensing. You can do compliance or force control, which lets the arm act as if it had a spring at the joints, so when it hits something the impact is nice and gentle, and importantly, so you can do things like e.g. a bolt insertion task where you have to control the position of the arm in x and y but you want to exert a small positive insertion force in z.

   • Finger tip force sensing: Is this for applications like picking up an egg?

Yes, but even for picking up rigid objects this turns out to be very useful. If you're picking up an eg, you want to exert a controlled positive grip force that's big enough so you don't slip but not so big that you crack the egg. If you're picking up a bolt, you definitely won't break it but many robots are strong enough to deform the threads. If you're picking up something slippery, it would be great to try to detect the slip by touch. And so on. Often, you don't know exactly how big the object is or how flexible/brittle it is and it's hard to judge by vision alone whether the fingers are even in contact with it, or if they are how much it's being deformed, so being able to control grip force is very useful. Add force and position sensing to the grippers and you can judge how deformable the object is and make decisions accordingly.

Or if you're folding clothes or handling cables or wires or anything else flexible, you really need to have a sense of touch. You can't really do these tasks very well with position sensing and vision alone.

Another idea: Maybe add a passive mounting adapter and power leads at the end effector so people can add their own vision or lidar sensors, and just let them connect via bluetooth, so you don't have to route signal cables?

FYI, I am a space roboticist by trade and I teach a graduate level class in robotics at the University of Maryland.