Today was one of those days. For the past 5 hours I have been discovering the limitations of my motor and trying to figure out where to go from here. Up till now things have gone very smoothly. I found a motor, sourced some gears, and tested the electronics all with surprising ease. Today though, while testing an h-bridge I realized that the motor I was able to resist the motor with relative ease when running at 3 volts. Perhaps I should have known that 3v is pretty low to be running most motors at, and so it seems, low even for 6v motors. This prompted a series of tests. I boosted the voltage to 6 volts, and then to 9, and still I could stop the motor with a pair of needle nose pliers without too much difficulty. Then I tried to lift a 15 inch, 1.2 kg spanner that was gripped onto the shaft. The spanner I estimated was about the length and weight of 4 modules, which is half of the length and mass it would have to lift in the most extreme case. The motor couldn’t dot it at even at 9 volts, not even with a bit of assistance. Argggghh! …why argggghhh, you ask. Because at least some of the parts I have researched and even bought were specified for this motor!! I may have wasted days of work, and a bunch of money. Ok, so I swapped the motor for this one, to see if another motor would fair better. The second one has very similar properties to the first. It is roughly two inches in length, and seems to have about the same torque at 6v. The one I purchased (no link), is stated to exert 14 kg*cm of torque, but it doesn’t seem to be that much. Neither do the job. I haven’t lost the option for these motors yet though. I have some gears on the way at a 2:1 ratio. So that will double the torque. And there’s always the opportunity to try even higher ratios. I will also most likely have to double up on a li-ion, li-poly battery pack for 7.2 volts instead of 3.6 volts (my original intent). That would increase the torque as well (though, as mentioned, even the higher voltage was not enough to do the work in my test).
If the gears don’t do it, then I’ll have to go with different motors altogether. After some searching today I found the following motors, all servos, all have a reasonably compact size, and all are very expensive. The Hitec HS-M7990TH is a very impressive motor. Most importantly it has a torque of 38 kg*cm or 528 oz*in at 6 volts! That’s triple or quadruple my current output. In addition to that it has Titanium gears, a magnetic encoder, and is brushless (which I think means less wear). Unfortunately, it costs over $200. I love that it runs at 6 volts meaning i won’t have to go crazy with batteries. I could still likely add gears as well, if necessary. Second option is a Robotis Biolotis, or a Kondo or something like that. These are both amazing motors that are meant for robotic applications. At the top end, they have torques around 64 kg*cm or 888 oz/in. Additionally, they have some very useful electronics built in, including torque feedback, position feedback (I don’t think the hitec outputs either of these), heat sensing, and they can also be daisy-chained. I wonder even what I would have to build in addition to these puppies. Additionally, they can be switched between the normal limits of servo rotation and full 360 degree rotation with modifying them. On the down side, these servos cost between $300 and $415 each!!!! Just the thought of that is insane. Furthermore, they need between 10 and 20 volts to run!!! Where am I going to get that? If I use say an RC battery pack than this machine is going to be one heavy beast! That’s the major reason I’m inclined to go with the hitec if any. Gad, it’s a lot to think about, and I wonder if I’m still missing the boat, and haven’t found and even better choice (if you know of one please pass on the info). One more note. In looking at these servos, I also came across the Uptech CDS 5500, a kind of cheaper knock-off of the low-end biolotis. But it has some very nice properties. To begin it has the same torque as my current motors if not more. Second, it can run off 6.5 volts, or two li-type battery packs. Third, it has all the bells and whistles of the Biolotis including various feedbacks, thus eliminating a bunch of work for me. I will likely order a couple just to test them out.
In other news, I finished assembling the electronics for the 2-birds contract which I’m also using to test electronics for ADB. I coded everything up on the Arduino in a very basic way, and it all works. The next step with that would be to create a more interesting and elaborate program, but that requires the input from my employers before I put in the time, and I don’t think they’re thinking about this right now. So I guess that test was a success and is concluded. I have to think what comes next. I’ll start by revisiting both my budget and my schedule.