So I got back and was able to pick up my 805s in the mail today. I opened one up, complete dissassembled it (without desoldering, yet) and put it back together. I’m rather happy with them, but some of the smaller gears in the drive train look really flimsy. I was wondering if there was a metal gear upgrade kit I could try out? I do have to say that I am much happier with the sound they make when working, almost none, the 5955s are much louder (probably due to the metal gears), so even if its possible I might not want to.
I was also wondering if I should get rid of the stop? I probably should hook my meter up to the pot first and see how much range it can handle before thinking about that. The stock configuration only gets around 190 degrees (physically) and though more range isn’t really needed, more is always better and the whole point of these experiments is to see how far I can push the 805.
Finally, I think this thread will be used for monitoring my OpenServo progress. I put in for the boards a week ago, so they should be coming fairly soon. I’ll post all results here.
Unfortunelty there is not metal gear upgrade for the HS-805.
They are a nice servo aren’t they. And yes, very quiet with gobs of torque. I simply love the simple design, big torque and huge bearings on the output shaft. These things just scream potential to be pushed.
It’s been almost 2 weeks and I haven’t gotten my OpenServo boards yet. I’m going to e-mail Jay and ask what’s the delay. Maybe the snow and/ or the fact its break and University postal staff is only working half days is to blame. Anyone else who ordered them get their boards yet? If so what was the wait?
EDIT: LOL. About half an hour after I posted this Jay e-mailed me (I hadn’t sent him a message yet) that he burnt the processors for my boards during reflow. Considering I didn’t want to build them myself essentially for the reason I though I might mess up during construction, I more than understand.
Okay, I finally got my OpenServo boards and took the plunge with an 805. After desoldering the motor and pot, I put everything together for a nice ensemble picture: http://sknauert.web.wesleyan.edu/ensemble.jpg
The three little gears are what I desperately want to replace with metal ones. If anybody thinks they know of a suitable gear, let me know. The two larger ones in the top servo shell I doubt will break unless I can crank the torque up significantly. However, those little ones look a tad fragile. Anyway…Here is a closeup of the geartrain: http://sknauert.web.wesleyan.edu/geartrain.jpg
I also took a picture of the motor and pot, showing how much of the inside of the 805 is just empty space: http://sknauert.web.wesleyan.edu/inside.jpg
Finally I took a close up of the board alongside my OpenServo board: http://sknauert.web.wesleyan.edu/boards.jpg
Please forgive the fact my camera can’t focus well close-up. The only really interesting thing about the board is it says Version 1.1, so evolution probably has some version 1.0 servos. I have half a dozen 805s, and they all appear to be version 1.1. Mine look like the “smd” version he found. The FETs are fairchild C2334-Y and A1010-Ys. I couldn’t find a definitive spec. sheet for them, but some posts online let me to believe they are capable of 7A. This would more or less dice any hope for the 805 being electronics limited right here, since these big TO-220 dwarf the SMD FETs on the OpenServo board, as from multiple people’s testing, the 805 fails at around 2.8A. While some sinks on the FETs might help, and SMD FETs might allow for better thermal design, I doubt this is going to give much of an improvement. Just in case anyone wanted to know, the pot registers 160 Ohm at zero and 4.35 KOhm at end. This would put it within tolerance for a 5K pot.
My plans now are to get an I2C test platform up and running with a SEEPROM and the OpenServo board. Once I get that, I can reassemble this 805 as a “stock” OpenServo and put it through its paces. However, since the base OpenServos I got have 3A FETs, I may get an initial reduction in torque. This is okay as I have compatible SMD 7A FETs on order, and am planning to stack them for 14A max current; although the small traces on the OpenServo board are starting to bother me.
EDIT: I found a good spec. sheet for the A1010-Ys. At 65 degrees (the temp of the internals of a hot servo) they can only give 3A at 12V. This is very close to the 2.8A observed, making me think electronics limiting might actually still be an option.
From my torture tests on a stock 805, the board can handle about 2.6 or 2.7 then it’s toast. That’s continuous, I’m not positive about burst.
The gears can take about 540 oz/in. When mine went, it actually split that huge primary gear in two and shattered the small first gear.
You could have metal gears made by a gear manufacturer but it would be costly. Your best bet would be to find a source for micro metal gears and order the ones you need. You may have to change the mounting shaft diameters but you always drill them out and add ball bearings.
Both have a 7A limit. Mind you, this is thermally limited, so in a small space like a servo so you might cook them a LOT sooner. Also, these are PNP and NPN transistors. You’re looking at these working not on/off, but in their analog region. You probably burn up more power in these transistors than in the motor!
No analog servo uses FETs, they’re all standard transistors.
If one attached an aluminum strip across the tops of the trasistors that went out the side of the case to a small thin heatsink, it would help to dissipate heat.
be careful which tabs you connect together on a single heatsink as the tabs are electrically connected to the center pin of the transistors.
also I don’t think you will find the transistors are operating in their analog region. they simply built an h-bridge from bipolars. it is just cheaper to use bipolars than MOSFETs and the update rate of ananalog servo is so slow there’s no harm other than a small loss in efficiency.
Here’s the thing though, if these stall at around 3A then the two transistors saturated are dissipating around 1 to 1.25W each. Using complementary MOSFET pair parts with an RdsON of 0.1 ohm you would be under 1W on each package, and it is easy to improve on that using better parts. The open servo should have no problem driving this load if they chose the right MOSFETs.
