Monday, 28 March 2011

Major Problem!

Ok, so having had the Mantis up and running for about two weeks, and gradually running more tests, I noticed one of the legs calibration point kept drifting. Upon further inspection it turned out that the clamp bolts were missing on the lower femur cheeks which clamp the main bearing shaft and lock it to the femur. With these loose the shaft is able to move slightly and hence the encoder drift. However, while isnpecting this I found something worse... much worse! I noticed the coxa assembly had some play in it around the bearing, so I thought the bearing nut may need tightening so removed the coxa encoder to take a look. To my horror the bearing was at a very jaunty angle, which can only mean the shaft was bent!





As you can see having removed the coxa plates the 32mm stainless steel shafts have buckled under what I would call moderate load testing, so its back to the drawing board for these!!!

bugger, could really do without this at this stage, not the kind of thing I want to find out a day before my birthday :(

Wednesday, 23 March 2011

Some initial test footage

Here is some test footage of the two types of gait with the Mantis hovering off of the floor on its jack stands.

video
video

Monday, 21 March 2011

The Mantis Lives!

Well a pretty monumental week for the Mantis project, for the first time I have had the mantis stood up and moving around, albeit slowly and very carefully. Initial thoughts are that the body translations need to be slowed down.. allot! and the overall hydraulic stiffness needs improving. I think there is still a fair amount of air in the rams which does not help! Also the feet seem to be too soft, so I may have to re-machine the inners in a harder foam.

Standing in Neutral Position
Standing in high position

Wednesday, 16 March 2011

More Encoder Issues!

I noticed something strange happening with a few of the encoders, after power up, occasionally some encoders would report an incorrect position, usually with and error around 90 to 180 degrees. What was strange was that if the encoder was moved slightly it would snap back to the expected position!

Having spoken with the SICK technical rep today, I decided to break out the scope once again and look at what was happening on the bus. Sure enough an encoder in an error state would suddenly flip back to its expected state when moved slightly as confirmed by monitoring the bit stream on the scope. At this point It was looking like an encoder fault, which was slightly worrying, however, I decided to slow down the clock rate to 450Khz which had been running at about 800Khz. This seems to have cured the issue, which is a little odd as the encoders SSI bus is supposed to run up to 2Mhz.

I have yet to discuss this further with the technical rep, but as long as the issue goes away I'm not to fussed. It could be something to do with cable lengths / termination etc.

Tuesday, 15 March 2011

Encoder Power Modification

The encoder relay power PCB's arrived this morning.

(great new service from Spirit Circuits, check their ebay web site out, £39 for 1 sqft of double sided PCB!)

I have been experiencing issues with encoders intermittently failing to power up, I thought this was probably due to the initial noisy power spike upon switch on, and so decided to run the encoder/SID power through a 12v relay that is switched from the 12v DC-DC on the VIC-E daughter board, this gives the 24V to stabilise as the DC-DC takes 100ms or so to power up.


Having fitted the PCB, to my dismay I was no having problems with the PIC not booting up! after spending an evening scratching my head and probing with the scope I realised it was due to an additional 220uF cap I added to the relay PCB across the 12V DC-DC supply which was causing the DC-DC to fail upon its first power up attempt causing a false start. removing the cap sorted the issue.

Thursday, 10 March 2011

New Metatrsus Bearings

I have had the metatarsus cheeks bored out (Thanks Mick & Chris at D&T Precision) and have fitted my test bearings which are igus igubal type. I'm using these to start with but I intend to replace them with maintenance free steel bearings.




After fitting the bearings and re-assembling the ankle I re-tested the load pin and it seems this has fixed the pre-tension issue, so now I just need to re-place these with steel bearings and repeat x 5!

Tuesday, 8 March 2011

Load Pin Problems

After spending the morning running a few tests, I have found an issue with some of the load pins. It seems that if the metatarsus cheeks that support the load pins are not precisely aligned with each other the misalignment pre tensions the load pin giving a false reading. Having taken one apart and talked to Josh it seems we both came to the same conclusion that we should have fitted spherical bearings to the metatarsus cheeks as well as the tibia bearings already fitted, this would allow for any misalignment in either assembly. Its a bit of a set back but I'm going to have to take the ankles apart and get the inside faces of the cheeks bored out to take a spherical bearing.. dull!

Friday, 4 March 2011

Hydraulic Installation Finished

I finished the hydraulic installation on the last leg today, great feeling to see all the rams and hose in place. Really glad I spent the extra time fitting the spiral wrap to the entire length of hydraulic hose (approx 120 meters) as it looks great. I'm not looking forward to the commissioning!