SX28 Driving Servos on RB0-3
Dr Frag
Posts: 6
Hello all, I need a little help figuring out why RB0-2 can't drive my micro servos.
I bought this controller and 3 of these servos. For some reason the servos connected to RB0-2 won't operate properly yet all 3 servos work on RB3. When a servo is connected to RB0-2 the control voltage drops from the normal 300-400mV to around 6mV. I noticed that RB0-2 are also comparator connections and believe there is some interface problem between RB0-2 and the YT5188 chip used in the servo. (here is the datasheet with 5188 circuit as the one on the YT5188 page only lists 5166 circuit)
This page has the controller schematic and source code (at bottom).
I am going to try to investigate the servo chip board more but it's really small and hard to work with.
If anyone has any ideas as to what is going on here both myself and the creator of the quad controller would really appreciate the help.
Thanx,
David
Post Edited (Dr Frag) : 3/22/2009 4:04:18 PM GMT
I bought this controller and 3 of these servos. For some reason the servos connected to RB0-2 won't operate properly yet all 3 servos work on RB3. When a servo is connected to RB0-2 the control voltage drops from the normal 300-400mV to around 6mV. I noticed that RB0-2 are also comparator connections and believe there is some interface problem between RB0-2 and the YT5188 chip used in the servo. (here is the datasheet with 5188 circuit as the one on the YT5188 page only lists 5166 circuit)
This page has the controller schematic and source code (at bottom).
I am going to try to investigate the servo chip board more but it's really small and hard to work with.
If anyone has any ideas as to what is going on here both myself and the creator of the quad controller would really appreciate the help.
Thanx,
David
Post Edited (Dr Frag) : 3/22/2009 4:04:18 PM GMT
Comments
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Shawn Lowe
When all else fails.....procrastinate!
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Shawn Lowe
When all else fails.....procrastinate!
Have you tried a different make of servo to see if it is indeed RB0-2 ?
Has the manufacturer of the board ever used the servos that you are seeing problems with?
If another brand or type of ·servo works on the RB0-2 and not the ones you presently have then possibly the spec for timing might be off·(board or servo).
If I was the designer of the board I would·be asking you to send the servos to me ( shipping on me) so that I ·could nail the problem down and not have a customer doing the work.
Sounds like a headscratcher.
Pete
Pete, I am disassembling them so I can see the chip/circuit in them and do some additional testing.
I do not have any other servos on hand but the board creator uses Hitec HS-311 I believe and tested the first board I sent back cuz we thought it might be bad but it worked for him. He has not used the servos I have and has asked me to send them to him and actually suggested we swap servos so I have some that he knows works but I want to use the smaller servos and frankly am really curious to see if I can figure out what the problem is.
Yes, I have a little bare patch already.
There could also be the possibility that the controller board is not getting the correct command(s) to move the servos on the pins in question.
Having a scope is about the best tool for looking at servo pulses and prevents damaging servos when driven into their extremes.
I'm just starting to get into the SX so not much help in the programming end but I have done a lot of servo control in timers (AVR) and before hooking up one I would look at my scope for the correct pulse and spacing of them.
I would definitely go to a hobby store and get the servo that the designer uses and verify that you can or cannot move it with the software commands on those pins before going thru the hassle of figuring out the inards of those $5 micros. If you can't make that servo work then my bet is in the way you are trying to send the commands.
Pete
For reasons known only to the webmaster at Parallax, links on this forum are in a color almost identical to normal text. This makes them very hard to see. I recommend making them a distinctive color as well. I use green, but anything that is bright should do.
Thanks,
PeterM
I have tested all outputs and they are changing when I press the buttons and the manufacturer tested the board I sent back so I have to assume it's working properly at this point.
I have a couple of oscilloscopes in storage along with some R/C equipment which I hope to pick up tomorrow. Hopefully that will help shed some light on what's happening on the pins.
Thanx PeterM, I will color my links and try to use a color that says 'click me'. [noparse]:)[/noparse]
Dave Hein
Here is another link to the source code. I'm not the programmer so I don't want to make any assumptions about what part to post.
Sorry, my bad. I didnt think you had that in your post. This is a weird problem.
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Shawn Lowe
When all else fails.....procrastinate!
There are J1, J2, J3, J4.· Is there 5_volts from pin_1 to pin_2 of each?
Post Edit -- You're measuring hundreds of millivolts, but where (at J1-J4·or at the SX-IC)?·
a.) Servo1-3 never work?
b.) Servo4 is OK till you plug in 1, 2, or 3 or it's OK regardless?
[noparse][[/noparse]Yes, it's millivolts because that's PWM on the DC_V.]
Post Edited (PJ Allen) : 3/22/2009 7:15:20 PM GMT
Same voltage at SX-IC as at pin 3. There's only a half inch of board trace between them.
Outputs RB0-2 have never worked for me with my servos. RB3 always works no matter what the configuration.
Dave, under a purely resistive load the output does not change significantly.
Had a brain storm and put a diode in series with RB0 and the servo works. There is some strange interaction with RB0-2 and the servo chip. Wish I had internal schematics for both.
Was unable to drive up to storage today to get my o-scope due to weather. Maybe tomorrow I can sneak away.