servo current sensing?

[robokoi]

I was looking thru the information Hitec put out about the RN1 servos. Looks like they're able to send back position, voltage, and _current_ information. It seems like this would be really useful for when a limb gets stuck against another body part or obstacle.

There was another post on getting the RN to react to arm movements, but I think that was using position changes thru MotorIn, no?

[i-Bot]

I went through the HSR-8498HB board and schematic again. This servo does appear able to sense position and supply voltage, but I see no way it can sense current. Maybe that is on the new servos.

I have had no success yet on the HMI protocol in the serial mode as described in the new data from Hitec. Will keep trying.

Interesting that they say all robot servos have overload protection. I guess a few owners would dispute that.

[Bullit]

I've had 6 HSR-8498HB's where the motors shorted and hence the H-bridge fried. There is definately no overload protection on these servos.

[robokoi]

I went through the HSR-8498HB board and schematic again. This servo does appear able to sense position and supply voltage, but I see no way it can sense current. Maybe that is on the new servos.

I have had no success yet on the HMI protocol in the serial mode as described in the new data from Hitec. Will keep trying.

Interesting that they say all robot servos have overload protection. I guess a few owners would dispute that.

Yeah, that's exactly why I'm asking. I already fried a hip servo in early oct and had to wait till this past weekend to install a new one since hitec took 1 month to take care of the warranty.

If the servo boards don't have the current sensing (whether for protection or not) I found an IC on Digikey that should be able to do that for us. Any idea how they can make this claim? There are supposed to be more advance features on the servos than can be handled by Robobasic, like daisy-chaining, etc.

btw... is there anywhere that tells how to pull out the servo controls? It looks like the board and motor are directly soldered. I looked into the case, but don't have spares yet (on order) that I intend to take apart.

[Bullit]

1 month! Wow you did well! The last ones they replaced for me took over 2 months. They claim the servos are on backorder to justify the time but I think its to discourage us from sending them back.

The circuit board on the servo can be removed only by unsoldering from the motor. I have been unable to remove the motor from one of these servos. It may have adhesive holding it in place.

I think the overload protection is that the H-Bridge will fail if the motor does. This may smoke and could perhaps cause a fire but generally just smokes a bit and then remains open. I guess that's almost as good as a fuse soldered on the board. I've replaced H-Bridges only to find that the motor was bad. I just think they have poor quality control on the motors. Tough to make a good servo for $60 I guess.

[robokoi]

1 month! Wow you did well! The last ones they replaced for me took over 2 months. They claim the servos are on backorder to justify the time but I think its to discourage us from sending them back.

What a lousy crock of .....

I think the overload protection is that the H-Bridge will fail if the motor does. This may smoke and could perhaps cause a fire but generally just smokes a bit and then remains open. I guess that's almost as good as a fuse soldered on the board. I've replaced H-Bridges only to find that the motor was bad. I just think they have poor quality control on the motors. Tough to make a good servo for $60 I guess.

Do you have a schematic from the control board? There's a couple guys around here that I can ask about it this/next week.

[Bullit]

From an old post from i-bot
http://web.ukonline.co.uk/r.ibbotson/files.htm

[i-Bot]

I will update the file above with the latest information on the HMI protocol. Though there are some erros in the new data from Hitec, the serial mode does work, and the servo does appear to sense the curent and allow it to be read over the HMI

[Bullit]

i-bot thanks for the update. As always your extensive doccumentaion is much appreciated. From looking over your findings and the new data provided by limor it appears that this protocol is really intended more for servo programming and testing then daisy chaining. I mostly base this assumption on the startup sequence requirements and the fact that if a checksum fault occurs the servo exits serial mode. It seems that this would be difficult to manage in a daisy chained configuration and is perhaps why Hitec hasn't released or used the serial protocol in the RN-1.
Do you agree? or am I totally off base here?

[i-Bot]

The HMI protocol for the HSR8498 is different to the other hitec servos. When the controller sends the header byte, then the servo interprets the serial stream after it. The stream is ignored as it is not a move pulse or feedback.

The position, current, voltage can be read, and the postition, speed, and on/off set.

Speed is only 19.2K, so speed is about same as Pulse commands, but load on controller should be less.



The following are fairly complete:

http://www.robonova.de/store/support/in ... ditemid=94

Note the Checksum calculation is wrong, the sum of the four bytes plus the checksum must be 0.

Also they do not tell how to set ID. Also we await the detail of how the D gain, dead band and P gain are set

http://www.robonova.de/store/support/in ... rticleid=5

[limor]

Seems like there's missing information in data1 and data1 for the Current&Voltage sensing request packet and also for the motor stop/start

It says data1 = 0 and data2 = 0 (in the "send packet")
but maybe it should have been data1 = ID
?

it would also be nice to see a circuit of how the 3 servo pins attach to the serial port. My guess it that there's something like a RS485 setup where the DATA line of the servo is connected to both Rx and Tx.

Code: Select all

PC -> voltage_convertor-> servo             
       |              |     |
  5V,GND            Battery(6V)


[i-Bot]

I can get all the commands to work except the motor start/stop. The position (both for read and write) is in microsec (550 to 2450). The speed is a value from 1 to 255. Current values seem quite small but do change under load, and voltage seems to be workin OK

It appears the commands (read position, current) can be performed while a motion is in progress without stopping.

I made a simple interface cable to PC. This converts RS232 to open collector ttl levels. Note the converter does not invert ( a 0 bit is high on the servo)

[limor]

hi i-Bot, can you please post circuit diagram, maybe a picture of the setup, and the program you used to retrieve the current and voltage ?


thanks!

[i-Bot]

Sorry for the delay on this, I tried to post the files, but had some problems.

What happens if I upload the files to this site ?

[limor]

go to the "Manage File Uploads" link on the left (User Info block).
upload files and link to them in your posts ( http://robosavvy/Builders/i-Bot/filename )