RoboSavvy Forum

AX-12 Settings

2007-01-24T14:26:18+01:00

Juha,

The way it was set up, it could reach the goal setting (more or less), but it would whine. If you set the goal position to the current position, it would just drop down a tiny bit and start whining more.

The servo was fine the way it was - the problem is, I'm going to be adding half a kilo worth of stereo cameras to the head, which was going to turn it from minor problem into a huge one.

Fortunately, with the Bioloid kit, you can reconfigure your robot however you like, and it only took about ten minutes to drop that servo from the design...

- Jon

Statistics: Posted by JonHylands — Wed Jan 24, 2007 2:26 pm

AX-12 Settings

2007-01-24T10:47:21+01:00

As Jon has been trying to say, the communication with DX vs AX is totally different. With AX one data wire is shared, information travelling there both ways. This is different from RS-485. Some places still have a bit missleading information about the protocol with AX, saying it would use RS-485.

On the original subject: Jon, so did I understand this correctly, depending on the parameters you use your AX-12 either bounces/swings back and forward, or it whines?

With the settings which give you the whining sound, does the servo reach a goal position within the set margins? Or does it stay in the moving status (0x2E = 1)?

If it's still trying to move, just as an idea, you might make a function which checks if the servo hasn't reached it's goal (+-margins) within certain expected time. After this you might have 2 choises.

Just set the goal position = current position. Ofcourse depending on how important it is for you to reach the goal position.

Another thing I was thinking about was if you could actually change slope/margin/punch _while the servo is moving_ (0x2E=1). For example, if you notice that the servo is stuck, increase the punch value. You might get some kind of hint on how much you should increase it from the current load parameter.

But, like I said, this is just an idea, and I haven't tried it myself.

Statistics: Posted by Juha — Wed Jan 24, 2007 10:47 am

AX-12 Settings

2007-01-24T03:54:34+01:00

The RS-485 bus for the DX-117 is a four-wire bus - the data is transmitted over two wires. The data on the AX-12 bus travels on one wire.

How do you plan to reconcile this?

- Jon

Statistics: Posted by JonHylands — Wed Jan 24, 2007 3:54 am

AX-12 Settings

2007-01-24T02:56:43+01:00

The protocol is identical (half-duplex, asynchronous, 8 bit, 1 stop bit and no parity). The difference is in the physical signaling (RS-485 vs. TTL). With the propeller chip I need to convert their 3.3volt signaling to 5 volt for the TTL. I expect to use open collector line drivers to "OR" the data received from either the Rs-485 or 5 Volt line driver. As far as the microprocessor is concerned, it will be a single two pin interface (data in/out, and direction)

I see no reason to deal with the complexity of the CAN interface since the DX-117 should meet all of my needs and as you say that is a totally different beast.

On the servo power side, the AX-12 is specified for 7-12 volts while the DX-117 is specified for 12-16 volts. I have not worked out the power supply yet but I expect I will use two LiPo 2-cell battery packs. They will operate in parallel for the AX-12 (7.4 volts)and in series for the DX-117 (14.8 volts)

I can't wait to get started on this but I got a few things to finish around the house first.

Statistics: Posted by lnielsen — Wed Jan 24, 2007 2:56 am

AX-12 Settings

2007-01-23T20:09:48+01:00

The poster who brought this up in the first place said this:

Currently I am planning on sharing a single communication bus so I don't have to worry about which servo I want to replace.

That was what I was saying woudn't work...

- Jon

Statistics: Posted by JonHylands — Tue Jan 23, 2007 8:09 pm

AX-12 Settings

2007-01-23T19:31:41+01:00

Although Robotis hasn't published schematics of the CM2+ I would think that electrically they are not on the same bus but the CM2+ is likely to see the DX and AX bus as the same bus. I think its likely that the exact same tx, rx and dir pins are used to different driver chips. The CAN bus would be altogether different of course.

Statistics: Posted by Bullit — Tue Jan 23, 2007 7:31 pm

AX-12 Settings

2007-01-23T17:14:13+01:00

Well, yes its easy to have an ATmega128 talk to both the AX-12 and the DX-117, but they aren't running on the same bus, I can pretty much guarantee that. The two bus protocols are electrically incompatible.

- Jon

Statistics: Posted by JonHylands — Tue Jan 23, 2007 5:14 pm

AX-12 Settings

2007-01-23T16:59:23+01:00

The CM2+ coming out from Robotis will have ports for all ax / dx & rx series servos including can.

http://www.robotis.com/html/sub.php?sub=2&menu=3

Statistics: Posted by Bullit — Tue Jan 23, 2007 4:59 pm

AX-12 Settings

2007-01-23T12:57:33+01:00

Its an interesting idea, but it would require another power source, which I am not ready to do.

As an aside, I don't understand how you plan on "sharing" the bus, given that the AX-12 bus is one-wire serial, and the DX-117 bus is two-wire RS-485 (which is a fundamentally different protocol).

- Jon

Statistics: Posted by JonHylands — Tue Jan 23, 2007 12:57 pm

AX-12 Settings

2007-01-23T05:17:10+01:00

Would you consider a DX-117? I know they are not cheap but it should give you the power you need.

I hope to start working on a Parallax Propeller controller that will support both types of servos for this exact reason. Currently I am planning on sharing a single communication bus so I don't have to worry about which servo I want to replace. The two things you need to deal with are combining the receive level converters and providing a different voltage for the servo power.

Statistics: Posted by lnielsen — Tue Jan 23, 2007 5:17 am

AX-12 Settings

2007-01-22T13:47:42+01:00

I don't think the current is the issue. This happened even if that was the only servo under power. I measured the current draw when the robot is "at rest", with the servos switched on but not trying to position themselves, and it was around 650 mA. So even if that one servo was drawing an amp, we're still way under the limits of what either the battery or the power supply can provide.

In any case, its a moot point now, because I have removed that servo from the robot...

- Jon

Statistics: Posted by JonHylands — Mon Jan 22, 2007 1:47 pm

AX-12 Settings

2007-01-22T12:26:52+01:00

may be way off track but... how much current are you drawing when this happening?

I have had a problem in past running a number of AX12's at high load with an inadequate power supply - some servo's would start oscillating. Fixed when switched to power supply better able to handle current draw.

just a thought

Jon

Statistics: Posted by jonkster — Mon Jan 22, 2007 12:26 pm

AX-12 Settings

2007-01-19T01:15:48+01:00

Its an interesting idea in general, but specifically for this robot it would be very difficult to fit an extra servo in place there. I would basically lose one of my yaw axis, and that is more important to me than the pitch axis.

The reason the yaw axes are important, in case you're wondering, is so that MicroRaptor can move his head left and right (to aid balance) while keeping his cameras pointing straight ahead.

- Jon

Statistics: Posted by JonHylands — Fri Jan 19, 2007 1:15 am

AX-12 Settings

2007-01-19T00:34:11+01:00

That's sometimes done in the RC world, but it's considered naughty because any two servos are not exactly the same performance

However!! Since we have feedback with the servos we can make sure they are mostly in sync.

Good idea. Might make for a good strong torso in a humanoid without having to buy the fancier servos.

Statistics: Posted by billyzelsnack — Fri Jan 19, 2007 12:34 am

AX-12 Settings

2007-01-19T00:11:36+01:00

Okay, I am not sure if the physics is right here but........

If the issue is a torque one that is not fix with a configuration/settings change, could you possibly put 2 AX-12s back to back and have a custom bracket with a driven horn at both ends.

As I say I am not sure about the physics but I think that should just about double the holding torque at that joint........

Well maybe

Statistics: Posted by Pev — Fri Jan 19, 2007 12:11 am