View Full Version : AX-12A and HS645MG

08-02-2012, 10:12 PM
Right now. I am looking at making a biped that is about 20 inches tall. Now, I know that the HS645MG isn't strong enough to hold up an biped and keep in balanced but I was just wondering about some of the specs between these servos. I noticed the AX-12A has quite more torque then the HS645MG but has a lower max current draw. Does the smart electronics really make the servo that much more efficient? I would think the new electronics would cause more current draw.

08-02-2012, 10:20 PM
Just curious, where are you getting your specs from? From the datasheets I've seen the AX-12A can run at 12V with a maximum of 900 mA, while the HS-645 can run at 6V and 450 mA.

In either case, you've also got to take into account the total power input, not just the current. The gear ratio inside the servo will also play a role in determining the amount of torque supplied, at the expense of speed.

08-02-2012, 11:34 PM
Yeah the maximum current is 900Ma for the ax-12a but no where in the HS645MG does it tell you the maximum current draw. 450mA is at no load which is very unrealistic. I have got my number for current from the servos I have on my hexapod. They seem to run around 1.3 amps under a good amount of load which is more than the maximum for the AX-12A.

08-02-2012, 11:57 PM
So if I power up a 645 and have it just sitting there, nothing attached to it (at idle/no load), then it'll be drawing 450 mA? That sounds more than a bit disturbing for both efficiency, and the fact that something as huge as an MX-106 with a lot more smarts is only drawing 100 mA at idle. I was reading the datasheet here (http://www.trossenrobotics.com/images/productdownloads/hitec/hs645.pdf), where the speed at 6V is specc'd at no load, but the torque isn't. Fair enough on the maximum load though, I suppose the datasheet is rather conservative in that regard.

08-03-2012, 12:24 AM
No that's not idle. If you read the data sheet that you even linked to it tells you idle/ no load is 9.1mA. Now seeing as you want to be smart about it and act like you don't understand if you hook up a HS645MG and have it moving (that means not idle) without any load it will draw 450mA. I'm really not in the need of someone trying to make a stupid arguement about I'm just wondering why the dynamixel servos seems to have more torque and use less current. If you don't know or have a good response please don't bother replying as I am simply looking for just some info.

08-03-2012, 12:36 AM
Sorry about that, it's been a long day here.

As for a "good response", was there anything wrong with the second half of my first post in this thread? Different motor quality (and hence efficiency) could probably be added to the list.

08-03-2012, 01:09 AM
The issue here is that you are comparing current rather than power, and that the H-bridge (and thus motor voltage/current rating) is not the number you are looking at (if the AX-12 motor is actually using [email protected], it will look like [email protected] at the power input point). For ease in tracking numbers, lets assume all numbers for the AX-12 are at 12V, and the HS-645 is at 6V, then:

AX-12 at 12V has stall torque of 900mA. P = VI = 10.8W
HS-645 at 6V has stall torque of 1.3A (based on your experiments). P = VI = 7.8W

In both cases, these power numbers disregard actual motor efficiency, or the standby power usage (the power consumed by the electronics when the H-Bridge is disabled, which will most likely be higher on the AX-12).

Inbreakable, in the future, please remember that sometimes things get lost in communication on the internets, and try to avoid lashing out with attacks that users are making "stupid arguments".


08-03-2012, 03:35 AM
Another issue is gearing. A servo that is geared slower will have higher torque. In hobby servo circuits, you'll see a "speed" rating, which is the speed to move 60 degrees with no load, and it typically ranges from 0.05 to 0.2 seconds. The same motor and servo would have four times the torque at 0.2 second speed as it has at 0.05 second speed.

Speaking of which -- what are the speed requirements for a walking gait, in everyone's experience?

08-03-2012, 09:09 AM
Sorry just didn't really like the snippy attitude I'll try to look over my post before I post them up. THank you for the info tho. Yeah I forgot about the fact one servo operates at twice the voltage haha... THanks for the input.

08-03-2012, 10:32 AM
Sorry just didn't really like the snippy attitude I'll try to look over my post before I post them up. THank you for the info tho. Yeah I forgot about the fact one servo operates at twice the voltage haha... THanks for the input.

There was no snippy attitude. Th232 is one of the more helpful posters on this forum and was simply trying to help. He certainly didn't need to apologize, quite the opposite, as you are in the wrong here.

Watch your own attitude, or you can find your way to the door. We're here to learn and share, not get upset and lash out because we don't understand something.