I’m trying to make a biped using the Dynamixel Ax-12A servos for the legs. Each leg will have a DOF of 3 and there will be two servos in the hips as well. The material that I’d be using the torso and the head is still undecided.
So in the user manual of the AX-12A it is clearly stated that:
Stable motions are possible with robots designed for loads with 1/5th or less of the stall torque
Does the above statement apply to just 1 of the AX - 12A Servos or should the total mass of the robot be 1/5th of the total stall torque of the 6 servo motors (3 for each leg) i.e. all of them added together, during the motion of the robot. Is the torque of the hip servos included as well?
It’s says in the servo manual that it has a stall torque of 15kg-cm and the recommended voltage for these servos is 11.1V. But i don’t know the torque the Ax-12A servo will have at this recommended voltage (of 11.1V). Is there any way of knowing the torques of the servo at different voltages?
I hope what I said made sense and any help will be much much appreciated.
The torque criteria would apply at each joint. Note that the torque while walking changes, and the torque at each of the three joints is different.
Unfortunately we don’t find any torque:current curve (though it would be nice). Just like a normal DC motor, voltage affects speed much more than torque.
Thank you so much for the prompt reply.
I totally understand that the torque at each joint will be different because the servo at each joint will be present at a different height. Hence the different torques at different joint. Please add to or correct me if I’m wrong, as I’m just a beginner.
And by
Do you mean the weight of the entire robot must be 1/5th of the servo’s torque mentioned in the manual of the servo or the servo’s torque at that particular joint?
To cut everything short, I want to use the Ax-12A servos, 3 of them in each leg and 2 in the hips, so that it’s strong enough to lift a mass of 3.5 - 4 kg.Will the these servos be suitable for this application.
As for how much weight it can hold? Not sure. I know that many up on Trossen have built Bipeds. I know that many have gone with the heavier duty servos. They have a pretty good chart showing torque and speed of many of these servos: trossenrobotics.com/images/p … _guide.pdf
Yes, I’ve seen and it give a lot of amazing information.
But my concern here is the weight these servos can lift, as the biped walks.
The Ax-12a servo’s stall torque is around 15kg-cm. So I don’t have any idea of how less this stall torque will be, when the biped is walking, that too stably.
Is there a way in which I could use the heavy duty servos at the greater height of the biped’s legs, I.e.the hips and the first servo of the three leg servos, from the top? This way the brunt of the 4kg weight will be handled by e heavy duty servos, while the remaining leg servos could just be the Ax-12a servos?
Hope the logic I’m trying to apply here actually works this way.
Warning, I am not a Biped person… I may still have a Brat around here, so others should be able to give you better answers. I do know with Hexapods and Quads, that often the Hip servos on these have the least needs, often it is the Knee and Ankles that need a the
To confirm, you are aware that the Bioloid Premium kit includes AX-12A servos and the robot almost everyone builds using this kit is the humanoid: robotshop.com/en/robotis-bio … t-kit.html
The controller, battery, sensors, R/C, frame components, software etc. come to $400 more than the 18 servos included.
I believe his question is, are these strong enough if your robot weighs something like 4kg… Currently the answer up there is it depends… Also suggestion at maybe looking at some Parallel leg Mechanism (PLM), that a few have built over there…
Essentially you want to create a Bioloid Premium humanoid, but have the torso weigh 4Kg?
The Darwin-Op is considerably taller, uses much more powerful servos and still only weights 2.9Kg: robotshop.com/en/robotis-dar … eluxe.html
Although calculations are needed, you will likely have issues using the AX-12A on a 4Kg robot.
Hi,
Thank you soo much guys for responding to my query.
Yes, I agree with CBenson, Ax-12A’s seem a little weak. So, I’ve moved on to Ax-18A servos.
The torque of these servos is higher, 18kg-cm and the 1/5th of this torque is 3.6kg. So, it gives my robot to be of about 3.5kg margin, and still can make it walk stably.Please correct me if I’m wrong.
But my Question is Can I control the Ax-18A servos using an arduino?
Will these arduino interfaces work for Ax-18A servos as well? robotshop.com/en/robotis-ax- … rface.html
What sort of arm and leg assembly should I have for the robot to be able to lift one of it’s legs to walk (instead of shuffling), along with the hip assembly (the hip shouldn’t twist), using dyamixel Ax-18A servos and the brackets used for these servos.
Any help will be really really appreciated.
Thanking in anticipation.
Torque does not directly equate to weight. Torque is force multiplied by distance. For example, if a servo can provide 5Kg-cm of torque, that means it can lift (from -270 to +90 degrees) a 5Kg weight at the end of 1cm. If you change the distance to 10cm, it can only lift 0.5Kg (5kg-cm / 10cm = .5Kg). The most torque a walking robot would need would be if it does a sit-up, which entails lifting the entire weight of the upper torso from 180 degrees to 90 degrees. For the torque calculations, you need to estimate the major weights and positions of each to calculate the torque needed at each position.
Yes, but you would not be able to use the sample Dynamixel code for their humanoid, you would need to either create it from scratch.
Can you please elaborate a little on this?
Will I be able to use the arduino library present for the Ax-12A servos?
Will I have to program the arduino in such a way that it gets linked with the arduino interface of the servo?
Or Can I even use this arduino interface (robotshop.com/en/robotis-ax- … rface.html) in the first place,when my servos are Ax-18A instead of Ax-12A?
Can I find any exapmles of the Ax-18A servos controlled by arduinos on the net?
Again if you look at the dynamixel chart I mentioned earlier, the ax-18 does not have any more strength than ax-12, but is faster. Yes you can control them exactly the same way.
Now they have some stronger servos like mx servos. Many of the servos have two versions, an RS485 version and a ttl version which you can control like AX. There are differences, like the resolution of servos, 1024 vs 4096…
Controlling each servo Dynamixel using Arduino code is not the issue - creating or adapting the code necessary to make a humanoid robot walk, turn etc. is.
