KHR-1 „Steelpump“ humanoid project:

As you know the Kondo KHR-1 Humanoid Robot has KRS 784 (blue) or KRS 786 (red) servos with 8,7kg torque at 6V. This is barely enough to carry himself. It is not enough to carry payload or stand on one leg.

When the KHR-2HV appeared I was happy to learn he had stronger and a bit faster servos, but 10kg torque is still not a lot. So I decided to rebuild the KHR-1 with stronger servos.

This is what this topic is about. It is all about making the KHR-1 stronger

I will describe the necessary issues to rework your KHR-1 to a 20kg version in the following chapters and guide you through the steps. Hope you like it.

first I had to decide which servos to choose. To keep most of the parts of the KHR-1, I decided to look for servos of pretty much the same shape and size. Also they must run with 6V and should be compatible with H2H and RCB-1 boards. Then they should have the digital position feedback for H2H teach in.
I checked Kondo servos, saw the KRS2346, found out they have been replaced by the KRS 2530. These servos offer 20kg/cm torque at 6V, have 180 degrees turn, metal gear and ball bearings, all at about 60gr per servo which is only a bit heavier as the standard KRS 786 servo.
Everyone who has already the KRS786 servo knows you might want to replace the standard reisin gears by the metal gears, but then the metal gears are only two of four gears. In the KRS 2350 all gears are metal, the final gear is metal too (where the hone sits). I like that.

I ordered some of the servos and (uff) price is very high – as with all servos as soon as you surpass 12 or 15 kg torque. It depends on the volume of servos purchased but there is a price tag of about 120 EUR / servo. Anybody remembers the 27kg YDA-EZH Robot from Kondo? This guy was about 4000 EUR a piece. Now you can have a KHR-1 with 20kg for less than the half of this price. I think this is a deal, if you can afford.

You can replace the servos joint by joint, you do not have to replace all, there is no use for a 20kg torque head, or?
Well what are the most important joints?
This would be a minimum of 6 strong servos for a rebuilt.
This would be a „best“ solution with 10 servos.
Skip the head and make it a maximum solution with 16 servos. (now we exceed 2000 EUR when you add the KHR-1 standard unit)

Here is now a more in depth discussion of the servo differences between the standard KRS786 servo and the strong KRS2350 servo:
First of all they both have 0.14mm cable, Futaba standard. The cable length is 32.5cm with the KRS786 and only 23.5cm with the KRS2350 which is a difference of 10cm and might lead to the need of adding an extension cord to reach the RCB-1 board. The KRS786 has plastic gears and no bearing:

The KRS2350 servo has metal gear and ball bearing:

KRS2350gear The hones of the KRS786 (ACTIVE SIDE !!!) fit onto the KRS2350 so they are compatible. Good.

The shape differs especially on the top:
You see the the different shape of the top? This leads to a redesign of the shoulder mount and chest frontplate, done here:
I had to change shapes a bit, now you can either use old shoulders or new shoulders with the new chest plate, holes are compatible. Old servos do not fit in new chest plate, use old chest plate and new shoulders if you only want to have strong arms.

Another difference is the size of the passive axis on the back of both servos. The KRS786 has a 6mm diameter axis and the KRS2350 has a 8mm diameter axis. See here:
With the 8mm axis the standard passive hones would not fit. This is why we need to swap parts a bit. First we open the servos:

servos_opened Then we put away: the original back of the KRS2350 the original screws of the KRS786.This is what is left over, now add my „KRS2350 passive hone extender“. It is a folded piece of aluminium to ensure the same height as KRS786 and to protect the inside of the KRS2350:
First click the passive hone extender to the KRS786 backplate:

Then use the screws and mount everything together. You will notice the screws will be more tighter and it is extremely helpful if you have an electric screwdriver, if you do not, then stay patient. Here is the result:

That’s it. The plastic parts to the left and right can be sewed off. (Remark: There is a special „knee enforcement kit“ which I came up with later, to mount this kit two of these plastic parts may not being sewed off) At most places they can stay as they are, they do not interfere with the KHR-1 structure. (Remark: so you could leave them as they are if they do not interfere with the aluminium structure. Be careful, they might touch cables because spacing is now reduced to a minimum).

Now you can mount this guy wherever you like in the KHR-1! I wonder who has done it before? I did hear about BreDoBrother Robosoccer team using a KRS2350 Servo for the knees of their robot. Do you know other people having done strong servo mods for the KHR-1?

I have just made a small „strong leg“ demo. The standard KHR-1 was modified with a new Chestplate and 7 servos of KRS-2350 all on the right leg and both hips (in the Chest). No care had been taken about cables, home position. I used the pose read in function of H2H and assembled some poses standing on his right leg and balancing heavily with his full weight on one or some joints. I did not care about beautiness or smoothness of servo transitions. I paused the movements at each step, this causes a bit of shaking but gives more time to look at it.

The result is shown in this little video

It works very well, battery drain is not percieved faster than before. I guess this mod is very useful for kicking and walking stairs and ramps. Also it might be good for some new tricks. He could carry a high payload now. Will try this later.

Knee supporters:
Here is an additional modification for the „Steelpump“ KHR-1:
Look at the weak point in the original design of the KHR-1 knee (fixed with KHR-2HV):

The orange arrow at the left shows the „open end which can be bent to open widely. The yellow arrow shows where the bend in the 0.8mm aluminium structure will occur. Here is a simple prototype solution:


A final note to this project: The modification took me about 1 month and was done in June/July 2006. The KHR-1 Steelpump has now a place on my shelf and waits to be reused if I ever have time again.


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