Hexapod tripod gait is bumping too much!

In robotics, it is generally good to have an acceleration/ deacceleration curve for all movements. If you had some success making a more gentle curve for the up/down motion, then perhaps you also need to add this for the front/back movement.

By the way, servos often have their own internal acceleration/ deacceleration curves. The best servos allow you to adjust this curve (by some sort of digital interface). There is also some "dead space" where the servo won't attempt to move. I've also played with cheap R/ C servos and been able to adjust some of its "tightness" by changing the rate at which I send pulses to them. Normally, a pluse lasting 1 to 2 ms is sent every 20 ms. But, by increasing or decreasing the

20ms value, you can adjust how quickly the servo acts to correct its position.

Joe Dunfee

Try one leg at a time, in the sequence (using DIP numbering, 1 = left front) 1 - 5 - 3 - 6 - 2 - 4 ... You still have your triplets (1, 5, 3 and 6, 2, 4) but now you have six small taps instead of two kalumps.

8-)

This person also asked the same question on a Lynxmotion support forum, at

The discussion there is more informative. He's apparently using a Lynxmotion CH3-R robot:

This is a nice piece of machinery, but has no useful feedback. For over $700, there should be more sensors on the thing, enough to do what Rod Brooks was doing twenty years ago with his insect robots. But no. There's no force feedback signal for the servos. There don't even seem to be foot/ground contact switches. It's a dumbbot.

The typical programming approach is to put a table of leg position values into an Excel spreadsheet and load those onto the robot's CPU. Given that you have no useful info about what the legs are doing, that's probably as good as it gets.

John Nagle

Could you put some soft rubber neoprene feet on the bot?

Bob

Some ideas:

- Find the maximum acceleration which doesn't cause spilling; use something below that to ramp up/ramp down the foot velocity curve.

- Move all three legs at once, but set the feet down one at a time (sort of a tripedal gallop?)

- Start swaying the stance legs before the swing legs touch down.

- Mount the cup on a spring/damper cushion. e.g. foam or a small pillow

- carpet the floor

- Place a lid on the cup. ;-)

Good luck, Daniel

John is correct ... feedback is the answer here. Yes, you can always deaccelerate at the point of motion where the leg is most likely approaching the ground, but this is a bit wasteful of the hardware. But, following John's link to the Lynxmotion forum I get the feeling hardware mods are not allowed, as the student groups get the use of the robot, and the next tgroup may not appreciate the hacks made to it. So that means going into the servos and tapping out the motors for EMF feedback, or maybe even attaching some rudimentary whisker sensors, may not be allowed.

This has me thinking what overall use the exercise might have. If it's purely a programmatical problem, then it's all in the gaits and accel/deaccel curves. But it seems to me this is like trying to build a fuel efficient car using steam. The robot will never do as well than one with even simple feedback.

I once experimented with the servo controller from NetMedia that touts force feedback using unmodified servos (unmodified for rotation and unmodified for picking off the EMF from the motor). It worked well enough that the concept behind how it works could probably be used here to achieve a substantial improvement for this application, without guessing at how the robot should walk - i.e., the robot will decide that in actual motion. And this approach would not require hardware modification.

-- Gordon

Hi Joe,

thanks for your reply.

I will try varying the frequency of servo pulsing and see if this has an effect.

The robot is actually not one produced by Lynxmotion, I was just looking up "robotics forum" in Google and this is where I was led and because of the fact that their hardware seems to be very similar to the hexapod we have, I thought I'll take a chance asking. An image of the hexapod we have can be found here:

Hardware modifications are unfortunately not allowed, so we have to live without any feedback.

At the moment, I am thinking that one of the main problems we are having is that the CPU speed of the µC located at the board is just too slow. When working with an incrementation of 1 for each cycle of the gait sequencer loop, the hexapod seems to stay relatively stable, but much too slow. The only way to make it go faster is incrementing by 2 for each loop cycle, as there is no artificial delay in our code anywhere and the µC is operating at its limit, but then (especially in the leg-up and leg-down sections) the movement gets too unexact. To solve this, we tried to pre-calculate the servo angles for steps in each direction we want, but then the program gets too large to fit on the µC.

Another question: Does anyone know a web site where a tripod gait that does not stop the body of the hexapod for changing the stance legs is explained in detail? I think I am having some conceptual errors in my code, too.

Thank you very much for your help!

Patrick.

cadcoke4 schrieb:

Hi D.,

D Herring schrieb:

We will probably spend our remaining time doing this.

This sounds like a good idea, I will try it! Thank you!

I am not completely sure if I got your idea, but if I did, wouldn't this even amplify the bump by doubling the speed at which the legs hit the floor?

Not allowed to do so :-)

Also not allowed.

...

Thanks for that!

Patrick

Art Kuo[1] did some work on the energy costs of human gaits. IIRC, one of his findings[2] was that you could reduce the energy lost on foot-ground impact by pushing off with the stance leg just before the swing leg struck ground.

Yeah, timing this wrong could be bad; but if done right, you should be able to the the torso moving _in anticipation of_ the foot impact, thus softening the blow.

- Daniel

[1] This paper might describe it especially figures 1 and 2.

Exactly. This is what irks me about most robotics hardware for hobbyists

- totally inadequate feedback. This is teaching kids the Wrong Stuff. If you want to deal with the real world, you have to have considerable feedback. This is NOT like programming a computer, where what you command happens reliably.

The pathetic thing is that even simple R/C servos have position and current info available within the servo, yet so few of them bring it out to where a controller can read it.

The Dynamixel servos have real feedback and a two-way digital interface, but are more costly than R/C pulse-coded servos. Bioloid, Troxxen, and Robotis robots now use them. Slowly, the hobbyist robotics industry is getting enough volume that the components become affordable.

John Nagle

However it would be counter-productive to add this to all the hardware because there is no one generic use of robotics components. It would generally be overkill, for example, to provide feedback (and therefore cost) for servos used on the typical line-follower. For kids price is paramount; you don't want to foist $300 kit of fancy parts when $100 will do.

To me this is an integration issue, but it won't get better any time soon. The Robotis servos are nice, but I bet the company is still working through the R&D cost. It's not like hobby servos where the R/C market - which does not require feedback - is MANY time larger than amateur robotics.

The ideal product is the one with a base model at an affordable price, and options that can be added on. It's a wonderful idea, but who has the money to create such a product line for what is ultimately a market of a few tens of thousands of potential buyers. Too rich for my blood!

-- Gordon

Hello,

I just wanted to say thank you once more for all your help, especially to Daniel who had the idea with the better timing and to Joe who made me pay more attention to the acceleration and deacceleration phase.

All in all the gait performed by the hexapod seems quite OK now, although not 100 % perfect. I'm curious about how the other teams will do tomorrow at the competition :-)

I'm going to bed now (it's 23:57 over here), so have a good night!

-- Patrick

The Linxmotion 6-legged thing is over US$700. At that price, there should be decent sensing.

Things are happening faster than you think. Take a look at "

" for what's going on in Japan. Some very interesting robotic toys will be shipping for Xmas 2009, too.

For some things, especially electronics, integration is cheaper than add-ons. The extra silicon is cheaper than the connectors and wiring for the accessories.

John Nagle

You're still missing the point. For R/C servos, 99% of the intended buyers would have no use for such a thing. The manufacturer is wasting his money, even if it's only incremental. There is no business sense in that, and this is business. They couldn't care less about the state of the art of robotics. (And those that do charge a lot for those servos, if you've ever priced them. Beyond what most of the kids you referred to a message ago can afford.)

-- Gordon

Some of Hitec's newer servos supposedly talk a "proprietary HMI protocol", which supposedly allows daisy-chaining. There's a document on this ("

")but it's almost useless. Somebody finally reverse-engineered the things and produced a document:

which at least describes how to talk to the thing via a serial port at 19.2KB.

Much to the annoyance of robotics people, the Atmel ATmega8 in the thing has been set to "mode 3, further programming and verify disabled", so you can't look at or replace the firmware.

John Nagle

of the participants got a circular hexapod robot

It doesn't really invalidate the points you are trying to make, but the original poster is not using the CH3 - just one similar in configuration. (see the 1st reply to cadcoke ... )

fwiw ... jcd

If you have to do this without feedback, probably the thing to do is to put in an IK solver and control the endpoints of the feet.

John Nagle

of the participants got a circular hexapod robot

By "IK solver" do you mean Inverse Kinetics ? I've never tried any of that at all, so I'll ask this: given the limited hardware, would it involve a look-up table or would it need to calculate everything

*completely* for every step ?

Just a thought... thanks! JCD

Inverse kinematics; given the end point (foot position), calculate the joint angles.

Here's a simple two-link 3D IK solver in Java, with a demo applet. The first joint has 2DOF, the second joint has 1DOF, which, I think, is what your robot has.

Doesn't even need any trig functions.

Once you have an IK solver, you can treat six legs as two triangular supports, each of which is moved as a unit.

John Nagle