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Posted: **Thu Sep 05, 2019 10:42 pm**

Hi Thomas,

I am wondering if there is a way to specify the initial height of the model for a simulation? My goal is to investigate a drop landing task.

I have tried using the standing high jump tutorial as a starting point, as I thought it might create a "drop jump" sort of effect so long as the initial pelvis_ty value could be specified. I first tried modifying the baseline .osim model to have a default starting pelvis_ty of a high value (e.g. 1.4m), but it did not create the effect I was after. I also tried specifying an initial states.sto file, which I pulled from the standing balance tutorial, and then modified the pelvis_ty to a higher value, again with no success.

I'll admit my knowledge of the scone environment is still quite limited, so I could be missing something quite basic. Could you point me in the correct direction if what I am trying to do is indeed possible?

Cheers,
Nirav

Posted: **Fri Sep 06, 2019 6:14 am**

Hi Nirav,

Your approach to increase pelvis_ty is correct -- however, by default, SCONE moves the model so that it touches the ground plane with a specific force. You can disable this behavior by specifying an 'initial_load' of zero in the 'OpenSimModel' section of your .scone scenario:

Code: Select all

OpenSimModel {
	model_file = data/Human0914.osim
	initial_load = 0
}

Let me know if this works!

P.S. I noticed this feature is missing in the documentation, I'll make sure it gets updated.

Posted: **Sun Oct 06, 2019 9:51 pm**

tgeijten wrote: ↑
Fri Sep 06, 2019 6:14 am
Hi Nirav,

Your approach to increase pelvis_ty is correct -- however, by default, SCONE moves the model so that it touches the ground plane with a specific force. You can disable this behavior by specifying an 'initial_load' of zero in the 'OpenSimModel' section of your .scone scenario:
Code: Select all
OpenSimModel {
	model_file = data/Human0914.osim
	initial_load = 0
}
Let me know if this works!

P.S. I noticed this feature is missing in the documentation, I'll make sure it gets updated.

Hi Thomas,

Thank you for the suggestion, that worked perfectly as I now have the model started above the ground as desired. I have been spending the last few weeks trying a few different constraints to create either a drop jump or drop landing (i.e. landing from a height without jumping up again), and have a couple more questions if you do not mind.

To make the model jump, I have included a BodyMeasure aiming to maximise the vertical velocity of the pelvis. However, from looking at the documentation, I have been unable to work out exactly how to do this. I have achieved reasonable results by including the following in my composite measure (along with a few other measures e.g. balance), but could you advise me on how to maximise vertical pelvis velocity if this is incorrect?

Code: Select all

BodyMeasure {
	body = pelvis
	axes_weights [ 0 100 0 ]
	velocity { min = 0 max = 0 abs_penalty = -100 } #negative penalty seems to produce the right sort of motion
	minimize = true
	weight = 1000
	threshold = 0
	start_time = 0.145 #this is approximately when the model lands
}

In addition, I have also tried minimising pelvis vertical velocity in order to try to make the model land (but not necessarily jump up again). I have tried writing this a few different ways, but it seems to produce identical results even if I change some of the settings (which leads me to believe I have not understood the documentation correctly).

Here was my first attempt for a measure to minimise vertical velocity of the pelvis.

Code: Select all

BodyMeasure {
	body = pelvis
	axes_weights = [0 100 0]
	velocity { min = -inf max = inf abs_penalty = -100 } 
	minimize = true
	weight = 1000
	threshold = 0
	start_time = 0.145
}

This produced a very "stiff" landing, so I tried to also minimise the "peak" vertical acceleration, but it made no difference.

Code: Select all

BodyMeasure {
	body = pelvis
	axes_weights = [0 100 0]
	velocity { min = -inf max = inf abs_penalty = -100 } 
	acceleration { min = -inf max = inf abs_penalty = -100 mode = highest } #changing the penalties did not change anything
	minimize = true
	weight = 1000
	threshold = 0
	start_time = 0.145
}

Since that did not work, I tried adding a measure to minimise the peak ground reaction force to the composite measure, but again my results were identical.

Code: Select all

BodyMeasure {
	body = pelvis
	axes_weights = [0 100 0]
	velocity { min = -inf max = inf abs_penalty = -100 }  #just velocity this time, left out the acceleration component
	minimize = true
	weight = 1000
	threshold = 0
	start_time = 0.145
}

#measure to minimise GRF, not sure if I have done this correctly. I tried to set a range similar to the BodyMeasure above, but kept getting errors.
ReactionForceMeasure {
	abs_penalty = -100 
	mode = highest 
	minimize = true
	weight = 1000 #I even bumped this up to 10000 with no difference. 
}

I am probably making basic errors, but any advice/suggestions you can provide would be appreciated.

Cheers,
Nirav

Posted: **Wed Oct 09, 2019 5:23 am**

Hi Nirav,

Good to hear you got it working. A couple of things that might help:

If you use BodyMeasure, you have to specify a valid min / max range (-inf / inf is not a valid range) -- the penalty will only be applied when a value is outside this range.
Look at the Standing Balance tutorial for a good example on how to define an objective for balancing. These should work for different initial heights after lowering the termination_height parameter in the BalanceMeasure.
The reflex-based control strategy used in the standing balance tutorial might work better for landing than just a FeedForward controller.
To penalize stiff landings, add a ReactionForceMeasure. Make sure to tweak the penalty weighting and the max of the range.
When combining different measures in a CompositeMeasure, you can see the individual contributions in the Messages Window after running the simulation (Ctrl + T). This can be very useful for tweaking the weights and the offsets.
If you want full control over your objective function (useful for complex tasks such as jumping after landing), using a ScriptMeasure is probably the way to go. See Tutorial 6a for a good starting point for writing a custom script Measure.

Hope this helps!

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Specifying initial height?

Specifying initial height?

Re: Specifying initial height?

Re: Specifying initial height?

Re: Specifying initial height?