Help with paycap method

[adriencholet]

Hello,

I am having difficulties using the paycap method for a ERobot model.
What I understand is that it should return the maximum payload wrench that my robot can handle.
I do not understand what is supposed to be input for the w parameter "the payload wrench" as it should be what the function is solving for.
Thank you in advance for your help !

[petercorke]

Hi @adriencholet, thank you for your interest in RTB, and apologies for the very long delay in replying. Various life events got in the way, and since May this year I've been doing a lot of work across RTB, Swift and SpatialGeometry.

Your confusion is justified, and the docstring is misleading. In paycap(w, tauR, q=q), w is not the answer: it is just the direction of the load you want to test (a 6-vector of force and torque at the end-effector). It is normalised internally, so only its direction matters, and the function tells you how far you can push along it before some joint hits its torque limit. For example, for a downward force:

puma = rtb.models.DH.Puma560()
tau_max = np.array([97.6, 186.4, 89.4, 24.2, 20.1, 21.3])   # joint torque limits
tauR = np.column_stack([tau_max, -tau_max])                 # columns are max, min

wmax, joint = puma.paycap([0, 0, -1, 0, 0, 0], tauR, q=puma.qr)
load = wmax.min()      # maximum force (N) along w; `joint` is the joint that limits it

On the Puma 560 at qr that gives about 4390 N with joint 2 limiting. I checked that by computing the joint torques at that load, where joint 2 is exactly at its limit.

Two things you should know. The return value is not the 6-element wrench the docstring says: it is the limit for each joint along w, so take the minimum, as above. More importantly, paycap currently fails for robots that don't have 6 joints, for example the 7-joint Panda, and it uses gravload, so it needs a model with dynamics (the DH models have them, the default URDF-based Panda does not). I have opened #728 for the crash and the docstring.

If you hit anything else, please post an Issue rather than a discussion thread. Thanks for the question.