How do I get my model to work correctly?

[mengysu]

Dear MuJoCo developers,
I am trying to use my humanoid model (21 freedom) for reinforcement learning, based on mujoco and mujoco-py. So far, I encountered some issues:

1. In my XML file, when I set the property armature = 0.01, It works fine if I just give the command, but if I train my model, it can not work well. When I set armature = 2.5, the model falls back if I give a direct command, but I can train for some bad effects with my model, like falling over after a single step. That confuses me a lot, what does ‘armature’ mean and how should I set the value?
2. In the process of learning the model, the agent interacts with the environment through step(action), where the code that interacts with the environment is do_simulation(ctrl, skip_frame). No matter if I set motor or position in the <actuator>, the joint only can move to a certain position, it seems like controlling the position. The difference is that the latter one is very shaky. I set the <actuator> as follows:

	<actuator>
		<motor name="waist"  gear="100.00000" joint="waist "/>
		<motor name="left_arm"   gear="100.00000" joint="left_arm"/>
		<motor name="right_arm"  gear="100.00000" joint="right_arm"/>
		<motor name="left_leg_1" gear="100.00000" joint="left_leg_1"/>
		<motor name="left_leg_2" gear="100.00000" joint="left_leg_2"/>
		<motor name="left_leg_3" gear="100.00000" joint="left_leg_3"/>
		<motor name="left_leg_4" gear="100.00000" joint="left_leg_4"/>
		<motor name="left_leg_5" gear="100.00000" joint="left_leg_5"/>
		<motor name="left_leg_6" gear="100.00000" joint="left_leg_6"/>
		<motor name="right_leg_1" gear="100.00000" joint="right_leg_1"/>
		<motor name="right_leg_2" gear="100.00000" joint="right_leg_2"/>
		<motor name="right_leg_3" gear="100.00000" joint="right_leg_3"/>
		<motor name="right_leg_4" gear="100.00000" joint="right_leg_4"/>
		<motor name="right_leg_5" gear="100.00000" joint="right_leg_5"/>
		<motor name="right_leg_6" gear="100.00000" joint="right_leg_6"/>
	</actuator>

I found that when I directly gave joint angles, it required a transformation, I got the relationship *100/7 through trial. I want to know why both motor and position are position control, as well as the actual process of control and their true transformation relationship.

[yuvaltassa]

1. Re armature inertia, see here.
2. Re position actuators and length scaling see Position actuators' control range stuck in degrees (even with the compiler set to radians) #249

[mengysu]

Thanks for your answer, but I also have some problems:

1. I have read the document but I still wonder why different armature has such different results, and how should I choose the correct value?
2. Position actuator means I should give radin/100, and motor actuator means I should give radin*100? However, the actual output is not correct, if I give the value radin*100/7 (maybe), the model will work well.

[yuvaltassa]

1. Try different values of armature to understand what it does. Maybe maybe make a test model with a single joint? I don't know how you should choose since I don't know what "correct" means for you. It depends on what you are trying to achieve.
2. Don't use gear for position actuators at all, use kp. Then the semantics will be what you expect.