Setup

1. Basic structure of skills
2. Make a skill config file
3. Compiling skills
4. Maintaining skill libraries

Basic structure of skills

Skills are essentially families of pressure trajectories defined by some channel-wise equations. Parameters can be defined and used in these equations to create dynamic “trajectory generators” which act like python functions.

Each skill is made up of four basic components:

1. context (list) - A list of control configuration profiles where the skill is valid to use (this is used during runtime to ensure skills are only run when the control system is configured correctly).
2. variables (dict) - A dictionary defining all the parameters of a skill as well as their default values.
3. postures (dict of list) - A dictionary defining all relevant vectors of pressures (what we call “postures”), where each element in the pressure vector is an equation which can utilize variables. (Each equation is interpreted as a string and variables are substituted into the text before evaluating.)
4. skill (dict of list) - A dictionary defining prefix, main, and suffix trajectories. Each trajectory is a list, where each list entry defines a time and posture.

Make a skill config file

This example will guide you through making your first pressure skill based on the “rotate4finger” skill:

“rotate4finger.yaml” on GitHub

1. Define the controller context (list of config profiles where your skill is relevant):

    context:
      - anthro8
      - anthro8mixed
   

2. Define the variables you want to use:

    variables:
      idle_pressure: -5     #[psi]
      grasp_pressure: 20    #[psi]
      twist_offset: 5       #[psi]
   

3. Define a set of postures to use in your skill. Each posture contains a vector of pressures with element-wise equations (which can use variables):

    postures:
      idle:
        - "idle_pressure"
        - "idle_pressure"
        - "idle_pressure"
        - "idle_pressure"
        - "idle_pressure"
        - "idle_pressure"
        - "idle_pressure"
        - "idle_pressure"
   
      .
      .
      .
   
      rotpos:
        - "grasp_pressure-twist_offset"
        - "grasp_pressure+twist_offset"
        - "grasp_pressure-twist_offset"
        - "grasp_pressure+twist_offset"
        - "grasp_pressure-twist_offset"
        - "grasp_pressure+twist_offset"
        - "grasp_pressure-twist_offset"
        - "grasp_pressure+twist_offset"
   

4. Define the skill in terms of postures vs. time. Trajectories are dynamically time-scaled later when they are compiled, so the times in this definition file only represent relative lengths of trajectory segments. Also, be sure to include default times.

    skill:
      default_times:
        prefix: 1.0 #[sec]
        main:   4.0 #[sec]
        suffix: 1.0 #[sec]
           
      prefix:
        - time: 0.0
          posture: idle
        - time: 1.0
          posture: grasp4
   
      main:
        - time: 0.0
          posture: grasp4
        - time: 0.25
          posture: rotneg
        - time: 0.75
          posture: rotpos
        - time: 1.0
          posture: grasp4
   
      suffix:
        - time: 0.0
          posture: grasp4
        - time: 1.0
          posture: idle
   

Compiling skills

Skills can be complied into pressure trajectories dynamically using variables.

#!/usr/bin/env python
import rospy
from pressure_controller_skills.build_skills import SkillBuilder

# Define the specific values of skill parameters and times to use when compiling a trajectory
variable_ovr = {'idle_pressure':  0.0,
                'grasp_pressure': 24.0,
                'twist_offset':   3.0,
                }

time_ovr = {'prefix': 2.0,
            'main':   20.0,
            'suffix': 2.0,
            }


# Define the location of the "rotate4finger" skill. Note the omission of the file extension.
filename = 'rotate4finger/rotate4finger'

# Define the context (i.e. pressure control setup) you are currently using.
context  = 'anthro8' # Use the anthro8 context

# Create a SkillBuilder object to load and compile the trajectory
node = SkillBuilder(context=context) # Omit the context to skip safety checks.
node.load_skill(filename) # Load a skill from a file
skill = node.generate_skill(vars=variable_ovr, times=time_ovr) # Generate the skill
node.save_skill(filename) # Save the generated skill

Maintaining skill libraries

It’s very important that skill files are saved in the correct location so that the package can compile them.

By default, skills should be saved in the “skills” folder within this package. If you would like to save skills in a different location, you need to explicitly define the location when instantiating a SkillBuilder object. For example, you could make a new ros package called “my_skills” to hold your skills, and contain them all inside the “pressure_skills” folder inside your new package. Then your code becomes:

skill_ros_package = 'my_skills'
skill_folder      = 'pressure_skills'
context           = 'anthro8'

# Create a SkillBuilder object
node = SkillBuilder(context=context, ros_package=skill_ros_package, folder=skill_folder)

# Proceed like normal -->