What this is

pam_interface provides a python interface to the Pneumatic Artificial Muscle (PAM) (pam) developped at the Intelligent Soft Robots Laboratory (Empirical Inference Department, Max Planck Institute for Intelligent Systems).

Robot overview

PAM is a robotic arm with 4 rotating joints. Each joint is controlled via 2 artificial muscles (agonist and antagonist muscle). Motion of the joint is obtained by tuning the pressure applied to these two muscles (i.e. applying contractions to the muscles). The robot is connected to a pressure source.

The control PC of PAM includes an FPGA which connects to the electronic box which will control the valves setting the pressures. The control PC runs CentOS 7, which is one of the OS supported by the FPGA driver.

The pam_interface provides a python API for:

• sending desired pressures to each muscle
• reading the sensors of the robot, which consist for each joint of:
  • the measured pressure
  • the encoder value
  • a boolean indicating if the encoder reference is found (more about this below)
  • the angular position of the joint
  • the angular velocity of the joint

The angular position of the joint is provided relative to the posture of the joint when the FPGA was started. This may not be very useful. The sensors of the robot will provide the absolute position of the joint once the joint crossed a "reference" middle position. If the "reference found" is true, then the joint crossed at some point this middle position and the angular position is absolute. If not, it is relative.

Installation

On ubuntu

pam_interface follows the general guidelines of IRS, and is provided by the treep project PAM.

Ubuntu is not supported by the FPGA controlling the robot. Installing pam_interface allows only to run the pam server over a dummy virtual robot (see below), useful only for debug.

On the control desktop

The control desktop runs CentOS and should have all the required dependencies already installed. You may therefore simply use treep to clone PAM, and compile as usual.

Usage

starting the server

After compilation and sourcing of the workspace, the pam_server is available, and can be started:

pam_server

The above will start the server over a dummy simulated robot (no realistic physics, just for debug). To start a server over the real robot:

pam_server real

The above obviously assumes you are working from the robot control workstation. See somewhere below for the instructions on how to start the robot (beyond the server)

printed data

Once the server started, it will print in the terminal, for each of the 4 dofs:

• the value of the encoder (if the reference has not been found), the angular position of the joint otherwise (in degree)
• the value of the observed and desired pressure for the agonist and antagonist muscles.

sending desired pressures and reading sensors

In another terminal, you may start a script using the following API:

segment_id = "pam_robot"

target_pressure_agos = [12000,14000,16000,13000]
target_pressure_antagos = [13000,15000,15000,12000]

# sending desired pressures to the robot
pam_interface.write_command(segment_id,agos,antagos)

# reading information from the robot
robot_state = pam_interface.read_robot_state(segment_id)

warning: "write_command" does not proceed any security check. If you send desired pressures that are very different to the current desired pressures, violent motions are likely to occur.

robot_state is an instance of RobotState.

See its documentation [here](to do: point to the right url)

Starting the real robot

Parts :

• the "electronic box", which is turned on via the power outlet
• the pressure valve (against the wall). Horizontal means closed
• the emergency stop button, which blocks the pressure
• the control software server (pam_interface or o80_pam)

Warning :

What is to be avoided is to have the pressure applied to the robot when the electronic box is off, or the electronic box is on but the fpga has not been initialized (the fpga is initialized by the control software server). The electronic box and the server ensure that the pressure applied to the robot is limited. If the robot is exposed to the pressure while the electronic box is off or the fpga has not been initialized, the maximal pressure will be applied to it and damage may occur. -> applying the pressure is always the last thing to do <-

What to do when in doubt : Press the emergency button.

Starting state:

• power outlet off
• emergency button pressed
• pressure valve closed (horizontal)
• 4th degree of freedom of the robot "aligned" (silver paint marking aligned with zero)

Starting steps:

1. Start electronic box (switch power outlet on)
2. Start server (pam_interface or o80_pam)
3. Turn on pressure on valve (vertical)
4. Release emergency button: pressures should reach their minimum values as set in the used json configuration file
5. Observe robot. A short motion (i.e. half a second) should be observed. If the robot keeps moving, press emergency button

Stopping steps:

1. Stop server (software has pressure decrease on exit)
2. Press emergency button
3. Close pressure supply valve
4. Turn off electronics

Checking the robot

The executable pam_check can be run once the server started and the pressure applied to the robot. It will apply pressure to each muscle one by one and generating plots of the observed/desired pressure.

pam_check

You may run the check over only (the two muscles of) 1 joint:

# only the first joint
pam_check 0