cassie_alip_mpc

Overview

This repository provides an implementation of a bipedal locomotion controller, described in the paper Terrain-Adaptive, ALIP-Based Bipedal Locomotion Controller via Model Predictive Control and Virtual Constraints(pdf)(arXiv). The controller has two components: (1) an Angular Momentum Linear Inverted Pendulum (ALIP)-based Model Predictive Control (MPC) foot placement planner and (2) a gait controller which takes the foot placement solution as an input. This controller enables improved stability for walking on a variety of sloped and textured terrains. The controller is implemented on the Agility Robotics Cassie Robot.

• Authors: Grant Gibson, Oluwami Dosunmu-Ogunbi, Yukai Gong, and Jessy Grizzle
• Maintainer: Grant Gibson (grantgib@umich.edu)
• Affiliation: The Biped Lab, the University of Michigan

View Shortened Results Video here

View Extended Results Video here

Abstract

This paper presents a gait controller for bipedal robots to achieve highly agile walking over various terrains given local slope and friction cone information. Without these considerations, untimely impacts can cause a robot to trip and inadequate tangential reaction forces at the stance foot can cause slippages. We address these challenges by combining, in a novel manner, a model based on an Angular Momentum Linear Inverted Pendulum (ALIP) and a Model Predictive Control (MPC) foot placement planner that is executed by the method of virtual constraints. The process starts with abstracting from the full dynamics of a Cassie 3D bipedal robot, an exact low-dimensional representation of its center of mass dynamics, parameterized by angular momentum. Under a piecewise planar terrain assumption and the elimination of terms for the angular momentum about the robot's center of mass, the centroidal dynamics about the contact point become linear and have dimension four. Importantly, we include the intra-step dynamics at uniformly-spaced intervals in the MPC formulation so that realistic workspace constraints on the robot's evolution can be imposed from step-to-step. The output of the low-dimensional MPC controller is directly implemented on a high-dimensional Cassie robot through the method of virtual constraints. In experiments, we validate the performance of our control strategy for the robot on a variety of surfaces with varied inclinations and textures.

Contents

• Overview
• How the Controller Works
• Simulation Tests
  • Networking (Simulation)
  • Building the ALIP-MPC Foot Placement Executable on the Linux Computer
  • MATLAB Simscape Mechanics Simulator
  • Default Test
  • Advanced Settings
• Cassie Hardware Tests

Repository Organization

The code is organized as follows:

.
├── codegen_alip_mpc
├── cpp_alip_mpc
├── external_packages
├── matlab_alip_mpc
└── media

1. codegen_alip_mpc
   • Contains MATLAB and CasADi code that was used to formulate and code-generate the foot placement planner for C++.
2. cpp_alip_mpc
   • CMake workspace used to build and run foot placement executable.
3. external_packages
   • Contains CasADi packages.
4. matlab_alip_mpc
   • Contains MATLAB/Simulink files used for simulating the controller in Simscape Mechanics and building the Simulink RealTime controller.
5. media
   • Miscellaneous images and files for this readme.

Requirements

• Hardware
  • Windows 10 Computer
  • MATLAB 2017b
  • Ubuntu 18.04 Computer
  • Ethernet Cables
  • Agility Robotics Cassie Robot (needed for experimental tests only)
    • Basic understanding of building/operating controllers on Cassie.
• Software
  • On Windows Computer
    • this repo
    • MATLAB 2017b
  • On Linux Computer
    • this repo
    • MATLAB 2017b or newer
    • Visual Studio Code

How the Controller Works

A schematic is shown below that describes how the controller and cassie system are integrated. The controller is separated into two components due to computational limitations of the Cassie Simulink RealTime computer (fixed frequency of 2kHz). The foot placement planner portion is run on a secondary Linux computer. CasADi was used to formulate and code generate an optimization problem described in the (paper)[]. The gait controller is run on the main computer and sends torques to the robot to execute. These commands are computed using the method of virtual constraints and inverse kinematics passivity-based control.

Simulation Tests

The following section describes how to run the controller in simulation.

Networking (Simulation)

• Create an ethernet connection between the Windows 10 computer (running the Simulink Cassie Controller) and the Linux Computer (running the MPC footplacement algorithm)

Set up Windows Ethernet settings

• On the Windows computer, go to Network and Sharing Center->Change Adapter options. Right-click your Ethernet connection and select properties.
• Make sure Internet Protocol Version 6 (TCP/IPv6) is unchecked.
• Check and double-click Internet Protocol Version 4 (TCP/IPv4). Make sure Obtain an IP address automatically and Obtain DNS server address automatically are selected.

Set up Linux Ethernet settings

• On Linux computer, navigate to Settings->Network->Wired. Create a new connection by clicking the + button.
• Select the Identity tab and enter a unique ID.
• Select the IPv4 tab and enter a valid Address. The first 2 sequence of digits should match the ethernet address from the Windows computer (you can find this by running ipconfig in the command prompt). The next 2 can be uniquely made up. The Netmask should be set to 255.255.0.0.
  • Example Windows Ethernet IP Address: 169.254.24.246 with matching example Linux Ethernet IP Address: 169.254.252.150.

• You should now have a working connection. Check this by pinging each computer from each other.
  • An example result from the Windows computer would return

  C:\Users\gibso>ping 169.254.252.150
  
  Pinging 169.254.252.150 with 32 bytes of data:
  Reply from 169.254.252.150: bytes=32 time<1ms TTL=64
  Reply from 169.254.252.150: bytes=32 time=1ms TTL=64
  Reply from 169.254.252.150: bytes=32 time=1ms TTL=64
  Reply from 169.254.252.150: bytes=32 time=1ms TTL=64
  
  Ping statistics for 169.254.252.150:
      Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
  Approximate round trip times in milli-seconds:
      Minimum = 0ms, Maximum = 1ms, Average = 0ms
  
  

Building the ALIP-MPC Foot Placement Executable on the Linux Computer

This section gives instructions for building and running the foot placement component of the ALIP-MPC Controller on a secondary Linux computer. The Cassie Simulink RealTime Computer is fixed at 2kHz so this portion of the controller must be computed on a secondary computer to satisfy computational timing constraints.

1. Build Install Casadi by Source and Install
   • Open terminal and enter the following commands

   cd external_packages/casadi
   sudo apt-get install gcc g++ gfortran git cmake liblapack-dev pkg-config --install-recommends
   mkdir build
   cd build
   cmake ..
   make
   sudo make install
   

   • Update library path by adding the following to .bashrc

   export LD_LIBRARY_PATH=LD_LIBRARY_PATH:/usr/local/lib
   

2. Build Executables with CMAKE
   • We use Visual Studio code to build the executables. Open a VSCode workspace with cpp_alip_mpc/ the top directory. To build
     • Press CTRL-SHIFT-B and select make. This will create a new build directory and call the cmake and make commands. You can alternatively do this via the terminal.
     • Selecting make clean with remove the build/ directory.
   • If this is your first build on the Linux Computer you need to re-create shared object libraries for the code-generated casadi mpc solvers in cpp_alip_mpc/src/solvers/mpc.

   cd src/solvers/mpc
   ./../../../build/generate_mpc_solver_libs
   

   • If you have run make clean you can simply copy the libraries into the build/ folder instead of generating them with

   cd src/solvers/mpc
   ./../../../build/copy_mpc_solver_libs
   

3. Check that the executable works
   • Navigate to the build directory and run the executable.

   cd cpp_alip_mpc/build
   ./cassie_alip_mpc simulator
   

   • If you are not connected you should see

   *************************************************************
   ** ALIP-MPC Foot Placement Controller for MATLAB Simulator **
   *************************************************************
   
   Error binding to interface address: Cannot assign requested address
   

   • If you are connected to the Windows laptop you should see

   *************************************************************
   ** ALIP-MPC Foot Placement Controller for MATLAB Simulator **
   *************************************************************
   
   --> ALIP-MPC Foot Placement Controller Initialized!
   --> Connecting to cassie...
   

   Make sure you see this before proceeding.

MATLAB Simscape Mechanics Simulator

1. Open MATLAB 2017b with matlab_alip_mpc as the top directory.
   • Run start_up_sim.m
     • Opens all files that may require additional edits (IP address changes, initial simulator configurations, reference commands, etc).
   • In CustomInitFcn_wth_standing.m change the udp_linux_ip_address variable to match the ip address of the linux computer explained here.

Default Test

The default test has cassie initially stand, walk-in-place, and then walk down a 5 degree lateral slope (shown below).

To re-create follow these steps

• On the Linux Computer
  • Make sure the foot placement controller has been built by following the steps above.
  • Open a terminal, navigate to the build directory (cd cpp_alip_mpc/build/).
  • Type ./cassie_alip_mpc simulator but wait to press enter until the simulator is running.
• On the Windows Computer
  • Open Mpc_SimMechanics_with_standing.slx.
  • Click the play button.
    • You should see the Simscape Mechanics Explorer window appear.
• Once the simulator starts running (you see the time increasing in the Mechanics Explorer window), run the executable on the Linux Computer.
  • You do not need to hit it immediately. If you run the foot placement controller too quickly the UDP connection can sometimes have issues.

Tips

• Make sure to wait until the simulator is running on the Windows computer prior to running the ./cassie_alip_mpc executable.

Advanced Settings

Modifying Reference Commands

• The RemoteSpoofer_with_standing.m matlab system is used to mimic radio commands that are sent from the Agility Robotics supplied Radio Transmitter.
• The 'RadioCommandInterpreter_with_standing` matlab system gives insight as to how these radio signals are converted to controller reference commands.
• Modify values inside the spoofer script to change references like velocity, slope, friction, step width, step clearance, etc.

Modifying Terrain Slope

• The ground slope can be modified by changing variables in the simulationInitFcn_with_standing.m file.
  • Change the alpha_x and alpha_y varibles to do so.
• You should also change the slope estimate that the controller uses. To do so, change RadioButton.RSA and RadioButton.LSA to values between -1 and 1.
  • By default the 1 corresponds to a slope of 22 deg.
  • To match the 5 degree lateral slope in the default example RadioButton.LSA is set equal to 5/22 = 0.2274.
• You can alternatively choose incorrect slope estimates to see how the controller reacts.

Modifying Friction

• In RemoteSpoofer_with_standing.m change the RadioButton.SCA variable.

Cassie Hardware Tests

Click images for videos of Experiments.

Building Simulink RealTime Controller

• Open Mpc_RealTime_with_standing.slx. Press CTRL-D and Press build.
• Create Bootable USB and copy controller files according to Agility Robotics documentation here.

Using the Controller

• Plug the bootable USB with the controller into Cassie's computer.
• Connect Ethernet cables between the main Cassie computer and a secondary Linux Computer.
• For experimental tests, an ethernet connection between the onboard Cassie Computer and secondary Linux Computer is needed. After starting up Cassie and connecting the cables, follow the instructions for creating a new connection detailed above.
  • The Cassie Computer IP Address is set to 10.10.10.3.
  • Set the new Linux Computer IP Address to 10.10.10.150 and the netmask to 255.255.0.0.
  • Check the connection once the robot is on and the radio transmitter has connected.

  ping 10.10.10.3
  

• Turn on Cassie and run the homing procedure according to the Agility Robotics documentation.
• Run the foot placement executable on the Linux Computer.

cd cpp_alip_mpc/build
./cassie_alip_mpc cassie

• Set the radio transmitter buttons to the default values used in the Remote Spoofer file. Adjust the LS and RS buttons such that they are 0 to represent flat ground.
  • The manual can be found here.
• Start the controller in standing mode by setting SB to 0 and enable the torques by setting SA to 1.
• Switch controller modes to walking by setting SB to 1 and modify the other buttons accordingly.