Welcome to bitbots_quintic_walk’s documentation!

This document shows how walking parameters can be tuned and describes how the bitbots_quintic_walking works.

Parameter Tuning Guide

The Quintic Walk tries to rely on as few parameters as possible and most of them should be independent from each other, meaning changing one does not require a change in another parameter. When tuning parameters following things should be kept in mind:

  1. Make sure the robot has no hardware problems, e.g. loose screws. Otherwise your tuning is kind of senseless

  2. Use slow motion videos from smart phones to debug the walking. This way it is much easier to see what is happening.

  3. Test different walking directions and speeds. Parameters may work perfectly when moving forward but will fail catastrophically when walking sidewards.

  4. When tuning completly new, first do it in rviz. This lets you tune at least to the point where it looks like it works and prevents damage to the robot by colliding legs.

  5. First tune open loop (without any PID control or phase reset). Otherwise problems from these parameters are maybe hidden by the stabilization approaches.

Tune the parameters in the following order:

  1. foot_distance: Set this so that the feet are under the hip or slightly outwards of them

  2. trunk_height: This is a trade off between high (legs are completely straight) and low (knees are bend). The more the knees are bend the more torque is applied to the knees (this is bad) but the maximal step length gets longer since the kinematic chain allows more movement of the flying leg (this is good). Good values are normally slightly bend knees.

  3. trunk_pitch: This should be set so that the robot is stable in pitch direction while standing still. Any pitch necessary from walking faster is coming from another parameter.

  4. foot_rise: This highly depends on the ground. For hard floors, just a centimeter should be enough. For artificial grass its around 5cm. Tune it so that the flying foot does not touch the ground while moving. Larger than necessary values are probably bad since they result in larger accelerations.

  5. freq: This determines how fast your steps are and thereby also how fast you can run since the maximum speed ist maximal step length times step frequency. Faster steps also mean a shorter absolute single support time which often results in a more stable walk. Since each step leads to a small odometry error, a higher frequency leads to more odometry errors. Typical values are between 0.5 and 1.5 steps/s.

  6. double_support_ratio: This determines how long per step both feet are on the ground. It should never be zero, since this will most likely not work because the accelerations for the body would get infinitive on step change (see “Introduction to Humanoid Robotics, Shuuji Kajita, 2015” for a detailed explaination). To large values will force the flying leg to move very fast during single support. This value can be fine tuned after setting the other values. Typical values are between 0.05 and 0.3.

  7. trunk_swing: This value determines how much the torso is moving sidewards during walking to put the center of pressure over the support foot. If the robot is falling to the inside of the support foot the value has to be increased, if it is falling to the outside it has to be decreased. The value of 1 means that it goes during maximal sidewards movement as far as the footDistance. Typical values are between 0.1 and 1.

  8. trunk_phase: This moves the trunk movement in relation to the leg movement in phase time. This is difficult to tune but often the value that you want to change when you don’t know why the walking doesn’t work. Typical values are between -0.1 and 0.1.

  9. trunk_pitch_p_coef_forward: This gives more trunk pitch when running forwards and less when running backwards. Increase this when the robot is falling to the front or back when getting faster.

The other values are mostly not that influential but can also be tried.

General Structure

The walking consists of three main parts:

  1. The ROS node (walk_node.cpp) which is the main class controlling the walking and connecting it to ROS

  2. The walking engine (walk_engine.cpp) which generates cartesian poses of the flying foot and the trunk in relation to the support foot based on quintic splines. It uses internally a finite state machine to track the state of the walking.

  3. The inverse kinematic which computes the necessary joint values to reach the computed cartesian poses. We use BioIK or KDL for this through the MoveIt! interface.


Quintic Splines

Splines are a mathematical method to describe a curve using spline points. Have a look in the internet for more details.

We use quintic splines to be able to define the position, velocity, acceleration and jerk at each spline point. This enables us to generate smooth trajectories in the cartesian space.

We currently use an ROSified version of Rhobans spline implementation from the bitbots_splines package. The x-achsis is always the time while the y-achsis is the value of position or rotation.

The Walking Engine

The walking engine consists of mainly two methods. One builds the splines for the next step and one gives the poses for trunk and flying foot at a given time point in the trajectory.

The engine uses a fixed time frame for the step which can be changed by the frequency parameter. In order to simplify the splines and make them independend from the frequency, a so called “phase time” is used. Therefore, changing the frequency just compresses or streches the splines in the time axis. The engine handles two trajectories for the flying foot and the trunk. This results in 12 splines, since we need each 3 splines for position and 3 for rotation. Additionally there are some splines having meta data like which foot is the support foot. Each step starts with the double support phase and ends with the single support phase.

When the next step is computed, the last positions, velocities and accelerations are used as start points for all splines, meaning as the first spline point at time 0. Afterwards, other spline points are added. Thoses points are pre programmed but their x and/or y values can be influenced by either parameters or the desired step length. Usually those points are on meaningful time points like the switch between double and single support.

Since these trajectories are only build when doing a new double step, changes in parameters or walking commands are only shown when the current double step is finished. The method which computes the poses at a given time point only takes the prebuild splines and computes the values for each of the splines using the methematical spline interpolation.

_images/FSM.png _images/engine_step.png _images/engine_params.png

The Walking Node

The walking node subscribes to the /cmd_vel topic which provides a Twist.msg commanding the velocities in x,y and yaw. If it recieves only zeros, it will completly stop. If there are values present, the node calls regularly the engine to get the next cartesian poses. Since those poses are in the support foot frame, it translates them to the base_link frame and calls the IK to get the corresponding joint values. These are then published as new goals for the servos. Furthermore, the node computes the walking odometry by adding up the changes to the robots pose from each step. This odometry is then published and used by the localization. Since the robot does not slide very much, the odometry is close to the truth. The node also publishes a debug topic where it provides information on the splines, as well as RViz marker messages.

To be able to change the parameters of the walking, the node provides a dynamic reconfigure interface. Each time a parameter is changed, the new parameters are handed to the walking engine.

The Stabilizer

The stabilizer uses a PID controller with feedback from the IMU to control the tilting of the upper body. This helps the robot to not aggregate errors over time and falling after several steps.

Indices and tables