# Combining Coarse and Fine Physics for Manipulation using Parallel-in-Time Integration

## TL;DR

This paper introduces a hybrid physics prediction method combining coarse and fine models using the Parareal parallel-in-time algorithm, enabling faster and accurate manipulation planning and control in robotics.

## Contribution

It adapts the Parareal algorithm for physics simulation, enabling parallelized, accurate, and fast predictions for manipulation tasks.

## Key findings

- Achieves similar success rates as detailed physics engine planning.
- Runs significantly faster due to parallelization across time.
- Validated in both simulation and real robotic experiments.

## Abstract

We present a method for fast and accurate physics-based predictions during non-prehensile manipulation planning and control. Given an initial state and a sequence of controls, the problem of predicting the resulting sequence of states is a key component of a variety of model-based planning and control algorithms. We propose combining a coarse (i.e. computationally cheap but not very accurate) predictive physics model, with a fine (i.e. computationally expensive but accurate) predictive physics model, to generate a hybrid model that is at the required speed and accuracy for a given manipulation task. Our approach is based on the Parareal algorithm, a parallel-in-time integration method used for computing numerical solutions for general systems of ordinary differential equations. We adapt Parareal to combine a coarse pushing model with an off-the-shelf physics engine to deliver physics-based predictions that are as accurate as the physics engine but run in substantially less wall-clock time, thanks to parallelization across time. We use these physics-based predictions in a model-predictive-control framework based on trajectory optimization, to plan pushing actions that avoid an obstacle and reach a goal location. We show that with hybrid physics models, we can achieve the same success rates as the planner that uses the off-the-shelf physics engine directly, but significantly faster. We present experiments in simulation and on a real robotic setup. Videos are available here: https://youtu.be/5e9oTeu4JOU