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Title: Discovering Motor Programs by Recomposing Demonstrations

Paper content:
In this paper, we present an approach to learn recomposable motor primitives across large-scale and diverse manipulation demonstrations.
Current approaches to decomposing demonstrations into primitives often assume manually defined primitives and bypass the difficulty of discovering these primitives.
On the other hand, approaches in primitive discovery put restrictive assumptions on the complexity of a primitive, which limit applicability to narrow tasks.
Our approach attempts to circumvent these challenges by jointly learning both the underlying motor primitives and recomposing these primitives to form the original demonstration.
Through constraints on both the parsimony of primitive decomposition and the simplicity of a given primitive, we are able to learn a diverse set of motor primitives, as well as a coherent latent representation for these primitives.
We demonstrate, both qualitatively and quantitatively, that our learned primitives capture semantically meaningful aspects of a demonstration.
This allows us to compose these primitives in a hierarchical reinforcement learning setup to efficiently solve robotic manipulation tasks like reaching and pushing.
We have seen impressive progress over the recent years in learning based approaches to perform a plethora of manipulation tasks Andrychowicz et al., 2018; Pinto & Gupta, 2016; Agrawal et al., 2016) .
However, these systems are typically task-centric savants -able to only execute a single task that they were trained for.
This is because these systems, whether leveraging demonstrations or environmental rewards, attempt to learn each task tabula rasa, where low to high level motor behaviours, are all acquired from scratch in context of the specified task.
In contrast, we humans are adept at a variety of basic manipulation skills e.g. picking, pushing, grasping etc., and can effortlessly perform these diverse tasks via a unified manipulation system.
Sample motor programs that emerge by discovering the space of motor programs from a diverse set of robot demonstration data in an unsupervised manner.
These motor programs facilitate understanding the commonalities across various demonstrations, and accelerate learning for downstream tasks.
How can we step-away from the paradigm of learning task-centric savants, and move towards building similar unified manipulation systems?
We can begin by not treating these tasks independently, but via instead exploiting the commonalities across them.
One such commonality relates to the primitive actions executed to accomplish the tasks -while the high-level semantics of tasks may differ significantly, the low and mid-level motor programs across them are often shared e.g. to either pick or push an object, one must move the hand towards it.
This concept of motor programs can be traced back to the work of Lashley, who noted that human motor movements consist of 'orderly sequences' that are not simply sequences of stimulus-response patterns.
The term 'motor programs' is however better attributed to Keele (1968) as being representative of 'muscle commands that execute a movement sequence uninfluenced by peripheral feedback', though later works shifted the focus from muscle commands to the movement itself, while allowing for some feedback (Adams, 1971) .
More directly relevant to our motivation is Schmidt's notion of 'generalized' motor programs (Schmidt, 1975) that can allow abstracting a class of movement patterns instead of a singular one.
In this work, we present an approach to discover the shared space of (generalized) motor programs underlying a variety of tasks, and show that elements from this space can be composed to accomplish diverse tasks.
Not only does this allow understanding the commonalities and shared structure across diverse skills, the discovered space of motor programs can provide a high-level abstraction using which new skills can be acquired quickly by simply learning the set of desired motor programs to compose.
We are not the first to advocate the use of such mid-level primitives for efficient learning or generalization, and there have been several reincarnations of this idea over the decades, from 'operators' in the classical STRIPS algorithm (Fikes & Nilsson, 1971) , to 'options' (Sutton et al., 1999) or 'primitives' (Schaal et al., 2005) in modern usage.
These previous approaches however assume a set of manually defined/programmed primitives and therefore bypass the difficulty of discovering them.
While some attempts have been made to simultaneously learn the desired skill and the underlying primitives, learning both from scratch is difficult, and are therefore restricted to narrow tasks.
Towards overcoming this difficulty, we observe that instead of learning the primitives from scratch in the context of a specific task, we can instead discover them using demonstrations of a diverse set of tasks.
Concretely, by leveraging demonstrations for different skills e.g. pouring, grasping, opening etc., we discover the motor programs (or movement primitives) that occur across these.
We present an approach to discover movement primitives from a set of unstructured robot demonstration i.e. demonstrations without additional parsing or segmentation labels available.
This is a challenging task as each demonstration is composed of a varying number of unknown primitives, and therefore the process of learning entails both, learning the space of primitives as well as understanding the available demonstrations in context of these.
Our approach is based on the insight that an abstraction of a demonstrations into a sequence of motor programs or primitives, each of which correspond to an implied movement sequence, and must yield back the demonstration when the inferred primitives are 'recomposed'.
We build on this and formulate an unsupervised approach to jointly learn the space of movement primitives, as well as a parsing of the available demonstrations into a high-level sequence of these primitives.
We demonstrate that our method allows us to learn a primitive space that captures the shared motions required across diverse skills, and that these motor programs can be adapted and composed to further perform specific tasks.
Furthermore, we show that these motor programs are semantically meaningful, and can be recombined to solved robotic tasks using reinforcement learning.
Specifically, solving reaching and pushing tasks with reinforcement learning over the space of primitives achieves 2 orders of magnitude faster training than reinforcement learning in the low-level control space.
We have presented an unsupervised approach to discover motor programs from a set of unstructured robot demonstrations.
Through the insight that learned motor programs should recompose into the original demonstration while being simplistic, we discover a coherent and diverse latent space of primitives on the MIME (Sharma et al., 2018) dataset.
We also observed that the learned primitives were semantically meaningful, and useful for efficiently learning downstream tasks in simulation.
We hope that the contributions from our work enable learning and executing primitives in a plethora of real-world robotic tasks.
It would also be interesting to leverage the learned motor programs in context of continual learning, to investigate how the discovered space can be adapted and expanded in context of novel robotic tasks.

assistant: text: We learn a space of motor primitives from unannotated robot demonstrations, and show these primitives are semantically meaningful and can be composed for new robot tasks.