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Potential-Based Reward Shaping for Intrinsic Motivation (Student Abstract)
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Extends potential-based reward shaping to arbitrary episodic shaping functions and proposes PBIM, which converts intrinsic motivation rewards into a potential-based form that preserves the optimal policy set, tested on MiniGrid DoorKey 8x8.
Problem
Intrinsic motivation reward shaping can change the set of optimal policies, causing suboptimal or 'hacked' behavior, and prior PBRS guarantees do not cover IM rewards that are trainable functions of many variables.
Approach
Generalizes the PBRS optimality condition to shaping rewards that depend on arbitrary variables in episodic settings, and converts IM rewards into a potential-based form via a terminal-state correction term (plus a mean-adjusted variant) so the optimal policy set is preserved.
Key results
- PBIM converged in 1.67E6 steps versus 2.88E6 for IM without PBIM at alpha=0.02, gamma=0.995 (table 1 setting), with mean episode length 51.8 vs 60.5.
- Without PBIM normalization, IM runs at alpha=0.02 and 0.25 failed to converge to the efficient policy (mean episode length ~635 steps), while PBIM converged (~52 steps).
- At alpha=0.005, gamma=0.99 PBIM converged faster than IM without PBIM (T=1.67E6 vs 1.46E6 is slower, but final episode length 36.4 vs 37.3 is better), so the method shows a speed/safety trade-off in the worst case.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Apply PBIM to the intrinsic reward | intrinsic motivation on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.005, γ = 0.99) | mean steps per episode after convergence: 37.3 -> 36.4 | Hurt |
| Apply PBIM to the intrinsic reward | intrinsic motivation on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.005, γ = 0.99) | time to convergence: 1.46E6 -> 1.67E6 | Hurt |
| Apply PBIM to the intrinsic reward | intrinsic motivation on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.02, γ = 0.995) | mean steps per episode after convergence: 60.5 -> 51.8 | Helped |
| Apply PBIM to the intrinsic reward | intrinsic motivation on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.02, γ = 0.995) | time to convergence: 2.88E6 -> 1.67E6 | Helped |
| Apply PBIM to the intrinsic reward | intrinsic motivation on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.25, γ = 0.995) | mean steps per episode after convergence: 62.0 -> 51.2 | Helped |
| Apply PBIM to the intrinsic reward | intrinsic motivation on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.25, γ = 0.995) | time to convergence: 6.07E6 -> 1.26E6 | Helped |
| Remove mean adjustment (normalization) (ablation) | PBIM on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.02, γ = 0.995) | time to convergence: 1.67E6 -> N/A | Hurt |
| Remove mean adjustment (normalization) (ablation) | PBIM on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.02, γ = 0.995) | mean steps per episode after convergence: 51.8 -> 635.9 | Hurt |
| Remove mean adjustment (normalization) (ablation) | PBIM on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.25, γ = 0.995) | time to convergence: 1.26E6 -> N/A | Hurt |
| Remove mean adjustment (normalization) (ablation) | PBIM on MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.25, γ = 0.995) | mean steps per episode after convergence: 51.2 -> 634.7 | Hurt |
| Use intrinsic motivation rather than no intrinsic reward | MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x8 (α = 0.005, γ = 0.99) | mean steps per episode after convergence: 35.9 -> 36.4 | Hurt |
| Use intrinsic motivation rather than no intrinsic reward | MiniGrid DoorKey 8x8 · MiniGrid DoorKey 8x0.005, γ = 0.99) | time to convergence: 2.95E6 -> 1.67E6 | Helped |
Surprising findings
- In the alpha=0.005, gamma=0.99 setting the normalized PBIM was not fastest to converge, though it reached a more efficient final policy than IM without PBIM.
Limitations
- Theory applies only under Assumption 1 that the shaping reward is constant with respect to actions at later times, which excludes some IM methods.
- Empirical validation limited to one small environment (MiniGrid DoorKey 8x8) with a tabular count-based reward.
- In the worst-case hyperparameter setting PBIM converged more slowly than IM without PBIM.
Experimental setup
Datasets: MiniGrid DoorKey 8x8
Benchmarks: MiniGrid DoorKey 8x8
Metrics: time to convergence mean steps per episode after convergence std of steps after convergence
Baselines: IM without PBIM PBIM without normalization no intrinsic motivation
Builds on: potential-based reward shaping (Ng, Harada, Russell 1999) Wiewiora et al. 2003 PBRS Burda et al. 2018 curiosity
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