Capstone: a planner for a warehouse mobile manipulator

This capstone brings the course together in a single design. A company is deploying mobile manipulators in a fulfilment warehouse: a differential-drive base, 0.7 m by 0.6 m, carrying a 7-joint arm with a suction gripper. Each robot drives to a shelf, picks totes or items from shelf bays and drops them into a bin on its back, then drives to a packing station. Aisles are 1.2 m wide, with people and other robots sharing them. The task is to choose planners for each part of the job, justify the choices with the methods of lessons 1 to 11, define how the system will be benchmarked, and decide what to do when planning fails. As in the capstones of ROB-310 and ROB-340, the result is a design document with numbers, not a single algorithm.

Requirements

Good planning systems start from measurable requirements. The operations team and safety engineers agreed the following for this deployment. They mix performance (planning time, path quality), reliability (success rate) and safety (clearance, behaviour near people), and they make explicit what happens when the planner cannot find a solution.

The base: global and local planning

The base is a differential-drive robot, so it can turn on the spot and a path in (x, y) is executable if the footprint fits (lesson 10). Its configuration space is effectively three-dimensional only in tight spots. The map is a static occupancy grid from the SLAM system of ROB-340, at 5 cm resolution, which gives \( 2400 \times 1200 = 2.88 \times 10^{6} \) cells. A 2-D A* on an inflated cost map is the natural global planner (lessons 4 and 5): the inflation uses the 0.30 m inscribed radius as the lethal band and an exponential decay out to 0.9 m so that paths prefer the middle of aisles. Because the footprint is rectangular with a 0.46 m circumscribed radius and aisles are 1.2 m wide, the robot can always rotate in an aisle, so footprint checks are needed only at shelf bays and doorways, where the local planner checks the full rectangle. With the octile heuristic, a cross-warehouse query expands a few hundred thousand cells in the worst case, comfortably within 100 ms in compiled code; weighted A* with w = 1.5 is held in reserve if profiling shows otherwise.

Locally, an MPPI controller (lesson 11) follows the global path at 20 Hz, using a 2 s horizon and the robot's acceleration limits, with people tracked by the perception system and predicted at constant velocity. People get a 0.5 m social margin, inflated further by their prediction uncertainty (lesson 11, chance constraints). If the local controller cannot make progress for 3 s, for example because an aisle is blocked, it reports to the global planner, which marks the blockage in a temporary layer and replans around it.

The arm: picking motions

The arm has seven joints, so grids are out (lesson 1) and the pick motions are planned with a sampling planner followed by optimization, using MoveIt 2 (lesson 11). Each pick is a sequence: a free-space transfer from the stowed configuration to a pre-pick pose 10 cm in front of the item; a straight Cartesian approach to the suction point; attaching the item to the robot model; a straight retreat; and a free-space transfer to the drop pose above the onboard bin. The free-space transfers use RRT-Connect (lesson 7) with a 0.5 s budget, followed by shortcutting and a short CHOMP pass (lesson 9) to smooth the path while keeping 3 cm of clearance from shelf edges. The collision model uses capsules for the arm links and convex hulls for the shelf and bin fixtures (lesson 8), plus an octree from the wrist camera for unexpected clutter in the bay. The padding budget follows lesson 8: encoder and calibration error, tracking error, camera error and base localization error at the shelf, combined by root sum of squares.

Because the shelf bays are fixed, the system can also learn. A PRM (lesson 6) built offline for each bay type gives a library of collision-free paths between common pre-pick and drop configurations; at run time the planner first tries a roadmap query and falls back to RRT-Connect only when the scene differs, for example when the octree shows an unexpected object. This multi-query reuse cuts the median planning time and makes motions more repeatable, which operators value.

Benchmarking

Planners must be compared on representative problems, not on a single demonstration. The team builds a benchmark of 2,000 base routes sampled from real order data and 5,000 pick problems taken from recorded shelf scans, including hard cases: items at the back of top shelves, bays with clutter, and drops when the onboard bin is nearly full. Each planner configuration is run on every problem with several random seeds, recording success, planning time, path length (or joint-path length), minimum clearance and number of collision checks. Results are reported as distributions, with medians and 95th percentiles, because averages hide the slow outliers that stall operations. OMPL's benchmarking tools and MoveIt's benchmark package support exactly this workflow, and the results drive configuration choices such as RRT-Connect's range parameter, the shortcutting budget and whether to enable the roadmap library.

import numpy as np, heapq, math
rng = np.random.default_rng(12)
# Benchmark a grid A* and a basic RRT on 40 random warehouse-like maps (rack rows with gaps); report success and path length ratio
def make_map():
    g = np.zeros((40, 60), bool)
    for r in range(5, 36, 6):
        g[r:r + 2, 4:56] = True
        for _ in range(3):
            c = rng.integers(8, 52); g[r:r + 2, c:c + 3] = False
    return g
def astar(g, s, t):
    D = [(dr, dc, math.hypot(dr, dc)) for dr in (-1, 0, 1) for dc in (-1, 0, 1) if dr or dc]
    h = lambda n: math.hypot(n[0] - t[0], n[1] - t[1])
    best = {s: 0.0}; pq = [(h(s), s)]; done = set()
    while pq:
        f, u = heapq.heappop(pq)
        if u in done: continue
        if u == t: return best[u]
        done.add(u)
        for dr, dc, c in D:
            v = (u[0] + dr, u[1] + dc)
            if 0 <= v[0] < 40 and 0 <= v[1] < 60 and not g[v] and best[u] + c < best.get(v, 1e9):
                best[v] = best[u] + c; heapq.heappush(pq, (best[v] + h(v), v))
    return None
def rrt(g, s, t, iters=800, eta=2.0):        # fixed budget of 800 iterations
    free = lambda p: 0 <= p[0] < 40 and 0 <= p[1] < 60 and not g[int(p[0]), int(p[1])]
    seg = lambda a, b: all(free(a + (b - a) * k) for k in np.linspace(0, 1, int(np.linalg.norm(b - a) / 0.25) + 2))
    N = [np.array(s, float)]; P = [-1]; T = np.array(t, float)
    for _ in range(iters):
        q = T if rng.random() < 0.1 else rng.uniform([0, 0], [40, 60])
        i = int(np.argmin([np.sum((n - q) ** 2) for n in N])); d = q - N[i]; L = np.linalg.norm(d)
        new = q if L <= eta else N[i] + eta * d / L
        if seg(N[i], new):
            N.append(new); P.append(i)
            if np.linalg.norm(new - T) < eta and seg(new, T):
                k, cost = len(N) - 1, np.linalg.norm(new - T)
                while P[k] >= 0: cost += np.linalg.norm(N[k] - N[P[k]]); k = P[k]
                return cost
    return None
ok_a = ok_r = 0; ratios = []
for _ in range(40):
    g = make_map(); s, t = (1, 1), (38, 58)
    a = astar(g, s, t); r = rrt(g, s, t)
    ok_a += a is not None; ok_r += r is not None
    if a and r: ratios.append(r / a)
print("A* solved %d of 40, RRT solved %d of 40" % (ok_a, ok_r))
print("RRT path length / A* path length: median %.2f, 90th percentile %.2f" % (np.median(ratios), np.percentile(ratios, 90)))

A* solved 40 of 40, RRT solved 28 of 40
RRT path length / A* path length: median 1.22, 90th percentile 1.31

Failure handling and fallbacks

No planner succeeds every time, so the design specifies what happens when one fails. For the base, the global planner reports 'no path' with the blocking region if A* exhausts the reachable cells, and the fleet manager reroutes or alerts staff; the local controller stops and waits if a person blocks the way, then asks for a replan. For the arm, a failed RRT-Connect attempt is retried with a different IK solution and seed, then with a larger time budget, then with the base repositioned 10 cm to give a better reach; after three attempts the item is flagged for human picking. Every failure is logged with the planning scene, so it can be added to the benchmark. Over time, this loop, deploy, log failures, add them to the benchmark, tune, is what drives success rates from the high 90s toward the 99.5 per cent target.