Automated material handling and storage: AGV, AS/RS

Why it matters

In a typical factory a part spends far more time being moved and stored than being processed, and material handling can account for a large share of manufacturing cost. Automated guided vehicles (AGVs) and automated storage and retrieval systems (AS/RS) take that work over in FMSs, automotive plants and distribution centres. Sizing them — how many vehicles, how many aisles — is a standard calculation that a production engineer is expected to do.

Key ideas

Material handling principles. Plan the flow first, unitise loads (pallets, totes), minimise handling and distance, standardise equipment, and integrate handling with storage and information systems. Automated handling earns its cost when flows are steady and traceable.

AGVs. Battery-powered, driverless vehicles that follow a path under computer control.

• Types: driverless towing vehicles (pull trains of carts), unit-load carriers (carry one pallet or tote, often with a roller deck or lift), and pallet trucks/forklift AGVs. Assembly-line AGVs carry a product through work stations.
• Guidance: imbedded guide wires (inductive), painted or magnetic tape, laser navigation with reflective targets, inertial guidance with magnetic floor markers, and natural-feature (SLAM) navigation used by autonomous mobile robots. Satellite positioning does not work reliably indoors and is not used for factory AGVs.
• Vehicle management: traffic control by zone blocking or on-board forward sensing (optical, ultrasonic, laser scanners), dispatching from a central computer, and automatic battery charging.
• Safety: obstacle detection with automatic stop, emergency-stop buttons, bumpers, warning lights and audible signals, restricted speeds near people.
• AGVs give flexible routes, can be re-routed without civil work, and deliver directly to machines; but they are expensive per vehicle and slower than conveyors for high, fixed flows.

AS/RS. A computer-controlled system of storage racks served by storage/retrieval (S/R) machines that place and retrieve loads in defined locations.

• Types: unit-load AS/RS (pallets, large loads), mini-load (bins and totes), man-on-board (the operator rides to pick items), automated item retrieval, vertical lift modules and carousels (horizontal and vertical).
• Components: storage structure (racks), S/R machine (a mast moving along the aisle and a carriage moving vertically, with a shuttle to push/pull loads), storage modules (pallets, bins), and pick-and-deposit (P&D) stations at the end of each aisle, all run by a control computer that keeps the location inventory.
• Single-command cycle: one store or one retrieve per trip — out to a location and back. Dual-command cycle: store a load in one location and retrieve from another in the same trip.
• Storage strategies: randomised storage (any empty location; fewer locations needed) and dedicated (class-based) storage (fast movers close to the P&D station).
• Benefits: high storage density (tall racks, narrow aisles), accurate inventory, reduced labour, security, integration with FMS and production control. Limits: high capital cost and a rigid structure.

Analysis assumptions. The S/R machine moves horizontally and vertically at the same time, so travel time to a location is the larger of the two; the AS/RS formulas below use the standard average values for randomised storage with the P&D station at the aisle base.

Formulas

T_c = T_L + L_d / v_c + T_U + L_e / v_e

• AGV delivery cycle time (min); T_L, T_U loading and unloading times (min); L_d loaded travel distance (m); L_e empty travel distance (m); v_c, v_e vehicle speeds loaded and empty (m/min).

R_dv = 60 · A · T_f · E / T_c

• Deliveries per hour per vehicle; A availability; T_f traffic factor (blocking losses, 0.85–1); E efficiency (1 for an AGV).

n_c = R_f / R_dv

• Number of vehicles; R_f required deliveries per hour. Round up.

T_cs = 2 · max(L / v_y, H / v_z) + 2 · T_pd

• AS/RS single-command cycle time (min); L aisle length (m); H rack height (m); v_y, v_z horizontal and vertical speeds of the S/R machine (m/min); T_pd pick-and-deposit time (min).

T_cd = 4 · max(0.75 · L / v_y, 0.75 · H / v_z) + 4 · T_pd

• Dual-command cycle time (min).

R_cs · T_cs + R_cd · T_cd = 60 · U

• R_cs, R_cd single- and dual-command cycles per hour; U utilisation of the S/R machine. Transactions per hour R_t = R_cs + 2 · R_cd.

Worked examples

Example 1 (standard). An AGV system has an average loaded travel of 300 m and empty travel of 200 m per delivery, speed 50 m/min both ways, and load and unload times of 0.5 min each. Availability is 0.95, traffic factor 0.90. The plant needs 40 deliveries per hour. How many AGVs?

1. T_c = T_L + L_d / v_c + T_U + L_e / v_e = 0.5 + 300/50 + 0.5 + 200/50 = 0.5 + 6 + 0.5 + 4 = 11 min.
2. R_dv = 60 · A · T_f · E / T_c = 60 × 0.95 × 0.90 × 1 / 11 = 4.66 deliveries/h per vehicle.
3. n_c = R_f / R_dv = 40 / 4.66 = 8.58 → 9 AGVs.

Example 2 (GATE level). A unit-load AS/RS aisle is 60 m long and 12 m high. The S/R machine moves at 120 m/min horizontally and 30 m/min vertically; P&D time is 0.3 min. Find the single- and dual-command cycle times, and the hourly throughput if the machine is used 75 % of the time with equal numbers of single and dual cycles.

1. L/v_y = 60/120 = 0.5 min; H/v_z = 12/30 = 0.4 min.
2. T_cs = 2 × max(0.5, 0.4) + 2 × 0.3 = 1.0 + 0.6 = 1.6 min.
3. T_cd = 4 × max(0.75 × 0.5, 0.75 × 0.4) + 4 × 0.3 = 4 × 0.375 + 1.2 = 2.7 min.
4. With R_cs = R_cd = R: R(1.6 + 2.7) = 0.75 × 60 = 45 → R = 10.47 cycles/h of each type.
5. Transactions: R_t = 10.47 + 2 × 10.47 = 31.4 storages + retrievals per hour per aisle.

Common mistakes

• Forgetting the empty return trip in the AGV cycle.
• Rounding the number of vehicles down — always round up.
• Adding horizontal and vertical travel times for an S/R machine; it moves on both axes at once, so take the maximum.
• Counting a dual-command cycle as one transaction — it is two (one store, one retrieve).
• Mixing m/s and m/min.

For GATE PI

Expect MCQs on AGV types and guidance, AS/RS types and components, and single- versus dual-command cycles. Numericals cover AGV cycle time and fleet size, AS/RS cycle times, throughput and S/R machine utilisation, and the number of aisles needed for a required throughput. Practise unit conversion and rounding up vehicle and aisle counts.

Quick check

1. Name three AGV guidance methods.
2. How many transactions does one dual-command cycle complete?
3. AGV: loaded 250 m, empty 150 m, 50 m/min, T_L = T_U = 0.75 min, A = 0.90, T_f = 0.85, demand 30 deliveries/h. How many vehicles?
4. An aisle is 90 m long and 15 m high; v_y = 150 m/min, v_z = 40 m/min, T_pd = 0.25 min. Find T_cs.
5. Why is randomised storage often preferred to dedicated storage?

Answers: 1. Imbedded guide wire, magnetic/painted tape, laser navigation (also inertial, natural-feature). 2. Two. 3. T_c = 9.5 min; R_dv = 4.83/h; 30/4.83 = 6.2 → 7 vehicles. 4. 2 × max(0.6, 0.375) + 0.5 = 1.7 min. 5. It needs fewer storage locations for the same inventory.