Flexible manufacturing systems and automated material handling

Why it matters

Most factories make many part variants in medium volumes: too varied for a transfer line, too many for stand-alone CNC machines with manual loading. A flexible manufacturing system (FMS) and its automated material handling (AGVs, conveyors, AS/RS, robots) fill that gap. Engineers size these systems with simple capacity models: which station is the bottleneck, and how many vehicles the plant needs. Getting the sizing wrong gives either idle capital or a line that cannot meet demand.

Key ideas

What an FMS is An FMS is a group of CNC workstations linked by an automated material handling and storage system and controlled by a distributed computer system, able to process different parts in the same family simultaneously with almost no lost time for changeover. Its three essential parts are:

1. Workstations: CNC machining centres, load/unload stations (where parts are fixtured on pallets), inspection (CMM), washing and assembly stations.
2. Material handling and storage: moves pallets between stations and holds a work-in-process buffer, often with pallet changers at each machine.
3. Computer control: workstation control and DNC programme download, production control (part mix, routing, scheduling), traffic control of vehicles, tool management, monitoring and reporting. Humans still load and unload parts, change tools and maintain the system.

Tests of flexibility. A system is flexible if it can (1) process different part styles in a non-batch mode, (2) accept changes to the production schedule, (3) keep running when a machine breaks down (alternative routings) and (4) accept new part designs. Types of flexibility include machine, routing, process (mix), product, volume and expansion flexibility.

Sizes: single machine cell (one machining centre with a pallet buffer), flexible manufacturing cell (two or three machines), and FMS (four or more). Dedicated FMSs make a limited family with a known mix; random-order FMSs handle a wide, changing mix and need more sophisticated control.

Layouts: in-line, loop (with secondary spurs to stations), ladder, open field (AGV networks, most flexible) and robot-centred cell.

Automated material handling equipment

• AGVs (automated guided vehicles): driverless, battery-powered vehicles following guide wires, magnetic tape, or laser/vision navigation with no fixed path (AMRs). Types: towing (driverless trains), unit-load carriers, pallet trucks and assembly-line carriers. Traffic is controlled by zone blocking, forward sensing and central dispatch.
• Conveyors: roller, belt, chain, overhead trolley, in-floor towline and power-and-free; best for high, steady flow on fixed routes.
• AS/RS (automated storage and retrieval systems): storage racks served by automatic stacker cranes; unit-load, mini-load and carousel types. Performance is stated as storage capacity and throughput in single- and dual-command cycles per hour.
• Industrial robots for machine tending and palletising; cranes and hoists for heavy loads.
• Identification: bar codes and RFID to track pallets and parts.

Principles of material handling: plan as a system, standardise unit loads, minimise moves and handling distance, use gravity, make flows straight and short, and keep work and storage space well used. Material handling adds cost but not value, so the best handling is often no handling.

Benefits and limits: higher machine utilisation than stand-alone CNC, lower WIP and lead time, fewer workers, consistent quality and quick response to mix changes. Costs are high capital investment, complex planning and software, and a limited part family.

Formulas

Workload of station i: WLᵢ = Σⱼ Σₖ tᵢⱼₖ · fᵢⱼₖ · pⱼ

• tᵢⱼₖ = processing time of operation k of part j at station i (min), fᵢⱼₖ = operation frequency (usually 1), pⱼ = fraction of part j in the mix. WLᵢ in min per part.

Bottleneck: the station with the largest WLᵢ / sᵢ; maximum production rate Rp* = s* / WL*

• sᵢ = number of servers (machines) at station i; s*, WL* = values at the bottleneck. Rp* in parts/min. Rate of part j = pⱼ · Rp*.

Utilisation of station i: Uᵢ = (WLᵢ / sᵢ) · Rp*

AGV delivery cycle: Tc = TL + Ld / vc + TU + Le / ve

• TL, TU = load and unload times (min), Ld = loaded travel distance (m), Le = empty travel distance (m), vc, ve = vehicle speeds loaded and empty (m/min).

Deliveries per vehicle per hour: Rdv = 60 · A · T / Tc

• A = availability, T = traffic factor (both ≤ 1, dimensionless).

Number of vehicles: nc = Rf / Rdv, rounded up

• Rf = required deliveries per hour.

Worked examples

Example 1 (standard): AGV fleet size. A plant needs Rf = 40 deliveries/h. Each delivery has TL = TU = 0.5 min, loaded distance 300 m and empty return 200 m at 50 m/min. Availability A = 0.95, traffic factor T = 0.90.

1. Tc = TL + Ld/vc + TU + Le/ve = 0.5 + 300/50 + 0.5 + 200/50 = 0.5 + 6 + 0.5 + 4 = 11.0 min.
2. Rdv = 60 × A × T / Tc = 60 × 0.95 × 0.90 / 11.0 = 51.3 / 11.0 = 4.66 deliveries/h per vehicle.
3. nc = Rf / Rdv = 40 / 4.66 = 8.58, so 9 AGVs are needed (always round up).

Example 2 (GATE level): FMS bottleneck model. An FMS has three stations: 1, load/unload (2 servers); 2, milling (3 servers); 3, drilling (2 servers). Part mix: A, p = 0.6, route load 4 min → mill 20 min → drill 12 min → unload 4 min; B, p = 0.4, route load 4 min → mill 30 min → unload 4 min.

1. WL₁ = 0.6 × (4 + 4) + 0.4 × (4 + 4) = 8.0 min.
2. WL₂ = 0.6 × 20 + 0.4 × 30 = 12 + 12 = 24.0 min.
3. WL₃ = 0.6 × 12 + 0.4 × 0 = 7.2 min.
4. WL/s: station 1 = 8/2 = 4.0; station 2 = 24/3 = 8.0; station 3 = 7.2/2 = 3.6 min. Station 2 (milling) is the bottleneck.
5. Rp* = s*/WL* = 3/24 = 0.125 parts/min = 7.5 parts/h, i.e. 4.5 A/h and 3.0 B/h.
6. Utilisations: U₁ = 4.0 × 0.125 = 0.50; U₂ = 8.0 × 0.125 = 1.00; U₃ = 3.6 × 0.125 = 0.45. Adding a fourth milling machine would raise capacity until load/unload becomes limiting.

Common mistakes

• Adding machine capacities of stations in series as if they worked in parallel; parts that visit every station are limited by the bottleneck.
• Comparing station workloads without dividing by the number of servers.
• Forgetting the unload operation at the load/unload station, or the empty return trip of the AGV.
• Rounding the number of vehicles down; 8.58 vehicles means 9.
• Treating availability and traffic factor as the same thing: A covers breakdowns and charging, T covers blocking and congestion.
• Assuming FMS means no people; loading, tooling and maintenance remain manual.

For GATE ME

Mostly conceptual: components and types of FMS, flexibility types, layouts, AGV guidance, AS/RS and conveyor types, and advantages over transfer lines and stand-alone CNC. Numericals, when they appear, are capacity calculations: bottleneck station and maximum production rate, utilisation, or simple vehicle and conveyor sizing. Practise building a workload table from a part mix and routing.

Quick check

1. Name the three basic components of an FMS.
2. Which FMS layout allows the most flexible routing of vehicles?
3. A station has workload 15 min/part and 3 machines; another has 6 min/part and 1 machine. Which is the bottleneck?
4. An AGV has Tc = 10 min, A = 0.9, T = 0.85. How many deliveries per hour can it make?
5. What does an AS/RS stacker crane do in a dual-command cycle?

Answers: 1. Workstations, material handling and storage system, computer control system. 2. Open field. 3. The second (6 min per server versus 5). 4. 60 × 0.9 × 0.85/10 = 4.59 deliveries/h. 5. Stores one load and retrieves another in the same trip.