A unit-load AS/RS crane is the workhorse of an automated storage system. It moves along a narrow aisle, lifts a carriage, and transfers pallets or large loads to and from rack openings. While the machine appears simple in concept, its behaviour depends on a chain of interacting components: rails, wheels, mast, hoist, shuttle or fork, drives, sensors, controllers, and safety circuits. Commissioning is the process of verifying that this chain works as intended under controlled conditions. Acceptance is the formal step where the site engineering team, after collecting and reviewing evidence, agrees that the crane is ready for routine operation. This article explains the practical sequence of checks, common findings, evidence collection, and the decision boundaries that separate a minor adjustment from a structural concern. It is written as independent technical education; the manufacturer’s documentation, site procedures, and the judgement of the responsible site engineer always take precedence over any generic guidance.
Purpose and Scope of Acceptance #
Commissioning and acceptance are often compressed into a single event by project schedules, but treating them as one is misleading. Commissioning is a technical activity: rotating drives, cycling interlocks, moving the crane in manual mode, and measuring positions. Acceptance is a decision: is the crane fit for the designed storage duty? A proper acceptance exercise does not only prove that the crane operates. It proves that the crane operates within the boundaries of the original design intent and that any signed-off deviation is understood and documented.
The scope of an acceptance test should be agreed before the work starts. In general, the following areas are included:
- Mechanical integrity of the rail system, mast, carriage, shuttle, and buffers.
- Correct and safe operation of all electrical, control, and safety circuits.
- Movement and positioning accuracy in all three axes: travel, hoist, and shuttle.
- Interface behaviour with the rack system, conveyor pick stations, and the warehouse control system.
- Recovery behaviour after a fault or an emergency stop, so operators understand what state the load is left in.
The boundary between commissioning and acceptance is not always clean. A test that reveals a serious misalignment may halt the process while corrections are made. The goal is to leave the site with a machine whose behaviour is known, measured, and reproducible. Any problem that cannot be reproduced reliably should be treated as unresolved, no matter how harmless it appears in a single cycle.
Documentation and Site Readiness #
Before any power is applied, the engineering team should confirm that the installed machine matches the design package. This is not a formality. A machine that has been installed with an older revision of a drive parameter set, or with a rail joint installed at the wrong shim thickness, can behave unexpectedly for years. The pre-power check list should include a review of mechanical drawings, electrical schematics, software revision records, and the manufacturer’s approved installation tolerances.
Site Conditions and Survey Data #
The crane behaves differently when the building and the aisle are not within tolerance. Floor flatness, rail gauge, rail elevation, column plumb, and rack alignment are all inputs that the crane controller assumes to be correct. Before the crane is powered, a third-party or supplier survey is often performed. The relevant records must be available and readable by the commissioning team. If the survey shows a rail elevation change that is outside tolerance but the decision was made to accept it, that decision must be recorded with a date, a signature, and a description of the operational impact.
In addition, the aisle itself must be clear of debris, temporary support stands, loose bolts, and construction material. The crane should never be moved by hand without following the manufacturer’s lockout procedure for motor brakes and drive engagement. Site lockout requirements and OEM instructions for manual handling always apply.
Mechanical and Structural Verification #
Mechanical inspection is the slowest part of commissioning, and it is the part that is most often rushed. A crane that passes its electrical tests but has a rail joint with a 2 mm step will eventually produce vibration, wheel wear, and load sway profiles that are difficult to diagnose weeks later.
Rail, Floor, and Column Checks #
The rail system should be checked for gauge width, level, and straightness along the full length of the aisle. Particular attention should be paid to the joints: expansion joints, welded joints, and end stops. Even a small difference in rail height between two adjacent rails can induce a lateral rock in the crane’s base. The floor beneath the rails should also be inspected for cracks, oil staining, or areas where containment was damaged during installation. These are signs of building behaviour, not simply crane behaviour.
The columns and the rack structure form the other half of the geometric relationship. The crane does not operate in an ideal space; it operates relative to the rack faces, load beams, and pallet openings. Any column plumb error that was compensated during installation by adjusting a rack beam may still show up as an acceptance issue when the crane’s shuttle tries to enter the opening from a slightly different angle.
Mast, Carriage, and Shuttle #
The mast should be checked for straightness in both the travel direction and across the aisle. The carriage guide rollers, typically running on the mast rails, must be adjusted to the designed clearance. A carriage that is too tight will cause motor overload and uneven wear; a carriage that is too loose will produce excessive sway and positioning scatter.
The shuttle or fork mechanism deserves special attention. Deck alignment, chain tension, and fork travel parallelism all determine whether a pallet will be picked cleanly. The mechanical check of the shuttle should include a visual confirmation that the fork home position sensor is correctly mounted and that there is no physical interference between the fork and the carriage under moving conditions.
Electrical, Control, and Safety Circuit Checks #
Once mechanical verification is complete, electrical checks begin in a logical order: incoming supply, power distribution, drive power, control power, then signal level checks. The incoming supply voltage and frequency should be recorded at the beginning of the shift and again under load. A small voltage sag that appears only when the travel drive accelerates can produce faults that are otherwise impossible to find.
Grounding is critical in an AS/RS aisle. The crane is connected to the building ground through the power cable and sometimes through a separate ground rail. Check that the ground is continuous and that no electrical noise from the variable frequency drives is returning through signal cables. Communication timeouts between the crane and the warehouse control system are frequently the result of a grounding loop that was unnoticed during installation.
All fieldbus cabling should be verified for continuity, correct connector termination, and physical routing away from power cables. Moveable cable tracks and festoon systems need to be observed while the crane travels the full length of the aisle. A cable track that runs smoothly for 80% of its travel and then lifts slightly will eventually cause intermittent communication fault.
Safety Function Validation #
The safety circuits deserve a separate and formal confirmation after the machine is already functioning. The purpose is to prove what effect an input will have and to verify that the safety response remains in force during a subsequent motion attempt. Emergency stop buttons, light curtains at load stations, aisle access doors, end-of-aisle overtravel switches, and anti-collision devices should be tested one at a time. Every functional test must follow the site’s lockout and hazard assessment procedures. The responsible engineer must not rely on a previous test result from the factory; machines behave differently when connected to their final rails, racking, and power.
It is never acceptable to bypass a safety device to complete a test cycle. If a safety device prevents the crane from completing a required motion, the correct response is to stop, investigate the cause, and obtain the OEM’s or site engineer’s instruction. A safety device that is bypassed during commissioning usually remains bypassed after commissioning, and it becomes part of the installed behaviour instead of the intended safety protection.
Motion, Positioning, and Load Handling Tests #
With safety checks complete, the crane is ready for controlled motions. The recommended sequence starts with slow travel in manual mode, checking that the direction of motion matches the controller command, and that limit switches are being approached at sensible speeds. Then the drives are tuned or verified according to the manufacturer’s settings, first on the empty crane, then under load.
Positioning Accuracy #
Positioning accuracy is measured in two categories: static accuracy and repeatability. Static accuracy is how close the crane’s position is to the commanded position at rest. Repeatability is how closely the crane comes back to the same position over many identical cycles. Both are important, but repeatability matters more for day-to-day operation because the rack opening position is usually taught or surveyed relative to the crane’s actual stopping floor.
A simple positioning test should include the following:
- Manual move to a target position 10 times; record the final position deviation each time.
- Semi-automatic move to the same target from alternating directions; record any direction-dependent offset.
- Full-speed move with deceleration and final creep; compare the settling behaviour against the drive tuning parameters.
- Hoist positioning at several height levels, including the highest and lowest openings.
Each recorded point should be compared to the design tolerance stated in the machine specification. The acceptance team should not alter the target position in the controller to hide a directional offset; instead, it should identify whether the offset is caused by the drive, the sensor, the braking system, or the mechanical structure.
Load Handling and Interface Verifications #
Load handling tests start with an empty shuttle, then move to a test load or a loaded pallet. The interface with the rack beam should be watched from two viewpoints: from the aisle floor and from the end of the aisle. A pallet should rest evenly on the load beams, and the shuttle should withdraw without dragging the pallet. If the crane is equipped with a fork, check fork extension and retraction at multiple rack openings. The same test should also be performed at the conveyor pick stations or transfer cars, where the crane’s departure from the station is as important as its arrival.
During automatic cycles, the crane’s behaviour after a fault is part of the acceptance. If a load is left partially extended on a rack beam after a fault, the system must have a defined recovery procedure. The acceptance team should verify that the controls log the fault correctly and that the operator can identify the state of the load before deciding how to resume. This is the practical boundary between the crane hardware and the higher-level warehouse control system.
Diagnostic Table: Common Commissioning Findings #
The table below summarizes findings that frequently appear during crane commissioning. It is intended for orientation only; the exact cause must be confirmed by site-specific evidence and the OEM’s own diagnostic documentation.
| Observed Symptom | Likely Cause Area | Suggested Next Step |
|---|---|---|
| Crane overshoots target position at high travel speed | Drive tuning, deceleration ramp, or brake engagement timing | Retune the travel drive unloaded first; review the speed profile; document deceleration curves before the load test. |
| Hoist drifts downward while parked with no load | Brake holding torque, brake air gap, or hoist mechanical brake delay | Inspect the hoist brake per OEM procedure; do not increase motor holding torque electronically to mask a worn brake. |
| Pallets sit off-center on the rack beam after a storage cycle | Shuttle/rack alignment, rail level error, or fork position feedback | Re-check rail and rack alignment before adjusting fork sensor positions; record the original raw offset. |
| Positioning repeatability degrades only at one aisle location | Rail joint step, column settlement, or rack structure interference | Re-survey the rail elevation and rail gauge at that location; compare with the commissioning baseline survey. |
| Communication timeouts occur only when the crane is at one end of travel | Fieldbus cable damage in the cable track, connector strain, or ground loop | Inspect the cable track routing and connector strain relief at that physical location; monitor the network error counter. |
| An aisle door interlock trips intermittently during the same cycle | Door interlock misalignment, vibration from the crane, or sensor contamination | Check the interlock’s mechanical alignment and mounting stiffness; review the safety event log for the time of the trip. |
Every finding should be reproduced at least once before a cause is recorded. A single event may be a random electrical glitch; a repeatable event reliably points to a physical condition.
Evidence Collection and Baseline Records #
The real output of an acceptance process is not a signed check sheet; it is a set of baseline records that will be used months later when the crane begins to behave differently. The engineering team should keep the following evidence as part of the handover documentation:
- Rail survey reports, including the original data and any re-survey performed due to a deviation.
- Torque records for the rail fasteners, mast bolts, and shuttle attachment points.
- Drive parameter settings, including acceleration ramps, gains, and any custom tuning values.
- Positioning verification plots or tabulated deviations for the travel, hoist, and shuttle axes.
- Safety function test results, showing each tested input, the response observed, and the person who witnessed it.
- Fault and event log snapshots from the crane controller and the warehouse control system.
Baseline data is useful only if it is stored somewhere that future maintainers can find. A file buried in a site server is as good as nothing. The format
