Crane control: drives, skew, load monitoring and interfaces

A crane drive does more than turn motors: it keeps a wide bridge square, knows the load before it lifts, and takes its power and its commands from a machine that moves. This lesson is course sections 4.2.3 to 4.4 and Tutorial 4.2.

From DC drives to vector control

For most of the twentieth century crane hoists used DC motors, because only the DC machine gave independent control of flux and armature current, and so four-quadrant operation with torque at zero speed. First came the Ward-Leonard system (a motor-generator set whose generator field set the hoist motor's armature voltage), then the static thyristor converter, a dual anti-parallel bridge feeding the armature straight from the grid, with the same four-quadrant control and no rotating plant. Wound-rotor induction motors with rotor resistance control were also common, for high starting torque and simple stepped speed, at the cost of poor low-speed efficiency and accuracy.

Today's standard is the cage induction motor on a vector (field-oriented) drive. Vector control separates the flux and torque components of the stator current, reproducing DC-drive control with a rugged, maintenance-free motor. With an encoder it holds full torque at zero speed and regulates speed precisely: every hoist requirement is met. The same drive family serves the travel motions, and an active front end returns overhauling energy to the network.

Tutorial 4.2 compares the two technologies criterion by criterion:

Criterion	DC drive (thyristor, Ward-Leonard)	AC vector drive (cage motor)
Zero-speed torque	excellent (native to the DC machine)	excellent (closed-loop vector with encoder)
Low-speed smoothness	very good	very good
Four-quadrant, regeneration	native (dual converter or M-G set)	needs a brake chopper or an active front end
Motor maintenance	high: commutator and brushes	very low: rugged cage rotor
Efficiency	good; M-G set poor	high
Speed range (field weakening)	wide	wide
Cost and footprint	higher, larger	lower, compact
Typical status	legacy, retrofit	current standard

The AC drive matches the DC drive on every criterion crane duty cares about, and removes the commutator. Its one missing native feature, regeneration, is added cheaply: a chopper for occasional lowering, an active front end for continuous heavy lowering. DC drives survive mainly as legacy installations.

Master-slave and gantry skew

A bridge whose two ends run on separate motors needs master-slave control: it keeps the girder square to the runway and shares the tractive effort evenly. Positioning and anti-collision systems (absolute encoders, laser or radar range-finders, area sensors) automate travel to target positions and keep cranes on a shared runway apart.

A wide-span gantry with independent motors at each leg has a harder problem. If one leg runs even slightly faster than the other, the gantry skews: the portal rotates in plan, the wheel flanges bind on the rails, and the lateral forces can reach several times the tractive effort. Small speed differences integrate into a growing position error, so the correction loop closes on the difference of the two legs' measured positions (an encoder on a non-driven measuring wheel, a laser to a fixed target, or a barcode tape along the rail):

$$\Delta v = k_\text{skew}\,(x_1 - x_2) \tag{4.6}$$

The gain must be high enough to keep the skew inside the flange clearance, and low enough not to fight the structure's own elasticity, which would make the portal oscillate about its vertical axis. In practice the loop is deliberately slow: seconds, not milliseconds.

Leg 2 slower by

0.5 %

A worn wheel or a slightly different gearbox is enough.

Skew-loop gain k

0.0 1/s

• Skew x₁ − x₂ (mm)
• Flange clearance (20 mm)

The skew exceeds the flange clearance: the wheels bind on the rails.

Skew after 60 s

180.0 mm

Largest skew

180.0 mm

Steady skew v·δ/k

∞

Δv = k_skew (x₁ − x₂) ; e_ss = v · δ / k

Predict first

With the loop at k = 0.3 1/s and leg 2 slower by 0.5 %, the skew settles at 10 mm. You double the mismatch to 1 %. What happens?

The skew still settles at 10 mmIt settles at 20 mmIt grows without limit

Load monitoring and overload protection

Every crane above a modest capacity measures its load: with a load pin or shear-beam cell in the rope anchorage, a strain-gauged link in the hook block, or from the vector drive's own torque estimate, at no extra hardware cost. The response is graded, not a single trip:

• 90 % of rated load: warn the operator;
• 100 %: block further hoisting, but still allow lowering;
• 110 %: trip the drive and apply the brakes.

The asymmetry is essential: a crane that stopped dead on overload would strand the load in the air with no safe way down. The same measurement gives snag protection (a sudden rise in rope tension when a grab catches on an obstruction, torque cut within milliseconds), slack-rope detection (the hook has landed but the drum keeps paying out), and load-dependent speed: an empty hook can return at two or three times the rated speed by field weakening.

Power and commands for a travelling machine

Two systems bring power to a crane that moves:

• the festoon: flat or round cables hung on small trolleys along a track, gathering and paying out as the crane travels. Simple and robust, limited in speed and travel length;
• the conductor bar (busbar): insulated rails along the runway, contacted by sliding collector shoes. Higher speeds, longer runways, a cleaner installation: the standard for heavy cranes.

The trolley is fed along the bridge in the same ways. The operator commands the crane from a pendant (a hanging push-button station wired to the crane) or a radio remote, which lets them move for the best view of the load. Both need a maintained emergency stop, hold-to-run (dead-man) controls so that letting go stops the motion, and, for radio, unique addressing and an automatic stop on loss of signal. More and more cranes are semi- or fully automatic, with a supervisory PLC running stored sequences under the same safety interlocks.

FoundationStart here if this is new to you

A long gantry on two motors is like two people carrying a long table through a corridor. If one walks a little faster, the table turns and jams against the walls. The fix is not to watch each other's speed but the table itself: if it starts to turn, the one behind speeds up a little. Speed up too eagerly and the table wobbles from side to side.

ExplorerGo deeper: derivations and open questions

Skew dynamics. With a speed lag $\tau$ on the trimmed leg, the skew obeys $\tau\ddot e + \dot e + k e = -v\delta$. For which $k\tau$ is the loop critically damped? Check your answer with the widget ($\tau = 0.5$ s).

Radio loss. A radio-controlled crane loses its signal while lowering a load. Write the sequence the drive should follow, using lesson 3's brake-lift handshake. How long may the stop take?