Sensorized bearings measure load, rotational position, velocity, and force to better control industrial and automotive applications. We'll review a few of their most common uses shortly.
The sensing parts are fully integrated into certain varieties of ball bearings, roller bearings, and electromechanical actuators. Four major categories exist.
Incremental position sensors
Incremental position sensors can detect position and direction of motion, as well as derivative speed. When integrated into bearings, the sensors precisely measure bearing rotation and detect even small changes in rotation, down to 5°. The rotational data can be translated to determine the movement and position of components making contact with (or supported by) the bearings.
Incremental sensor bearing designs vary, but the major components usually include sensor body, a magnetized impulse ring, and connecting cable.
In the most common design, the sensor body is attached to the bearing outer ring, and two embedded Hall cells determine the direction of bearing location. The two cells are set in a small integrated circuit and offset from each other in the sensor body. A magnetized impulse ring is attached to the bearing inner ring; it is divided into a number of north and south poles. Depending on bearing size, the number of pulses per revolution normally ranges between 32 and 80 — which can be decoded into 128 to 320 distinct positions. A multi-wire cable connects the bearing unit with signal processing electronics.
Mast height control units in forklifts are a major application for incremental position sensor bearings. These produce an electronic signal that gives information on the vehicle's mast and fork position. The sensor bearings interface directly with the vehicle controller, so forklift operators can maximize speed when cornering the vehicle while avoiding any risk of tipping.
Absolute position sensors
Incremental position sensors restart their count from zero when electrical power is switched off and then on again. More sophisticated are absolute position sensors, also called true power-on sensors. They remember their last output value, regardless of power fluctuations. This capability equips them for use in more demanding steer-by-wire applications, where the sensors do two things:
As feedback sensors, they provide accurate position information about the steered wheels to the vehicle operator (as in 4 in the figure to the left)
Act as a steering input (as in 1 in the figure to the left)
Some absolute position sensor bearings provide closed-loop control and steer-by-wire functionality with accuracy to less than 1°.
These vehicles have a kingpin, around which their front (or rear) wheels rotate. The sensor measures the kingpin's absolute position and provides feedback to the operator. A through-shaft version fits around the shaft, and another version fits at the shaft's end.
Absolute position sensor bearings also improve the motion of off-highway vehicles that have a rotating cabin supported by a large slewing bearing. Here, the slewing bearing contains an integrated absolute position sensor. The sensorized bearing measures the cabin's rotation and provides that information to the vehicle controller, which allows the controller to correctly align the cabin with the vehicle's boom.
Speed sensors are incremental-type sensors in which speed is derived from measurements of bearing rotation. One application is automotive: Hub units fitted with speed sensors can measure wheel speed as part of anti-lock braking systems, or ABSs. Speed sensors can also be used for motor control in direct-drive systems, in which there is no gearbox between the motor and wheels.
In addition, speed sensors are used in moving walkways and people movers, such as those in airports. Here, bearing units with integrated speed sensors support the belts powering the moving walkways and handrails. Lack of synchronization between handrails and walkways can pose a safety hazard, so the sensors help synchronize the walkway and handrails for safe operation.
The newest sensor bearings are wheel bearings with load sensing capabilities. Currently in development, load sensing hub unit bearings may soon improve vehicle stability and control in ABSs and electronic stability programs, or ESPs.
Current stability systems are reactive, based primarily on wheel or vehicle motion sensing. Wheel speed sensors in current-generation ABSs, for example, detect wheel locking at the moment it occurs. The control system then releases brake force, which reinitiates wheel rotation. With ESP, accelerometers detect unstable vehicle body acceleration caused by conditions such as under or oversteering as it takes place.
In contrast, load-sensing hub-bearing units measure the force between a vehicle's wheel and the roadway. They detect unstable situations almost instantly, before there's wheel blockage or reactive under or oversteering, to allow ABS and ESP controllers to react before unstable vehicle motion can occur.
Design engineers can also use newer load sensors to test rotating equipment such as wind turbines and large gearboxes, for example. (One tip: When installing load sensors in bearing rollers hollowed out for testing purposes, allow the sensors to measure the forces acting on their rollers and shafts during testing.) Resulting data — helpful to engineers looking to optimize components for new designs — can be transmitted by these sensor bearings wirelessly.
For more information on sensorized bearings or the units pictured here, visit www.skfusa.com or call (800) 541-3624.
Lifts benefit from integrated sensor bearings on their kingpins; the closed-loop control from the former allows automatic adjustments to make lift operation safer.
A pair of offset Hall cells are embedded in the bearing and report its position.
Allowing lifts to zip
Mast height control units are incremental position sensor bearings that act as a control input, and measure the motion direction and acceleration so that operators can maximize their speed without tipping their vehicle.