Monitoring the absolute position of side-facing forks helps keep forklifts on the narrow path...er, aisle.
Edited by Robert Repas
Most people are familiar with the front-facing load-bearing forks found on standard forklifts or lift trucks. Under normal operation, the truck driver swings the forklift to face the material to be moved, then drives forward to place the forks into the lift area of the pallet. This means, of course, that the aisle where the forklift operates must be wide enough to handle the overall length of the truck from the tip of the forks to the rear bumper. This minimum-aisle-width limits the number of shelves in the warehouse or storage area. Reducing aisle width permits installing additional storage shelves, boosting warehouse capacity and efficiency.
Raymond Corp.’s (Greene, N.Y.) 9000 Series Swing-Reach lift truck lets warehouse designers shrink aisle widths with forks that turn side-facing or front-facing. This lets operators work in less aisle space — as little as 66-in. wide — because the truck never has to face the material it moves.
While the Swing-Reach truck has been manufactured for decades, the 9000 Series has a number of industry firsts. These include extending battery life by recovering energy as the load is lowered, accurate lift positioning, and user-optimized travel speed and acceleration. These advances contribute to greater operator productivity and safety. Pallets move down narrow aisles, then up 45 ft, while the acceleration, maximum speed, and deceleration of travel, lift, lower, and load handler are programmed to the truck driver’s personal skill level and other factors.
Motion control becomes a critical task when moving a 3,000-lb load. The design of the 9000 Series centers around Raymond’s Intellispeed control and ACR system. Intellispeed coordinates multiple-axes travel speeds based on load weight and height, while ACR provides quick acceleration, smooths lift-and-lower speeds, and smooths direction changes.
Central to the load-moving motion is the mast that the lifting forks move along vertically. Raymond calls this the mini-mast. The mini-mast is part of the system that lets operators move loads up, down, and side-to-side at lateral speeds up to eight inches per second. It uses hydraulic cylinders and steel rails attached to lift forks to move loads. Tracking the position of the mini-mast is key to efficiently and repeatedly performing material handling tasks.
Raymond engineer Dan Driscall describes the motion control aspects of the 9000 Series, “The truck is a Swing-Reach truck with a mast and set of forks. This system swivels and travels back and forth. It is especially important to know the center crossing and approaches to end points for stability reasons. The mast and forks move via a rack-and-pinion steering setup making a multiturn measurement necessary. Velocity is also important to know.”
Additional position sensor specifications included absolute-position sensing to monitor mast position, noncontact technology for long life, and have the ability to count up to 12 turns. The search led the Raymond engineers to Novotechnik in Southborough, Mass., and its recently launched RSM2800 Series of multiturn angle sensors.
The RSM2800 Series features 12-bit resolution, a size of 38-mm center-to-center between mounting flanges, and repeatability within 1 least significant bit. It measures angles up to 5,760° (16 turns) using noncontact technology with an independent linearity of 0.25%. The sensor is also sealed to class IP65 standards. Through the RSM2800, Raymond can monitor position down to 0.078 in., as well as speed and acceleration.
One additional feature desired of the sensor was an ability to work in absolute position mode from start-up, even if the sensor was turned without power. The RSM2800 Series captures position and turn counts without power or batteries using a quantum physics phenomenon that occurs in thin film structures. Applications that use the RSM2800 can lose power or shut down, then resume right where they left off as if the power loss never happened.
The technology behind the RSM2800 is made up of several parts. A free-standing permanent magnet attaches to the rotating portion of the application. Two independent detectors in the sensor monitor the position of the magnetic field as the magnet turns.
The first detector measures rotation angle while the second detector counts turns. A microprocessor, memory, d/a converter, and associated circuitry store the magnetic field readings, calculate the angles and convert them to analog values that are output as a ratiometric voltage. The turn count provided by the second detector, along with the angle, are stored in nonvolatile memory. If power is removed, the RSM2800 continues to count turns and both the turns and angle are available when power is restored — including turns made with no power applied.
Without power of any type, including batteries, how does the sensor continue to count turns? The answer is magnetoresistance, a quantum physics phenomenon demonstrated by thin-film structures. This phenomenon occurs when two ferromagnetic layers are separated by a thin nonmagnetic film, a technology used in hard-disk drives for computers.
When the two magnetic layers are parallel, resistance drops to a minimal value. As the magnetic layers turn so they are no longer in alignment, electrical resistance rises. The amount of increase can measure absolute position values or full rotations. The result is a combined multiturn rotation counter and position sensor that senses without contact or power. In addition, the sensed turn count can be held without power for years.
The physics of this sensor technology is described in three elements. The first is called a Domain Wall Generator (DWG). A magnetic DWG is a group of atoms aligned in parallel to magnetic north and south. The second element is a specific number of ferromagnetic sensor tracks arranged in layers, separated by a thin nonmagnetic spacer layer. The sensor tracks begin at the DWG. The third element is a reference track that is another ferromagnetic layer placed above or below, and parallel to, the sensor track layer.
The track segments are arranged in a spiral structure and act as a narrow tunnel that forces alignment of the magnetic domains to be only 0 or 180°. This is due to an effect called shape anisotropy. The DWG area is large enough that shape anisotropy cannot occur and, therefore, the magnetization direction in the DWG is the same direction as the magnetic field of the rotating magnet. The reference layer has a fixed magnetization direction that doesn’t change.
As the magnet passes over a segment, the DWG pumps out groups of magnetic domains aligned to either 0 or 180° to the first segment depending on the position of the magnet. The two directions either align with (0°) or oppose (180°) the reference layer. At each 90° turn, the DWG pumps more domains into the sensing track forcing the magnetized segments to advance.
However, as the magnet rotates through 180° from the start, its magnetic alignment forces a 180° change in the direction of the domains the DWG produces. The first segment becomes magnetically aligned in the opposite direction to the prior segments. This creates a domain wall between segments 1 and 2. The process repeats at 270°, creating a second opposing-direction domain wall. The cycle starts again at 360° except that the inner spiral segments are affected.
Measuring the resistance determines the total number of turns made by the sense magnet, and the count accurately determined. Because this is a purely magnetic phenomena, the process occurs and information stored in the segments whether there is power applied or not.
Once the resistance is measured, a custom integrated circuit stores the change of resistance and applies a predetermined offset to count the number of turns. The number of spiral tracks determines the number of rotations that can be detected.
To handle partial turns from 0 to 360°, a second sensor measures the specific angle of the sense magnet within a single-turn range. Its value is then added to the turn-count sensor to determine the absolute position of the sensor.
Though the RSM2800 integrates a high degree of technology such as angle-and-turn detection sensors, signal pre and postprocessing circuitry, and a microcontroller, the use of the sensor is as simple as using a potentiometer. Three connections supply power, ground, and output — an analog voltage that corresponds to the rotation angle. The voltage represents the absolute angular position of the shaft within a full-scale reading of 2 to 15 turns without mechanical gearing or buffers.
For example, a 10-turn application (3,600°) with the sensor connected to a 24-V supply produces a 0 to 9-V output that represents angles from 0 to 3,600°. A lookup table that maps voltage-to-angle provides the total angle. In this example, if the output voltage is 4.5 V, the angle is 1,800°. Dividing the measured angle by 360° provides the number of turns (5). A second lookup table provides the fractional rotation of the sensor if needed.