At California Expanded Metals Co. (CEMCO), a mechanical line shaft would stretch and perforate coils of cold-formed steel, creating a mesh pattern in the steel. CEMCO manufactures steel-framing and metal lath products. The versatile mesh metal material is often used as reinforcement for all types of construction, including walls, ceilings, and as an aid in the fireproofing of steel beams and columns.
The steel, which is 24 in. wide and 1/16 in. thick, is unwound into a line-shaft machine that has six sections that create openings in the metal to expand the steel into the needed mesh patterns. Each section performs a different expansion or perforation task to expand the metal. One might stretch the metal sideways, another lengthwise, another might stamp in perforations, with the final section cutting the mesh to length. For this type of application, coordinated motion is crucial.
Each machine section includes a motor and gearing components connected to the line shaft, which was powered by a drive. This main drive coordinated all motion.
Age and time, though, took their toll on the line shaft, to the point where mechanical problems were constant. Pattern changes were frequent, which required speed changes, as well as alterations and maintenance of the coupling and sprocket and gear settings. The time needed to make these changes continually increased until finally management knew it was time to make a change.
They called in application engineers at Kaman, a distributor for Yaskawa motors and drives, to recommend a better way to execute the needed tasks. The application engineers saw that this was a perfect application for an electronic line shaft.
Electronic line shafts use software to maintain the position, or phase alignment of two or more independent machine sections, process lines, or other mechanical systems. From tenter frames to printing applications, this software will synchronize driven sections to within one quadrature encoder count.
The Kaman engineers began the changeover at the back of the machine with the follower sections, replacing all the individual drives, motors and gearboxes. Then they removed all of the mechanics in each section driven from the main drive shaft. After that, they focused on the main drive.
In the electronic upgrade, the engineers replaced the old drives with Yaskawa F7 series, and set them up to run in vector mode. The new master is a 30 hp drive, and it controls four followers that range from 2 to 20 hp. As the main drive, it is now the master speed reference and is controlled with a potentiometer. It retransmits a digital encoder signal to the first follower, which then transmits an encoder signal to the second follower, and so on down the line.
With an electronic line shaft, any electronic device that can supply a quadrature signal can be used as the master signal, including an encoder or another drive. The main requirement is that the machine sections to be synchronized are mechanically isolated and separately driven sections, so that the software can maintain a constant position relationship of the machine sections to within a few encoder counts.
The follower sections are aligned and synchronized to the master section. The follower drives monitor the pulse feedback from the master encoder and compare it to their own encoder data. The follower motor rotor position is compensated, in relation to the master signal, to maintain position (phase) synchronization with the master signal. This arrangement achieves near perfect alignment between the master encoder and the follower motor with no accumulation of position error over time.
Electronic gear ratios allow the follower to operate at a programmable ratio of the master speed, “simulating” mechanical belts or gearboxes. The gear ratio can be set by parameter (2 preset), analog, MOP, and network communication. There is zero drift with the system, even with repeating or non-repeating ratios.
One of the main advantages of electronic line shafts is their precision. The range for the master-follower gear ratio is large, an 8 digit numerator divided by an 8 digit denominator. The programming can be set so that ratios can be changed with a button (digital input) or with network communications. Acceleration and deceleration are adjustable during gear ratio changes. Maximum follower to master ratio is 99999999:1. Speed control range is 1 to 1000.
The software can also be run in a speed (velocity) follower mode in which the follower tracks the master signal speed and velocity, but not position (phase). This provides a very accurate, real time speed reference.
Filed Under: Motion control • motor controls