Master-speed reference: What is it?
The master drive is the timekeeper/pacer of all other drive motors and elements in the line.
David Roisum, Ph.D. Consulting Technical Editor -- Converting Magazine, 3/1/2009 12:00:00 AM
Most web machines have one and only one master-speed reference. This master drive is the timekeeper/pacer of all other drive motors and elements in the line. When the master moves, everyone else must move. When it comes to a stop, everyone else must stop. It's the one drive that directly follows the speed command input by operators. When the operator hits the go button, that command tells the master to construct an “S-curve.” It is this reinterpreted command that is ultimately sent to most of the other drives in the line.
The reason is simple. Drives can't follow a step change such as “go from thread speed to 300 fpm;” that would require infinite torque. Instead, the master must construct an S-curve that has three parts: round, ramp and round. The rounding at both ends of the s-curve is to limit the jerk (first derivative of acceleration) to the system. It is set up to be around 2-3 secs for many machines. The rounding is far more important than most people think. Many of the troubles during the ramp (speed changes) are in fact at the ends of the speed change rather than in the middle. Slowing the accel/decel rate or inching the speed are largely ineffective patches to the root problem of inadequate rounding.
The acceleration rate is very machine-dependent. A rate something like 10-20 fpm/sec is ordinary. Thus, it might take one min to reach 1,000 fpm. There's no hurry to get to speed if you're going to run for a whole shift. In contrast, many offline rewinders must not dawdle because they start and stop many times every hour. Here, accel rates can be upward of 100-150 fpm/sec. Obviously, this jackrabbit acceleration puts very high demands on the mechanical and electrical designers. Failure to master the details will result in tension-related troubles. On a rewinder, you'll note an “acceleration offset” on the side of the wound roll at speed changes. This has nothing to do with acceleration per se. What this is telling you is that the drive system is not holding tensions well during the demanding speed-change condition.
The other drives are fully informed of everything the master is doing and what is expected of them. The master passes on its own speed; the other drives then do inertial compensations. By this, we mean that the motor calculates the additional effort required to just make the speed change and makes the motor compensate before being told by the load cell that a tension upset has already occurred. This is one way to tell whether your drive techs know what they're doing. Ninety-eight percent of the work of the motor is calculated and only 2 percent is trimmed by the dancer or load cell. Opening trim to accommodate sloppy drive-program design and tuning too much has one result: tension oscillations.
Which motor has this special position? This can be arbitrary because the truly expert programmer can sometimes make even an idler roll the master-speed reference. Modern drives even allow you to switch masters, perhaps in response to a new threading path on a large coater. In all cases, however, traction is essential because the web will often break down within seconds if it slips on the master.
920/312-8466drroisum@aol.com, www.roisum.com
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