Category: Roll forming Applications

Closed Loop vs Open Loop

Closed Loop Flying Die


Open Loop Flying Die


In the world of Roll forming, length control discussions usually come down to, “Do we run an open loop or a closed loop system?”  In some cases, both types of length control might be used on the same production line.  Above, you can see two animated versions of the concepts depicted in examples of a flying die application, but both types are also available for feed-to-stop applications, as well.

Open or Closed?

For the most part, when someone talks about Open or Closed Loop, they are specifically denoting the presence of a servo system.  The servo system is looking for a command to tell it what speed, direction, and position go to on a millisecond-by-millisecond basis.  In this context, the controller tells the servo go here and the servo responds with feedback I’m there. Continue reading “Closed Loop vs Open Loop”

Length Variance and Encoder Tracking

Encoder Assembly Out of Parallel with Material

When someone has a problem with length variance in roll forming applications, the prognosis is almost always encoder tracking.  If you look at length control from the perspective of the computer that controls the process it’s the brains of the machine.  It’s telling the roll former when to go and stop, and it’s telling the shear and punch when to fire.  The encoder and wheel are the eyes and ears of the brain.  Everything the computer knows – distance, speed, direction – all come from a very small contact point between the material and the encoder wheel. Continue reading “Length Variance and Encoder Tracking”

Continuous Press and Servo Feed Calculations

If you’re setting up a punching operation with pneumatic or hydraulic presses, the servo feeds are relatively simple, because you have full control over the presses and when they’re fired.  Mechanical presses running in continuous require a balance of timing and accelerations.  If your timing is off, you could break tooling or miss your target.  If your accelerations are off, the feed rolls could slip and induce a variance or even leave marks on the part.

Timing is directly impacted by production rate and press angle.  The higher the rate, the faster the press has to run.  The faster the press runs, the more acceleration you need to make servo moves.  Ultimately, production rate is capped by the capability of the press motor/drive and the maximum acceleration rate of the servo motor.

The most efficient way to dial in a particular production line is to setup the servo to handle the most extreme production rate and minimize setup changes for less-demanding applications.  The demands of production – balancing speed, quality, wear, and cost – might require a unique set of parameters for every job a line might run.  This post will walk through the math of using press speed and part/punch distance to calculate the speeds and accelerations required from the servo.

Continue reading “Continuous Press and Servo Feed Calculations”

Press Consistency

Unless you’re using a servo, press consistency is crucial to length accuracy in flying punch and cut applications.  Without servo positioning, timing variances in the press will directly relate to length tolerance error, and the faster you run the line, the worse even small timing variances will affect length.

For instance, if you’re running a roll former at 400 fpm, that’s 80 ips.  At that speed a timing variance as small as ±0.003 s will cause a total length variance of ±0.24″!

Even the best 24 VDC relays have a switching time of ± 0.003 s.  Thus, running a press fire output through an electro-mechanical relay could easily induce a total length variance of almost 0.5″.  That assumes you have no encoder tracking issues.

Continue reading “Press Consistency”