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.

Open Loop

Open Loop length control systems rely on timing and consistency to achieve length accuracy and precision.  The better you understand the timing, the better your accuracy – regardless of speed.  The more consistent the timing, the better your precision (repeatability).

In an open loop system, the computer that runs the machine is typically looking at a rotary encoder and then firing a press.  The computer receives no feedback in terms of the position of the tooling with respect to the position of the original target point.

In flying die applications, the carriage/platen/die might have a pneumatic or hydraulic cylinder that “boosts” the die forward, but this cylinder is not typically capable of having its pressure adjusted on-the-fly to compensate for speed changes, though some systems do provide support for proportional valves, but if the cylinder doesn’t match speed or position for any reason, the control system has no way to verify the error.

Closed Loop

Closed Loop length control systems use a servo (usually in conjunction with a rotary encoder) to account for small variances in timing and speed to ensure length accuracy and precision.  In most cases, a servo will be capable of quickly compensating for speed changes without suffering unacceptable tolerance errors.

In feed-to-stop applications, the rotary encoder would be used to account for slip between a set of feed rolls and the material.  Servo feed systems that don’t require a material encoder can achieve extremely tight tolerances.

Which is Better?

Deciding which type of length control is better than the other is a loaded question.  Closed Loop systems make it easier to achieve tight tolerances at higher speeds, but the servo systems and their associated components (couplers and linkages needed to tie the servo to the system) are more expensive than Open Loop systems.  Servos don’t guarantee tolerance.  Even with a servo, the accuracy of the finished part is dependent on the material encoder and its tracking accuracy.  There are no magic wands in Roll forming.

Accuracy and Repeatability

A good rule of thumb is to base the decision on the length accuracy requirements of the finished part.  If acceptable length tolerance is ± 0.0625″ or greater, then you are probably fine to use an Open Loop system.  The caveat to this is line speed and cut/punch quality.  Above 250 fpm you might find that the material suffers damage due to the weight of the press and tooling applying back pressure on the steel.  Obviously, heavier gauges will suffer the issue better than lighter gauges.

Open Loop systems are capable of holding tolerances ±0.03125″, but the machine will require constant maintenance to ensure mechanical slop is eliminated and that the hydraulics/pneumatics are consistent and free from leaks or pressure irregularities (i.e. temperature variance).  Most companies don’t have the resources to pour the effort needed to keep a machine in peak condition all the time.

Setup and Troubleshooting

As long as the problem isn’t actually in the servo (parameter, component failure, tuning), then troubleshooting Closed Loop systems can be very simple.  Down stream from the servo motor shaft, you should expect to find tight mechanical couplings all the way to the tooling.  If you don’t, you’ve probably found the problem.  The trouble here is many Maintenance Technicians lack servo experience, and there’s a tendency for people to want to point at the black box where all the magic happens, as opposed to looking for the problem in the real world.  If you have Maintenance Technicians who have servo knowledge and experience, they’re likely to be more expensive assets, but if your company relies on a lot of servo equipment, it’s worth the expense to keep the expertise in-house.

Even though Open Loop control is simpler in nature, the places where variance can be introduced are greater and more varied.  Temperature, pressure, speed, and encoder tracking can all contribute to problems.  I’ve seen many situations where the plant air wasn’t sufficient for all the machines and running too many lines at once caused problems.  There’s also a certain amount of mathematics required to dial such a system in to begin with.  Even though the math involved is no more complicated than high school algebra, most Maintenance Technicians lack the context to know when and how to apply such formulas to the equipment.  To throw an additional curve at your technicians, changing anything about the system – rebuilding a press cylinder, replacing a solenoid, or even replacing a hose – can change the timing of the system and now requires the math to be run again to accurately dial the system back in to tolerance.  This is really where experience and training can make a big difference.

Need Help?

Having trouble deciding which length control system makes the most sense for your application?  Looking for training for your Maintenance group?

I’m available for phone/email support, as well as on-site support and training.  I can train your people to solve their own issues, as well as provide phone support to help them troubleshoot and solve problems without incurring travel expenses.

Contact me today for a free consultation.

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