9.1 Closed-Loop Control

A load cell rollers, also called tension rollers or transducer rollers, work on the same principles of any load measuring device. If you can weigh yourself on a bathroom scale, then you can understand a transducer roller.

How does a bathroom scale work? You stand on the scale and the needle moves to indicate your weight. Inside a bathroom scale is a spring. When you step on the scale, you compress the spring. The needle is mechanically linked to the spring. The heavier you are, the more you compress the spring, the more the needle moves, and your higher weight is revealed.

How do you know a bathroom scale is reading correctly? You need to calibrate it. There are two parts to calibrating a scale: zero offset and gain. The zero offset is used to counter the weight of the scale’s components, such as the plate you stand on. The gain sets the relationship of the spring deflection to the applied force.  Since all bathroom scales are designed to work under Earth’s gravity and expect you to use them on a flat surface, the gain is preset at the factory, leaving you to only set the zero for a reasonably calibrated scale.

A load cell roller is a roller mounted on two transducers, one supporting each end of the roller, that convert the applied force into an analog signal. Typically, the transducers are installed between the shaft and the equipment side frames or support structure. One company, Dover Flexo, has a unique and patented tension measuring roller design where the load cell is inside the roller between the rotating shell and the roller shaft.

Like bathroom scale, the load cell or force-to-current transducer measures load by monitoring the deflection of a spring (in this case, the spring is a bending beam). Load cell rollers measure three forces: the weight of the roller, the tension of the incoming web, and the tension of the outgoing web. However, the load cell roller does not measure these three forces independently; instead, it measures a component in the sensing direction of the vector sum of the three forces.

How do you know a load cell roller is reading correctly? You need to calibrate it. Like a bathroom scale, there are two parts to calibrating a load cell roller: zero offset and gain. The zero offset is used to counter the weight of the roller. The gain adjusts for the relationship of the output signal to the load cell geometry. The gain will change with different wrap angles and the orientation of the wrap angle and gravity relative to the load cell sensing direction.

I’m a big fan of load cell rollers. I like load cell rollers because they measure tension. Dancer rollers have their place and unique function, but they don’t tell you what the tension is. Since understanding tension (and strain from tension) is so important to resolving web handling problems, I always recommend load cell roller for all but the simplest web handling processes.

I’m also a big fan of having a fast responding analog or graphical display of the measured tension. Monitoring your tension and tension variations is like measuring the heartbeat of a patient. You can learn a lot from watching the pulses of tension, when they happen, and how big they are. If you only have a digital readout, especially one that is filtered and updates at a slow rate, you are missing out on valuable process information.

Many equipment suppliers will by default tell you to use 45 or even 90-degree wraps on load cell rollers. More wrap creates a stronger force on the roller, making is easier to measure the web tension. However, in many cases, it can be difficult to create even a 45-degree wrap. Does this mean you can use a load cell roller? No, I like to think about a minimum acceptable wrap in terms of signal to noise (or maybe I should call it load-to-sensitivity). By signal to noise, I’m referring to the force exerted by the web’s tension relative to the roller’s weight. If you have a high tension web and a light roller, then you can use a small wrap angle. However, if you are running ultra-light tension, you will need more wrap angle. I’d like to see the vector sum of the web tensions be greater than 40-50 percent of the roller weight.

Regarding load to sensitivity, many load cells are accurate to less than one percent of the full range, so don’t buy an over-sized load cell if you want to run with small wrap angles. You can use a smaller load cell and smaller wrap angles if you set up the sensing direction perpendicular to gravity.