Web Tensioning Solution Note

High performance motion trace is the key to optimizing the design of a general purpose web tension controller.

Application Challenge

Web tension control is a common task for both manufacturing and packaging machines. These controllers can be mechanical or electronic, however, the trend is toward electronic control because this allows much higher acceleration and deceleration of the web spool.

How do we analyze the motion associated with these complex dynamical systems so that we can build a high performance web tension controller?

Application Considerations

Figure 1: System Diagram

Motion Control Solution

In this application PMD Corp.'s Pro-Motion® development software is used with a three axis Magellan®Motion Processor IC and two Atlas® Digital Amplifiers to provide very accurate and high speed analysis of the combined mechanics and control system performance. An ultra-low mass dancer arm is connected to an encoder to provide a measure of linear tension. This position data stream is fed into axis three of the Magellan IC for tracing, and is converted to tension via a simple equation during post-trace analysis.

The overall configuration of the web tensioning system is shown in the main diagram. The motion control system can be configured to control either DC Brush or BLDC motors. Several candidate motors from each type must be investigated, and the optimum motion profiles as well as PID and feedforward parameters need to be determined.

For each motor/hardware configuration, we will go through the same series of motion characterizations using Pro-Motion. Once we are done, we can compare overall results to determine the highest performing hardware configuration.

Figure 2: Pro-Motion® Software from PMD

• Step 1: Without the web material threaded through the spools, run Pro-Motion's Axis Wizard to get the motors set up and to allow the auto-tuner to develop basic parameters for servo control of both the drive spool motor and the tensioning spool motor. Once completed, thread the web material so that the spools are connected.

• Step 2: Setup an aggressive forward direction accelerating move in the drive spool motor. Use a traditional PID filter with acceleration feedforward and constant forward bias torque to control the position of the drive motor. Setup an equal and opposite constant torque command for the tensioning motor, along with cross linked acceleration feedforward to the axis #1 command profile.

• Step 3: Set the Pro-Motion trace variables to capture the drive motor velocity, the commanded drive motor torque, the commanded tensioning motor torque, and the measured linear tension (encoder position axis #3).

• Step 4: Repeat and optimize the various gain settings, particularly the acceleration feedforward terms. The overall goal is for the linear tension (measured via encoder #3) to stay the same even while the spools and web material are accelerated and decelerated aggressively. Once the optimal gain values are recorded, this data set is saved for comparison with the other motor types and hardware configurations being explored.

Going Further

Pro-Motion allows the entire configuration setup to be saved to a named file so that various motors as well as various servo control schemes can be explored. It is possible that the Magellan Motion Processor's bi-quad filters may also be used for further optimization, in which case after optimizing the parameters described in the basic sequence above, an extra trace/optimize session should be undertaken to optimize frequency-based filtering via the dual bi-quad filters provided by Magellan Motion Processors.

Pro-Motion is an easy to use Windows-based application designed to accelerate motion system development by providing ready-to-go capabilities that can be shared by the entire development team. It features a step-by-step axis wizard that allows designers to quickly and easily tune position loop, current loop, and field-oriented control motor parameters. 

You may also be interested in:

• Optimize Control of Syringe Dispensing Application
• Use Torque FeedForward to Get More From Your Motion Controller
• Field Oriented Control (FOC) - A Deep Dive
• S-Curve Motion Profiles - A Deep Dive
• Servo Tuning Deep Dive: Black art, rocket science, or walk in the park?

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