Swift & steady
When cranes move loads, the resulting pendulum effects make it difficult to reach the destination with both speed and accuracy. Conventional approaches to countering load sway are effective, yet costly. An elegant and inexpensive software alternative is now available for B&R's Automation Studio development environment.
Production hall or container port – time pressure is a constant factor. Unproductive idle times are a luxury no one can afford. A logical solution is to accelerate transitions between production steps such as loading, unloading and repositioning. Yet, increasing speed at the cost of precision would risk damaging both the load and the destination.
Crane loads hang from hoist ropes or chains, and this pendulum construction generates new oscillations with every acceleration and deceleration. In outdoor applications, the additional factor of wind compounds the challenge. To delay downstream production steps until the load has stopped swinging is out of the question. Slowing down the movement to reduce swinging would be equally unacceptable.
Systematic approaches to sway reduction
Various types of anti-sway systems are implemented to counteract the pendulum effects. These systems have traditionally been comprised of mechanical equipment that uses auxiliary ropes to dampen the motion. The hydraulic winches used to control these systems place additional constraints on dynamics and are expensive both to purchase and to maintain. Alternatively, the crane bridge, trolley or main hoist can be driven into the swing to dampen the pendulum effect.
Many approaches to electronic anti-sway solutions have very limited applications. One such method is input shaping, which is largely confined to small oscillations and nearly constant rope lengths. Input shaping cancels out the load's own oscillation with a sequence of superimposed impulses.
A path following controller, on the other hand, influences the set movement of the trolley and thereby the set angle of the load for any prescribed movement profile. These closed-loop techniques generally generate motor torque set values, which precludes the use of a speed controller integrated in the motor control unit and may cause unwanted changes to the rope length, which results in considerable power dissipation.
"What we implemented was fundamentally a combination of path following control with selective sensor support," explains Dr. Gernot Grabmair. "By controlling the set velocity and acceleration with the option of using torque, force or velocity as the manipulated variable, we were able to achieve a sufficiently smooth path – even with a variable rope length."
The control concept was developed by modeling the load on a rope or ropes of various lengths, as well as by modeling the dynamic mechanical system including the trolley, portal and drives.
A systematic reduction of the nonlinear mathematical system model provides the option of using the trolley speed as the set value. Port-based modeling eliminates the need to derive differential-algebraic equations for the simulation model. Tools that use symbolic model formulation, such as MapleSim, generate equations that can easily be reused for further analysis.
"Today's port-based modeling tools – such as Simulink, MapleSim or the license-free Scilab/XCos tool developed at the University of Applied Sciences in Wels for use in industrial controllers and embedded systems – provide efficient modeling, validation and optimization of open and closed loop controllers for tasks such as suppressing oscillation in cranes," says Grabmair. "Redeveloping the underlying algorithms from scratch each time would be quite a waste of resources."
Flexible programming of B&R automation systems in Automation Studio using the widespread C programming language makes is possible to integrate many different modeling and simulation tools. Model data from these tools can be automatically converted to C-code and transferred to Automation Studio.
For some time now, the specially developed "Automation Studio Target for Simulink" module has provided convenient interaction between Automation Studio and Simulink. In May of 2012, B&R introduced a similar interface for MapleSim. Seamless communication between these tools ensures an uninterrupted flow of development – from configuring the project to testing it on the actual target hardware with a real-time capable simulation model or performing a hardware-in-the-loop emulation. Access to the controller directly from the simulation system allows engineers to generate source code and automatically integrate it into Automation Studio projects and also brings considerable advantages during commissioning.
Five times higher productivity
In tests for fine positioning movements, the presented approach achieved up to five times higher productivity. There is especially promising potential for improvement in the area of intralogistics. Automation Studio users have a collection of powerful function blocks at their fingertips to implement vibration damping for trolley movements or a multi-rate observer for integrating sensors with a slow sample rate.
"B&R customers benefit from this development in an inexpensive and easy-to-use form with professional support during implementation," reports Grabmair. "They can develop cable conveying applications with minimal oscillation – quickly, simply and without requiring an in-depth mathematical background."
"The solution I've developed gives B&R Automation Studio users a quick and simple way to implement cable conveying applications with minimized oscillations, without requiring them to have an extensive mathematical background." Dr. Gernot Grabmair, Professor of Electrical and Control Engineering and entrepreneur.