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Stop, hold and go safely: Motion tuning for vertical loads

16 Aug 2022
Kollmorgen Experts

When designing motion for applications such as vertical gantries and hoists, you need to take special care to ensure operator safety and operational efficiency. Let’s discuss best practices for meeting the particular challenges involved.

The problem: Gravity never compromises

When a load in a servo system stores potential energy—either by gravity or spring effect—one or more servo motors must hold part or all of the load with absolute stability and reliability even when the motor is not moving. In most industrial applications, the weight and thus the potential energy of that load can vary widely from day to day or even moment to moment.

Decelerating these variable loads to a stop or accelerating them from a stop greatly magnifies these forces compared to a horizontal application in which the load is supported primarily by structural elements such as beams, struts and bearings rather than by a motor, gearing, brakes or other mechanical elements.

You absolutely can’t afford for the load to drop, possibly resulting in wasted product, operational interruptions or even human injury.

Moreover, as inertial loads and potential energy vary throughout and between production runs, you want the system to maintain consistent throughput. You want safe brake control, so that operators can interact with the production line as needed without shutting down equipment or giving in to the temptation to bypass safety features such as gates and cages. And you want a way to routinely test for safe braking capabilities with minimal disruption.

All of these needs are made more challenging by the relentless nature of gravity. Because gravity doesn’t care about safety, you have to make safety your top priority—without compromise.

Tune your axis right the first time

When tuning and setting brake parameters—such as deadband and brake engagement time—it’s easy to make hypothetical assumptions that don’t hold true under real-world conditions. These false assumptions lead to an overly iterative process of tuning and testing that can drag out your development cycle and potentially even lead to safety issues.

For example, it would be wrong to assume that the ideal brake delay time could be zero. Zero isn’t possible, as there is always delay and reaction time involved in brake actuation. When a servo system transitions from moving a load to holding it in a fixed position, there is an infinite dither as the feedback device reports the axis position and the servo drive responds with voltage and current commands intended to bring it to the target position. This happens continuously and very quickly. However, by its nature a servo system is a dynamic system, always making tiny adjustments and never coming to complete stasis.

At a certain point, you will want to interrupt this dithering and hold the load stationary while the machine completes its next process. At this point, the servo system must remove power from the axis and employ a holding brake to stop the motion and hold the vertical load absolutely stable. When it’s time to move the load again, the opposite must happen: The system must remove the brake and apply torque seamlessly to accelerate the load without a jolt—and, certainly, without dropping it.

Neither the control commands nor the brake application happen instantaneously. Accounting for the delays in the control and brake systems—no matter how miniscule—is critical when designing for vertical load applications. While initially tuning the drive, the biggest challenge is often that design engineers can’t foresee the many different types of loads that will be carried by the system.

Because they can’t know the physics involved in handling a complete range of lighter and heavier loads, engineers struggle to tune the drive so that it can brake and accelerate these loads without significant delays or jarring motions. Additionally, the axis may need to lock and remain solidly in place for an entire shift or weekend, so the control methodology must be absolutely trustworthy.

Given these challenges, engineers must size for the correct maximum load while also accounting properly for lighter loads. To get it right the first time—minimizing risk as well as excessive testing and rework—it’s crucial to factor in the correct deadband and brake engagement times. Kollmorgen engineers have considerable experience tuning vertical loads, and they can help you apply methods that have proven successful in a variety of vertical applications.

Ensure reliable operation every time

To ensure continuous, reliable operation, you need to specify a reliable motion system. This encompasses both the system’s features and its components.

For example, the Safe Brake Control functionality incorporated into Kollmorgen’s next-generation AKD2G servo drives allows the motion system to move an axis into position, remove power from the servo motor while employing the brake in a coordinated manner, and hold the load securely in place with no need to shut down the equipment. This is a huge productivity gain, for example when an operator needs to safely open a gate and take something on or off the production line. The axis can then be restarted as quickly and smoothly as it had been stopped, and production can continue with virtually no interruption.

Kollmorgen’s Safe Brake Control also includes a server safety signal that can be used to deploy two brakes for a single motor. This provides the redundancy needed to ensure reliable braking in any circumstance, in accordance with customer needs as well as industry guidelines and regulatory requirements.

Safe Brake Test functionality allows for testing of motor brakes by applying torque from the motor with the brake engaged, monitoring the system to ensure that no movement occurs. Because this testing capability is incorporated into the AKD2G drive, operators can test the braking system on their preferred schedule with minimal disruption. On-drive functional safety features also eliminate the need for external solutions that depend on complex and potentially error-prone integration between the controller, safe PLC and drive.

Beyond the drive, the entire motion system matters for reliable operation. For example, cables are often specified almost as an afterthought. But poor-quality cables on the communication bus can be susceptible to electromagnetic interference that causes feedback issues and position errors. Brake power cables that aren’t specified with the right wire gauge can cause the brake to spontaneously revert to closed, causing unexpected brake run-through and operational problems.

For vertical load handling, absolute reliability is the only option. A motion partner that can deliver dependable components designed to work together, simplify integration, and provide powerful tools for motion programming and tuning can help you achieve reliable success.

Get the support you need, on your terms

Don’t go it alone. When you source products from motion vendors rather than a true motion partner, you are the sole expert for fitting the products together and making them work. If they should fail, you’re on the hook. Collaboration with motion experts is a better and, frankly, safer approach.

Kollmorgen has more than 100 years of motion experience, proven in the industry’s highest-performing, most reliable motors, drives, programming tools, automation solutions and more. Our engineers have unrivalled experience helping machine and robotics designers achieve their goals in virtually every motion-dependent industry.

Count on our self-service support options such as our online design tools, e-learning opportunities, knowledge base and the community of experts in the Kollmorgen Developer Network. And when you need one-on-one help, know that you can contact us anytime to chat online or over the phone with engineers who know our products—and your industry’s applications—inside and out.

We want you to succeed—on your terms.

About the Author

Kollmorgen Experts

This blog was a collaborative effort among a team of motion and automation experts here at Kollmorgen, including engineers, customer service and design experts. Wherever you are in your project, we’re here to help.

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