Point-to-point acceleration with linear motion

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Hardware and software have equal impact on linear-motion applications. The technology can bring accuracy and precision to many applications, but selecting components requires a bit of forethought. Our panel of industry experts explains how to move point to point faster.

What can I do with software or hardware to make my linear motion application faster when moving point to point?

Julius Rurangirwa, Heidenhain: To the reduce the time it takes a linear axis to move from point to point, one certainly needs a well-sized motor with a correct winding, but also a complete solution where linear motors, position controllers and active isolation system are highly integrated and calibrated to deliver a minimized move and settle time. In other words, to move faster from point to point, one has to solve not a single-axis problem but a system-level problem.

Julius Rurangirwa is Etel motion systems applications engineer at Heidenhain.

Clint Hayes, Bosch Rexroth: Regarding hardware, for the drive mechanisms, look for belt or linear motor options, if speed is the primary target.

Ball screws with driven nuts are a good alternative, offering precision positioning while at the same time eliminating the critical speed concerns that typically limit a more traditional screw-driven configuration.

For linear guides, manufacturers offer varying degrees for speed performance. Look for models that will meet the application criteria—for example, cam roller linear guides offer lower load, but higher speed capabilities, up to 10 m/s. Profiled rail systems, often referred to as square rail, ball rail or roller rail, typically offer a very wide range of performance options and sizes.

Regarding software, using today’s advanced controls, motors controlling linear motion can be precision-tuned for maximum performance. For example, controls can use a high-speed Sercos servo control loop to precisely control motion while maximizing performance.

Industry 4.0 ready

Figure 1: Integrated linear measurement/scale system with ball screw and roller rail linear guide adds precision and Industry 4.0 capability to linear motion systems.

(Source: Bosch Rexroth)

Speed, acceleration and positon can easily be controlled in varying process conditions. The best controls offer auto-tuning, a built-in drive feature that handles many applications. They also allow custom tuning to be easily performed for highly dynamic systems that require optimized motion.

Motion system designers should look for controls that include algorithms within the software to handle maximum loads, maximum accelerations and maximum torques of a linear axis, while maintaining maximum repeatability.

If your motion technology supplier offers both mechanical and electrical motion components, often the control software can automatically identify the connected motors and allow easy pairing and integration of mechanical components.

Most importantly, make sure to take advantage of the knowledge and expertise of your linear-motion-system suppliers. Hardware and software are frequently upgraded to enhance performance, as well as Industry 4.0 connectivity (Figure 1).

Clint Hayes is product sales manager, linear motion technology at Bosch Rexroth.

Chris Bullock, Bishop-Wisecarver Group: A properly sized motor and properly tuned PID control system will help you to get the best performance from your system. Inertia ratios between the motor and actuator become very important when you need fast and precise motion.

Chris Bullock is applications engineer I at Bishop-Wisecarver Group.

Broc Grell, Nexen Group: On the hardware side, better materials, hardening of materials and better bearing systems allow for faster movements. Motors are also advancing to allow higher torque at higher speeds, as well as higher-inertia motors for better inertia mismatch. All of these new additions can make controlling a linear motion system easier. This saves time and money.

On the software side, there are a lot of good tools out there to help with motor and gearbox sizing using system analysis. There are also great tools for safety-related functions, a big one being Sistema. These tools, if used correctly, can save a lot of time when designing a system to make sure all the component sizing makes sense for the application and that all the correct safety features are in the design to minimize the risk level of a machine.

As with all technology, the software side of driving linear motion is moving forward in leaps and bounds each year. Controls and drives are faster than ever, making the systems they drive even faster. The limitations with most systems now are the mechanical components, where 15 years ago the controls couldn’t work fast enough to touch the capacity of the mechanical components.

Broc Grell is applications engineer at Nexen Group.

Josh Teslow, Curtiss-Wright: Put the load on low-friction linear bearings. And modify the motion profile to accelerate more aggressively. Utilize what torque the motor has.

Josh Teslow is applications engineer at Curtiss-Wright.

Aaron Dietrich, Tolomatic: If you are torque or current limited with the servo drive or motor, the best way to make faster point-to-point moves is with a triangular profile. This is because the acceleration/deceleration rates do not have to be as aggressive as a typical trapezoidal, with or without jerk, profile. Accel/decel areas of the motion profile require higher spikes in current—torque—to complete the motion. With triangular profiles, the motion is always accelerating or decelerating, which actually leads to smaller spikes in current, but the peak speed is only momentarily achieved.

Another way to optimize time when moving point-to-point is to size the servo drive/motor with respect to the peak operating conditions for faster peak speed acceleration and decelerate only right at the end of the move. By optimizing the servo system like this, a trapezoidal profile can be used to move the mechanical system at peak speed for a longer time achieving a faster overall move time.

Aaron Dietrich is director of marketing at Tolomatic.

Matt Prellwitz, Beckhoff Automation: Some platforms can handle all motion control alongside PLC, HMI, advanced measurement and more, providing the foundation for lean system architectures. Dynamic motion-control projects can be adjusted, and functionality such as collision avoidance is easily accessible in the software. The configuration of motion systems can completely handled in this software. There are easy steps to generate motion profiles, optimize functionality and generate documentation and parts lists for assembling the motion system.

Matt Prellwitz is motion product specialist at Beckhoff Automation.

Brian Zlotorzycki, Heidenhain: For hardware, most motors have alternate windings which reduce Back-EMF and allow for an increase of speed at the cost of requiring more current to operate. Also, a more powerful motor would allow for less dwell time so the motor can move for more of its cycle.

Brian Zlotorzycki is Etel motors product specialist at Heidenhain.

Jay Johnson, Sick: The chosen feedback interface has a great impact on the timing ability of the controller to read position and generate trajectory paths. With hybrid digital - sin/cos (HIPERFACE) and real-time high resolution pure digital (HDSL) encoders the position information is generated in the sensor synchronously with the processing of the motion controller.

Regarding hardware, the elimination of inertia and friction will yield the greatest gains. Linear motors make for an efficient system as the rotary components are removed (motor rotor, screw, pulleys, belts etc.). Much higher rates of acceleration, deceleration and top speeds can be achieved. However, consider the environmental conditions and packaging design of the motor.

Jay Johnson is national product manager at Sick.