Drive-based vibration and slosh control can help improve throughput in machines that assemble medical devices and products.
Bipin Sen • Ed Lasch
Bosch Rexroth Corporation
Simply increasing the speed of a system that assembles medical devices, or packages medical products, seems like an easy way to boost throughput – until unwanted vibration (either induced or parasitic) appears. That requires deceleration of key actuators to avoid damaging the device or product, such as liquid medications, catheter kits, or electronic devices.
In today’s high-speed automation systems, the inherent speed of machine movement is often not the limiting factor for throughput. The time required for the product to settle or for the machine to stabilize after a move limits the rate of production. Controlling these unwanted movements (oscillations and vibrations) offers an opportunity for productivity gains in medical manufacturing. Our company addresses this with motion-control technology that reduces oscillations in a wide range of applications, with the potential to increase process throughput by as much as 50%.
Common industrial problems include liquid medications that oscillate inside moving packages, and objects that vibrate or sway as they move through multiple process steps via robots or linear transport systems. For example, in automated diagnostic tools, samples can be damaged and test results compromised if actuators impart too much motion to the micro plates or tubes.
In these applications, vibration can be imparted to the object by movement of the object itself, as well as by machine resonance – the low-frequency hum or ringing inherent in mechanical systems. The typical solution is to set an upper limit on the speed of motion. However, longer transient times to dampen oscillations can reduce productivity, while quality and dimensional accuracy are limited by the mechanical effect of the oscillations.
When a mechanical system resonates, its frequencies can be captured by the intelligent drive system. Control functions for damping and avoiding can then be introduced to reduce resonance. Less vibration reduces dimensional variation and damage to the product and permits shorter stabilization and transient times. Another plus: reducing vibration lower mechanical stress on the manufacturing equipment itself, which extends machine life.
When packaging contains fluid, acceleration or deceleration of the package induces motion – or slosh – in the liquid. Sloshing liquid negatively affects filling, sealing and measuring processes – for example, there are strict regulations governing the tamper-proof sealing of liquid medications (over-the-counter, prescription and those used in clinical settings), so automated filling systems must work along with the container design to ensure that every container is correctly sealed with the proper amount of ingredients. To combat slosh, more reserve space in the container, more wait time for the contents to settle and extra space to allow for package movement are commonly required in liquid handling applications.
Because package movement is controlled by servo drives, using adaptive technology to minimize slosh provides several benefits. For instance, it reduces reserve space required in the container, which allows container height to be lowered to reduce the amount and expense of packaging material. Greater stability in the fluid meniscus also allows the use of optical level sensors to provide feedback to further optimize motion control.
In the process, anti-slosh technology shortens stabilization times, which cuts down on transient (i.e., process pause) time needed to allow contents to settle. The result is faster filling and sealing rates and machine cycle times. Packaging lines that implement anti-slosh motion control typically improve throughput from 10% to 50%.
Why oscillation occurs
Vibrations or oscillations are simply motions that repeat over a period of time. Controlling slosh and vibration involves factors of wave physics. Oscillations in medical manufacturing systems and automated diagnostic and lab tools, for example, would typically take the form of asymmetrical sinusoidal waves that vary in amplitude and frequency.
Oscillations can be classified as free vibrations, in which objects vibrate on their own after an initial disturbance to the system (much like a tuning fork), and as forced vibrations, in which objects vibrate when an external force disturbs the system (such as an out-of-balance washing machine).
Oscillations that occur as smooth sinusoidal harmonic waves can be further classified as linear vibrations, which are typically observed in small amplitude vibrations of flexible shafts and long slender objects, and as non-linear vibrations that are distorted sinusoidal waves propagated by interactions between the object being manipulated and the internal resonance of the machine itself.
The complex physics of waves often make traditional methods of controlling oscillations ineffective. For example, adding stiffness or mass to increase machine bulk can help reduce vibration, but it is costly and cannot completely eliminate machine resonance caused by servo drives and other components.
Increasing time to allow contents or objects to settle is a simple tactic that lets wave energy disperse, but from a production standpoint, causes waste in the form of waiting time.
Vibrations can be minimized by smoothing the motion with jerk-limitation or S-curve motion-control profiles that avoid abrupt acceleration or deceleration. But programming can be complex and can accomplish only a limited amount of correction. Too much limiting to create smoother motion will slow acceleration to the point that machine cycles are reduced, further limiting productivity.
A better way to control vibrations
An alternative is to use a software-based motion control that can settle or filter vibration by adapting to changes in the material, the load, and the environment. To control oscillations, motion-control programs can utilize two techniques: Avoiding and damping. Avoiding technology filters the vibrations caused by high dynamic movements. Damping technology uses external feedback to employ counteracting kinetic energy to settle vibration. This feedback can be provided by the motor, an external encoder, or an accelerometer.
These functions require high intelligence and rapid processing in the motion controller to mathematically model and apply the appropriate value to the servo axis control system. With vibration avoidance by filtering, open-loop control is used without external feedback. The resonance frequency mode of the system is modeled, so all that is required is for the motion controller to send position commands to cancel vibration.
This is different from merely doing an S-curve or jerk limiting to the motion profile. This technique can be applied in assembly applications for catheters, needles and other “sharps.” For example: Catheters have a wire drawn from a spool to a precise length, then cut and inserted into the catheter sheath. The wire needs to be kept at a precise, constant tension, an application that can be improved with vibration avoidance techniques.
With active vibration damping, closed-loop control is employed with a position interpolator and sensor feedback to send force or acceleration signals to the motion controller. Then the drive generates the torque position or velocity offset to cancel the vibration and suppress externally induced vibrations. A self-tuning, proportional-integral-derivative (PID) controller using adaptive intelligence, adjusts the drive based on interaction with the machine and the load.
Active vibration damping is highly applicable to most automated, high-speed pharmaceutical and medical product packaging systems. It can also be implemented on large-scale lab automation tools, which may utilize a pick-and-place motion where a large number of samples in liquid form are being dispensed into wells on a microplate.
The plate can move in multiple directions (up, over, down, and more) and because the plate is often uncovered, preventing the samples from sloshing is essential to ensure accurate results in lab experiments and diagnostic procedures. Through vibration damping, it can be possible to improve automated lab device throughput rates, or increase the number of microplate wells, achieving greater efficiency (and target accuracy) without comprising the results.
The two techniques – both vibration avoiding and vibration damping – can also be implemented together, which is a capability offered uniquely by Rexroth.
One way to leverage drive-base motion control efficiency
Because vibration damping requires a motion control system that can adapt quickly to changes in process, material and application, Rexroth developed its Adaptive System technology to operate at the servo drive level, rather than at the control level. Drive-based motion logic and command value processing speeds up event handling and avoids the delays inherent in a control system’s cycle and update times.
Rexroth Adaptive System motion control can handle high-speed inputs and deterministic events with closed-loop response times that are unsurpassed by any other solution. For example, current closure can be accomplished in 62.5 µs (microseconds), speed loop closure in 125 µs, and position closure in 250 µs.
Drive-based firmware also allows more flexibility in implementation. Firmware options are available that provide base performance and high-precision performance. If the packaging machine or production tool generates induced or parasitic vibrations, open-loop vibration avoidance can be implemented as standard on the drive; when encountering non-linear, non-repeating disturbances, closed-loop vibration dampening are easily added.
In addition, development of Adaptive Systems for specific applications is simplified by Rexroth’s Open Core Engineering (OCE) automation software and programming platform that allows personnel unfamiliar with PLC/HMI programming to create and integrate highly customizable automation applications. OCE provides the software tools, function modules and libraries that allow programmers who know C++, Java and other high-level languages to translate applications development into ladder logic for motion control, including Adaptive Systems vibration damping sequences.
The Rexroth Open Core Interface also provides libraries for LabVIEW Software – the popular graphical programming platform – to provide data acquisition on a Rexroth human machine interface, thus eliminating the expense of a separate PC.
The precision machine control and increased throughput offered by drive-based Adaptive Systems – such as vibration damping and anti-slosh – combined with easy-to-use programming tools provide great potential for productivity gains in medical applications.