Motion Control Requirements for Medical Instruments
- Friday, 01 April 2011
Medical equipment motion control runs the gamut from electric wheelchair motion to heart assist pumps. This article will focus on the segment consisting of medical laboratory instruments. Even in this segment, motion control ranges from precision liquid handling and dispensing, to sample- handling robotics and automated sample storage and retrieval systems. We’ll delve into precision liquid handling and dispensing and related functions, and the interaction between the mechanical system and the motion control system. High-pole-count permanent magnet AC servo motors can simplify system design and improve system performance and reliability when mated with control systems capable of bringing out their full potential.
Common to many liquid transfer systems is the need for repeatable operation, minimal down time, and robust fault detection. Many of the motions also require “gentle” motion profiles. Harsh, jerky motion profiles can generate air bubbles in both samples and reagents, or worse, generate unwanted aerosols from patient samples. The SilverDust™ and SilverSterling™ motion control systems from QuickSilver Controls incorporate advanced control techniques to improve motion performance through added damping techniques, including synthetic inertial dampers and motor driver impedance control.
Precise liquid delivery of discrete aliquots (portions) is commonly done using either the forward sampling or the reverse sampling methods. In the case of forward sampling, the pump first aspirates air, allowing any backlash in the piston mechanics as well as the associated seals to be taken up before the critical sampling motions occur. The tip is immersed in the liquid and the net sample volume is aspirated. The tip is removed from the source liquid. An optional extension to the forward sampling method aspirates a small volume of air following the sample to isolate the liquid at the probe tip to allow the sampling probe/tip to be washed without diluting the contained liquid. The tip is then moved to the dispense container, either against a wall or under the liquid to avoid remaining drops on the tip after dispensing. The pump then dispenses a slightly larger volume than was aspirated to make sure that all of the sampled volume is delivered. This method is most useful for liquids that do not adhere to the sample tip/probe as any adhered liquid that remains in the tip reduces the delivered volume.
The volume captured in the forward method is related to the piston motion between immersion into the source container and the removal of the tip. The motion of the sampling piston must be smooth to avoid pressure surges or overshoot that will change the aspirated volume. A consistent meniscus is needed to make the aspirated volume consistent, which will not occur if the liquid flow motion is not well controlled; ringing or overshoot are to be avoided. Moving the piston in a unidirectional manner allows the backlash in the system to be taken up while aspirating air. If the direction does not reverse, the backlash of the system does not enter into additional motions in the same direction. The critical volume determining motions should thus be in the same direction as the previous motion. The presence of overshoot or ringing disrupts this unidirectional motion sequence, reducing the accuracy of the system. Of note, the backlash in the volumetric system is not just the position of the piston, but includes any changes in the shape or position of the seals as well.
Reverse Liquid Sampling
Reverse sampling is more useful for liquids that wet the sample tip and leave a residue upon dispensing. To overcome the effects of the retained liquid, an excess of sample is aspirated. According to preference and or contamination considerations, the piston can be reversed to take up backlash and some surface tension effects while still in the source container. The tip is then moved to the dispense location, either under the liquid surface or against a wall to prevent a drop of the dispensed liquid from adhering to the tip. The volume to be delivered is then pumped to the delivery container, with some excess liquid left remaining within the tip.
The dispense phase must be carefully controlled. The dispense motion again must be unidirectional. The rate of sample delivery must be shaped to avoid sudden pressure change. The liquid flow must be smooth and allowed to stop gently. It is easy to stop a pump cylinder rapidly, but a column of liquid will not respond as quickly, with an excess of liquid being delivered with some of the delivery container liquid picked up into the tip on the rebound if the pump stops too quickly.
Careful control of the probe motion is also needed to achieve high accuracy, especially for smaller samples. An example of probe motion that can affect delivery is moving the probe down into a delivery receptacle. If the deceleration of the probe is too abrupt, the liquid column can continue to move while the probe is coming to a stop, causing some of the aliquot to exit the probe before any motion of the pump. This can change the amount of liquid delivered in a reverse pipetting delivery. In the case of a forward pipetting system, lifting the probe following aspiration is also a critical motion, as this could reduce the captured volume if some of the metered liquid stays behind as the probe lifts too suddenly.