Practical techniques for feedback and control of laboratory automation systems

Summary Sample preparation has been the weak link in chemical analysis. The preparation step has been time-consuming, expensive, and a major source of errors. Laboratory robotics has become a popular alternative to preparing samples manually. Many users are getting better analytical results — faster, safer, and at less cost than before. The industry recognizes the importance of reducing human exposure to a wide range of chemicals and biologically active materials, while isolating sensitive procedures from human contamination. Automated sample preparation systems normally run unattended. While increasing productivity, unattended operation creates situations that must be accommodated if the system is to run reliably. The modern robotic system uses both feedback and feedforward techniques to assure reliable operation. Feedforward is used to confirm that the operation is possible. The heart of feedforward control is the system image. The system image consists of a set of data variables for every possible container, or tube, position in the system. The usual variables are container volume and capped status. Tube tare and net weights can also be tracked. As the containers move and volumes change, the stored image is kept updated. In the example of removing a tube from a rack, the computer program can compare the intended operation with a stored record, or image, of the system before the action starts. In this example, it would confirm that the robot's hand is empty and that there is a tube in the target location. Feedback is used to confirm that an operation was performed as expected. Feedback control relies mostly on robotic tactile sensing and position sensing switches in associated mechanisms. It is important for the laboratory robot to be able to sense the force with which it is pushing in all of its axes, not just the grip. For example, after the robot has removed a test tube from a rack, the force exerted by the gripper mechanism can be used to confirm that there is a tube in the fingers and that the operation was successful. Additionally, laboratory balances are used to gravimetrically confirm transfers. Weight confirmation is important with other automated preparation steps such as membrane filtration, solid-phase extraction, and evaporation where the final sample amount may be in question.