Precision non contact dispensing of small liquid volumes requires using a 500µL or larger volume syringe barrel or liquid head, although 1000µL typically works best. These larger volumes allow for the creation of enough fluid velocity behind the dispense if the acceleration, running speed and deceleration of the motor is rapid enough to clear the droplet with a high level of precision.
The resolution of a stepper motor-driven pump will limit its ability to accurately and precisely deliver the correct amount of liquid at low dispense volumes. Compounding this problem, stepper motors are not capable of high enough acceleration; running speed and deceleration to reliably eject the droplet, leading to poor fluidic dispense precision.
Stepper motors are limited in their resolution, which is measured in how many steps the syringe or plunger length can be divided into. Most are limited to 30,000 steps or fewer full stroke. Stepper motor-driven pumps will often have stall condition issues and motor torque limitations that result in high resolution set points combined with high speeds. The size of existing syringe and piston stepper motor pumps requires that they be mounted deep in the instrument, necessitating long transfer lines to transport fluid to the probe. These lines, averaging nearly a meter in length, rob the pumps of fluid metering performance, increase wasted liquid and decrease critical throughput time.
Servo motors are capable of much higher acceleration, running speeds and deceleration than stepper motor-driven pumps. This makes it possible to obtain sufficient flow velocity behind the dispense to reliably eject droplets, while still having enough resolution to precisely and accurately deliver the right amount of fluid to the desired site. Additionally, the servo motor-driven pump design is small and light enough to be mounted on a motion system right at the point of dispense, thereby completely eliminating the need for a transfer line and associated drawbacks.
Article contributed by Donald S. McNeil, senior product manager, Parker Hannifin Corporation, Precision Fluidics Division
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