High Accuracy Fluid Delivery
Posted on October 22, 2024 Instrumentation & Equipment Design
Previously, we discussed how syringe pumps operate with microstepping stepper motors and the resulting large flow rate inconsistencies. How much this matters depends on the application; in an intravenous application, the time-average being consistent on a minute-to-minute basis is good enough. In an inline mixer with a one-second residence time, flow variations must have a period much less than a second.
Coanda can design and build fluid delivery systems that combine exact flow rates with precise volume deliveries rates at conditions not obtainable through commercial systems. We’ll explore the high pressure and high flow rate regime in a future post, but here we’ll focus on delivering consistent flow in the μL/min range.
While the most visually obvious departure from constant flow rate on standard syringe pumps comes from uneven microstepping, even the size of the ideal microstep is prohibitively large. Further, stick-slip motion allowed by the softness of the toothed belt drive make just replacing the stepper motor insufficient. Overcoming these limitations requires a full redesign, as opposed to retrofitting some components.
Using a servo motor with encoder feedback allows precise positioning. The stepper motor has 200 real steps per revolution, and with 128 microsteps per step, a total theoretical resolution of 25600 microsteps per revolution, but an accuracy of only about 1/400th of a turn. The servo Coanda uses has a true positioning accuracy of 1/524336ths of a turn.
Following the servo, a gear train—finishing with hypoid gears for maximal smoothness—drives a ball screw. Contact between the ball screw and ball nut is through recirculating, pre-loaded ball bearings, removing the backlash inherent with a conventional screw. Lastly, the syringe is coaxial with the ball screw, making the coupling rigid enough to eliminate stick-slip.
Even with a larger diameter syringe and less than 1/100th of the flow rate where the commercially available syringe pumps fail, Coanda’s syringe pump delivers uniform flow.