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Controlling a micro dosing pump

APPLICATION NOTE

Introduction

This application note shows how the Pulsed Flow Sensor (PFS) behaves if air bubbles are trapped in the liquid, or if only air passes through the sensor. An ideal liquid flow sensor would only measure liquid and neglect air/gases completely. The following experiments show the behavior of the PFS in this context.

Used Material

  • Pulsed Flow Sensor V3-B1

  • PFS controller with user Interface

  • Nordson EFD55ml pressurized cartridge with piston

  • Syringe Pump

 

Sensor specifications

  • Flowrate: 3 – 30 ml/min

  • Response time: 1 ms

  • Calibration media: water

  • Sensor principle: differential-pressure flow sensor

controlling 1

Experiment setup

The Pulsed Flow Sensor was connected to the aspiration side of the pump. In a vertical setup, single droplets were dispensed at a frequency of 1Hz. The tube between sensor and pump was kept as short as possible to prevent damping.

Stroke and droplet Volume

The stroke of the FMM-20 pump is varied by turning an adjustment screw from 0 to 100°, while in figure 1, 100° is the minimal stroke and 0 the full stroke height of the pumps plunger.

Measurements

For 35 different plunger strokes, the droplet volumes were recorded by the Pulsed Flow Sensor at a sampling rate of 20 kHz. The data shows:

  • Small plunger strokes show an earlier raise in flow than big plunger strokes.

  • Big plunger strokes show a plateau and an acceleration before closing.

  • The closing event at 50 ms and the post-pulse oscillation is very reproducible.

Implementing a control circuit

With a PID control loop the droplet volume was guided to follow a Target volume of 0, 12 and 7 microliters per shot. The flow sensor signal was used as the control variable. As a second reference, a balance measured the droplet volumes.