A Look at High-Precision Fluid Dispensing

Fluids all have different density, viscosity, solubility, conductivity, and more. When precise and accurate measurements are required, these varying physical properties can cause problems for pumps and flowmeters. High-precision fluid dispensing uses motion control techniques to overcome these challenges.

 

Figure 1. High precision fluid dispensers require precise and accurate measurements. Image courtesy of Michael Jarmoluk.

 

This article introduces the concept of high-precision fluid dispensing, introduces types of precision pumps used in these systems, and explores motion control's role in and applications for high-precision fluid dispensing 

 

What is High-Precision Fluid Dispensing?

When the amount of liquid to be dispensed is in the range of microliters (μL) with a requirement for 99.xx % accuracy, and standard deviation in the range of nanoliters (nL), most pumps and flowmeters are inadequate to handle the task. High-precision fluid dispensing techniques are employed for such microfluidic applications. Microfluidic applications use relevant pumping techniques for the use case along with a complimentary motion control technique.

 

Types of Precision Pumps

Piston or Syringe Pump

A piston or syringe pump is the most commonly used apparatus for high-precision fluid dispensing. As shown in the figure, the plunger of the syringe is connected to a motor using a lead screw. The motor can be moved in both clockwise and counter-clockwise directions. Fluid can be ejected or drawn into the syringe depending upon the direction of motor rotation. 

 

Figure 2. General schematic of the syringe pump. Image courtesy of Precision Motion Devices. 

 

The maximum amount of fluid that can be dispensed by the syringe pump is equal to the volume of the syringe. Once the fluid is completely ejected, it needs to be refilled to dispense more fluid. If there is a need for continuous flow of the fluid in volumes greater than the volume of a single syringe pump, multiple pumps are used synchronously. 

When one syringe is empty, the other starts dispensing fluid and the first one starts drawing in the fluid. When the second syringe is empty, they switch their roles. This will ensure a smooth continuous flow of fluid. In such a configuration, check valves can be used to manage the direction of flow and to ensure smooth, continuous flow.

 

Diaphragm Pump

The diaphragm pump is a variation of the syringe pump. Here the piston for the mechanisms moves in a reciprocating manner like the pistons in an automobile engine. This displaces the liquid in the chamber. It can be used in conjunction with check valves to ensure flow in one direction.  

 

Figure 3. General schematic of the diaphragm pump. Image courtesy of Precision Motor Control.

 

The diaphragm pump is a variation of the syringe pump. Here the piston for the mechanisms moves in a reciprocating manner like the pistons in an automobile engine. This displaces the liquid in the chamber. It can be used in conjunction with check valves to ensure flow in one direction.  

 

Peristaltic Pump

Another design for the precise displacement of fluids is the peristaltic pump. It is used in instances where the fluid to be dispensed is highly sensitive to contaminants.

 

Figure 4. A look at how peristaltic pumps work. Gif courtesy of Njmcca for Watson-Marlow. [CC BY-SA 3.0]

 

In peristaltic pumps, a roller squeezes liquid in a flexible tube, as shown in the figure above, and displaced the fluid in the direction of movement of the roller. In both syringe pump and diaphragm pump, the plunger comes in contact with the fluid to be displaced and has the potential to contaminate the fluid. This possibility is eliminated in peristaltic pumps. Due to this advantage, it is widely used in medical and chemical applications.

 

Motion Control

All the forms of pumps used for precision fluid dispensing use a servo or stepper motor. The liquid is dispensed by the plunger in the case of syringe pump & diaphragm pumps, and with the roller in the case of peristaltic pumps. The precision that can be achieved by these pumps is directly correlated to the resolution of the motor’s positioning. Higher the positioning resolution of the motor used, higher the precision achieved by the pump. The position control is translated by using encoders, and PID controllers (when servomotors are used).

For movement from one position of the motor to another, a motion profile that minimizes jerk should be chosen. Another reason to choose a motion profile is that a constant rotary velocity of the motor is not directly translated to a constant volume of fluid dispensed. In piston pumps and diaphragm pumps at the extremes of piston motion, the volume of fluid displaced is zero and is maximum in the middle. It doesn’t exhibit a linear pattern at different positions of the piston motion. Without a motion profile, the dispensed liquid will be erratic in output volume and velocity.

As a generic solution, motion profiles like a trapezoidal curve or s-curve (velocity vs time) can be used to reduce the variability in output. When compared to each other, the s-curve velocity profile causes less jerk during operations. As the transition in velocities is also smooths out compared to the trapezoidal curve. 

For specific cases, it is better to use a cam profile to control motion. Motor position and velocity to ensure a smooth constant flow of liquid is calculated during calibration and setting up the motor for a fluid with specific density, viscosity, and other physical characteristics. This information is encoded as a lookup table for the cams. The data from the encoder is checked against the lookup table and the corresponding velocity is engaged by the motor. 

 

Applications for High-Precision Fluid Dispensing

High-precision fluid dispensing has a wide range of applications in medical, chemical, laboratory, and manufacturing scenarios.

Drug infusion, dialysis, and platelet extraction are the most common procedures where high-precision fluid dispensing techniques are widely employed. A mentioned earlier medical equipment like artificial hearts use pumps with motion control enabled. 

Automated liquid handling is becoming popular in laboratory applications too. Biotechnology and various life sciences laboratory activities rely on precision fluid dispensing. Processes like sample preparation, dilution, and transfer for DNA sequencing, gene analysis, cell sorting use precision liquid handling for highly accurate and precise tests and results.

In manufacturing too fluid dispensing techniques are used. Adhesives to be applied during various manufacturing procedures are handled by such techniques to ensure precise application. Application of coolants and solder paste for electronics assembly and manufacturing can also be done with precision fluid dispensing. Conductive ink dispensed with high accuracy can be used to draw customs circuits very fast.