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By David Bach
BST-Bach Solutions & Technology, LLC
Accurate dispensing affects a great deal of drug development and delivery. For instance, leading-edge pharmaceuticals, bio-tech “designer molecules,” and high-potency compounds are tremendously expensive and come with difficult safety concerns that require accurate dispensing and aspirations. Many of these compounds are composed of proteins or synthetic molecular chains that are extremely fragile and highly susceptible to shear. In addition, setup time must be reduced, and maintaining the product within a dispensing tube must meet or exceed safety and contamination concerns. For these reasons, peristaltic pumps have found wide application.
Hospitals use peristaltic pumps to circulate blood during bypass surgery through the pump and as a critical part in heart-lung machines because the pump’s design does not cause significant hemolysis – the rupture or destruction of red blood cells. The design uses a compressible polymer tube that holds the fluid that requires dispensing.
The arrangement allows pumping aggressive chemicals, slurries with a high solid content, and other materials when it is critical to isolate the fluid from the environment. However, the peristaltic pump suffers from pulsations because its rollers move on and off a pressure shoe as the tube is compressed.
The pulsations plotted in the graph 3 Roller Dispense (Figure 3) can be reduced by adding more rollers to a peristaltic pump…But there is a better idea.
If the peristaltic pump can start each dispense with one roller in the same starting position, the amount of fluid-dispensing variance can be greatly reduced. If the pump has three rollers and the starting position is repeated every 180°, the graph Half Revolution Indexes – 3 Roller (Figure 4) shows the two positions representing half and full revolutions.
A solution for fluid accuracy
The Liberty MDrive integrated motor solution includes signal inputs and outputs for roller positioning. The design allows dispense volumes made from multiple revolutions plus some fraction of a revolution. Roller positioning takes into consideration the fraction of a revolution and ignores the complete revolutions. External valves are used so the peristaltic pump can dispense fluid and then control the starting position of the next peristaltic roller without dispensing.
Connecting a Liberty MDrive, as shown in the electrical wiring diagram below, lets it dispense and then control the valves so the rollers are positioned to the same starting point for each dispense cycle. This allows dispensing a precise and repeatable quantity.
When the number of peristaltic rollers does not divide evenly into the number of microstepping motor counts, a rounding off error occurs. For example: a three-roller system will not divide without a fraction into 51,200 microsteps per revolution. The number of steps between rollers would be 17,066.6667 steps. The motors will only recognize 17,066 steps for each of two rollers and need to add 2 steps to the final roller making it 17,068. Each dispense must end with the total movement of rollers equaling 51,200 steps per revolution. Each move during the dispense cycle can be verified by using the encoder within the Liberty MDrive. Set to MS = 180, it will use 36000 steps per revolution for even roller divisions.
Roller positioning in peristaltic pumps
System (A) in Figure 6 describes where an external pinch valve is present as a normally open dispense and normally closed position valve. One of the rollers is positioned relative to the encoder Z pulse, through the pump dispense is selected and fluid would then be dispensed. A drip retention is used to bring the fluid back into the tube so that the closing of the dispense valve does not cause a drip. The dispensing valve is closed, and the position valve opened. The next roller is moved into the same position as previously used for dispensing. This will result in a small amount of fluid to waste. After the first roller position move, the next roller can be positioned either in a positive or negative direction minimizing fluid waste. Using the various directions for roller positioning minimizes fluid waste.
System (B) represents a two tube peristaltic system where the rollers are offset and the two tubes combined. A six roller system effectively becomes a 12 roller system, minimizing pulsations. Both of the peristaltic tubes are combined using an “x” connector where one leg becomes the waste tube. The dispense valve must be after the dispense tube “x”. The roller positioning takes place with the dispense valve closed and the position valve open. The pressure shoes could be different, so it is recommended that one tube and shoe combination be used for roller positioning.
System (C) is used to aspirate fluid into a pipette tip, complete a drip retention, and move to the same roller position using an air volume. Since the tube contains filtered air and the tip contains the fluid, a roller is positioned when the fluid is dispensed. It is recommended that the system blow out the fluid, the velocity controlled and adequate air used for roller positioning.
4 roller configuration
A four roller Watson-Marlow 313 pump is used with a NEMA23 Liberty MDrive integrated smart motor and two Bio-Chem pinch valves. The on and off switch allows the system to be in a standby configuration. The integrated motor drive is coupled to the pump using a Helical coupling, or equivalent, so that reverses do not adversely affect performance while the mounting block between the motor and pump controls any heat transfer.
2 roller configuration
This two roller configuration uses a Watson-Marlow 520 pump. The plots show an embodiment of the system in which a NEMA34 Liberty MDrive integrated motor drive is used. A small tube was used along with valves above and a similar coupling.
BST offers roller positioning for any peristaltic-pump system that needs greater dispensing accuracy. The use of peristaltic pump systems, pinch valves, and Liberty MDrive integrated motors for motion control lets the BST approach solve accuracy problems and opening up new peristaltic opportunities.
BST has filed a United States patent for the use of roller positioning. The Liberty MDrive systems can also be configured for use as optical or proximity devices for correct roller position control.