New liquid dispensing pump technology that uses a solenoid rather than a dc motor precisely delivers fluids up to 20 times per second for accurate metering applications. The design of the pump also offers high reliability, normally an issue with solenoid-operated pumps and valves, and an operational lifetime of up to 1 billion cycles.
"Our pumps typically use a standard dc, brushless dc or ac motor to push the diaphragm up and down," says Stephan Kaufmann, head of development and design at KNF Flodos AG. A pump controlled by a dc motor or brushless motor can work very well in continuous applications, but to dispense a very accurate amount of liquid, Kaufmann says it is difficult to control and precise metering solutions are expensive.
Even though KNF has more than 20 years experience with liquid diaphragm pumps, this is the company's first solenoid pump. Solenoid pumps and valves are typically limited in lifetime, and the limiting part of the design is usually the armature, check valves or diaphragm.
"Experience with existing designs has shown that, after 10 million cycles, the armatures are falling to pieces and the valves are not working anymore," says Michael Davies, application engineer for KNF. "This pump is designed for a billion cycles, or 100 times what is typical in other products. Because of our experience with a variety of motor-driven pumps, we are used to designing for high frequencies and long lifetimes."
The key to achieving long life for the product was finding a surface treatment and thin layer of polymer to reduce friction and wear. Special coatings on the armature reduce friction when the armature is going up and down. If every shot of the pump is 10 micro liters, the distance the armature moves is only 2/10 of a millimeter and the armature must be guided very accurately to achieve reliable operation. The coating and the way the armature is pulled down by the solenoid and pushed up by the spring is critical to the accuracy and lifetime of the pump.
Operation of the FMM20 pump is simple. With each electrical impulse, the solenoid moves the diaphragm up and down once to deliver a calibrated shot of liquid. "Because the solenoid is very controllable, you can accurately deliver one stroke versus utilizing a rotating dc motor," says Kaufmann. "We are sending shots of liquid out at 20 times per second, compared to existing designs that typically only support a frequency of 2 Hz."
In dosing applications, higher speeds and frequencies can create operational flexibility. With a priming function, for example, the faster you can pump the air out of the system and get it primed with liquid the better. Because the pump can deliver up to 20 shots per second, it creates both a larger range of flow and more accurate dispensing of fluids.
"With one product, it is possible to serve a wide range of applications in terms of flow at 20 Hz while precisely dispensing exact volumes at low frequency," says Kaufmann. "Some customers are designing it into systems as a dispensing pump, where they utilize 10 shots to achieve a specific volume of liquid. Others run the pump as a continuous metering application."
A unique advantage of the product is the ability to easily make stroke adjustments in the field. By turning an Allen key at the bottom of the pump, the user can increase or reduce the stroke length which, in turn, adjusts the amount of liquid dispensed per shot. Many factors influence the amount of liquid a pump dispenses including the vacuum, suction height, pressure in the system and viscosity of the liquid.
A dosing pump, by definition, needs to be calibrated to achieve the desired flow rate. Applications can have hard or soft tubing, a large diameter or thin tubing, no pressure or a lot of pressure, so every application has a unique set of system parameters that affect flow rate.
"With the solenoid-operated pump, there are two different factors that can be adjusted," says Kaufmann. "The ability to change the stroke length, along with the length and frequency of the electrical impulses, creates new dimensions that affect the pump's characteristics, performance and provide better overall control."
KNF adjusts the pump at the factory so every shot delivers 20 micro liters, but the pump is calibrated using standard tubing and test equipment. Kaufmann says the customer might have a liquid that has a higher viscosity, and get less flow from the unit. But changing to a longer stroke compensates for the application differences.
One challenge during the project was the complete head redesign. The initial design actually used the head of a transfer pump that uses a standard dc motor. The design team thought that the special anchor valves used in this pump worked well at low speed, as the flow curve was very linear. That led to the idea of using a solenoid instead of a motor.
"The precision of the product is required to achieve both the accuracy and lifetime requirements," Kaufmann says. "Using the solenoid versus the dc motor complicated the design, but in the final result is a more sophisticated control capability."
During pump operation, an electrical impulse energizes the solenoid coil and creates a magnetic field, which pulls the armature and diaphragm down and compresses the spring. When the electrical impulse is turned off, the spring pushes the armature and diaphragm upward to create the pumping action. On the downward stroke, the vacuum created pulls air or liquid into the pump. As the spring pushes the diaphragm up, the pressure created opens the outlet valve, dispensing the liquid. If the pump is off, the spring pushes the diaphragm up into the pump head, sealing the pump, and eliminating the need for a separate return check valve. View full size