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Driving Pump Efficiency
Pump can infuse for eight to 10 hours on a single change of batteries
Al Presher, Contributing Editor -- Design News, October 19, 2008
The human body can contain as much as 85 percent water, so it's not surprising one of the first lines of treatment for trauma patients like wounded soldiers is controlled by fluid replacement. The problem is conventional infusion pumps typically deliver small amounts of fluids at high accuracies, 100 mL over the course of an hour. When the U.S. Army went looking for a portable field unit that could run at 100 mL/min, they turned to a high-efficiency pump and efficient, high-torque motors to provide an answer.
The Army wanted a pump that was battery-operated and provided no electronic signature and minimal noise to inhibit detection. The result was the Power Infuser developed by Zoll Medical, a 5 x 4 x 2.3-inch unit that weighs just 0.6 lb. In addition to pumping 100 mL/min, the unit can also administer 250 mL in a single burst and stop until commanded otherwise.
The design consists of a modular pump, a disposable cartridge and six AAA batteries for a total of 9V. The goal was for a change of batteries to infuse one patient, but the finished device can operate for eight to 10 hours at full speed, infusing multiple patients on a single battery pack.
The design consists of an aluminum frame topped by a yoke with two flanges on the underside in a fork configuration. The cartridge has two transparent plastic tubes, each with passive check valves causing unidirectional flow as the tubes are compressed. The fork straddles a rotating eccentric cam that forces it to rock back-and-forth. A ridge compressing the tube of the cartridge above it expels the fluid.
To maximize battery life, each time a tube is compressed, the unit stores potential energy. Once the liquid is expelled, the potential energy converts to kinetic energy as the expansion of the empty tube boosts the action of rocking the yoke to the other side so that it can begin to compress the other tube.
“It's incredibly efficient,” says Michael Loughnane, now president of Instech Labs. and one of the designers of the unit. “If you were to just use one tube, you'd get a certain flow at a certain power. If you add the second tube, the power requirements only go up slightly but the flow doubles. We use this balanced pumping action to get the best efficiency out of the mechanism.”
To power the yoke, the group needed a gearmotor to compress the tubes, but in a small enough package. “Using a mathematical model, we homed in on the best gear ratio, the best speed and then picked the appropriate motor and gear ratios to meet those criteria,” says Loughnane. They chose a 13-mm-diameter, 31-mm-long servo motor from MicroMo Electronics fitted with a 15-mm-diameter gearhead for a 76:1 reduction ratio. The group modified the gearmotor by adding an external bearing to the distal end of the shaft to provide additional support.
For control, since the device could not have an electronic signature, it ruled out use of a conventional microprocessor. Instead, the group opted for discrete logic and analog control based on back-EMF feedback. The selector switch that sets the pump rate establishes a voltage that the amplified back-EMF of the motor must match. The analog circuit varies the drive signal to the motor depending on the results, providing closed-loop speed control.
To save energy, the speed controller also only drives the motor forward. It does not attempt to hold the speed of the motor steady throughout the cycle. So if the motor is going a bit faster after peak compression as a result of stored potential energy, the circuitry allows it to free wheel, electrically speaking.
