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Advanced Servo Control Helps Pump Accurately Perform Under Pressure

Thu, 10/09/2008 - 6:05am
The Project: Resolve the start/stop action of a hybrid piston syringe pump to achieve a steady and regular flow stream.
The Solution: Use a servo controller and calibration table to dynamically vary the pump velocity as a function of requested flow rate and pump dynamics.

By Don Labriola and John Beard

Don Labriola is the president of QuickSilver Controls Inc., a company that specializes in servo motion control systems. John Beard is the president of Car-May, a producer of precision fluid metering products. Labriola can be reached at 909-447-7417 or don_labriola@quicksilvercontrols.com, and Beard can be reached at 970-330-4575 or jbeard@car-may.com.



Figure 1: Car-May Encynova continuous flow pump.
Positive displacement pumps have the advantages of accurate delivery with high-pressure capability while handling a wide range of materials. However, the standard reciprocating piston or syringe pump alternates between fill cycles and dispense cycles, resulting in a start/stop of the flow stream or 100% pulsation. The Car-May Encynova pump is a hybrid piston syringe pump with four cylinders. It has the advantage of continuous flow with greatly reduced pulsation, produced by using four cylinders coupled to a single rotating crankshaft.

Figure 1 shows a single rotating input shaft actuating the four cylinders, as well as the four sets of sliding ceramic plates. As the crank rotates, at least one cylinder is being filled, and at least one cylinder is dispensing at any time, with two doing each function for the majority of the revolution. This design produces a continuous output flow from the pump. The resulting flow, however, still has a slight pulsation of ±15% when operated at a constant speed of crankshaft rotation.

Figure 2 shows the idealized flow rate from a four-cylinder pump operating at constant angular velocity. An actual pump also has cylinder-to-cylinder variations. The pump displacement is a repeatable function of crank angle and is individually calibrated for each pump.

The SilverDust controller from QuickSilver Controls Inc. uses this calibration table, loaded into onboard non-volatile memory, to dynamically vary the pump velocity as a function of requested flow rate and
Figure 2: Pump cylinder contributions to output.
pump dynamics. The resulting NovaFlow system produces a nearly constant flow rate (‹1% of total flow), even into varying pressure heads. The system may also be operated in volumetric mode to deliver requested volumes at requested delivery rates, with the volumes not limited by cylinder sizes. The intelligent servo controller uses these same calibration tables to determine starting and ending positions needed to dispense the desired volume. The use of a continuous operating pump allows increased throughput in systems by eliminating the aspirate time normally required by piston or syringe pumps. Precision is improved by the valve-less design which uses ceramics to channel the flow and prevent the backflow associated with other pump technologies, as well as reducing undesired pulsations in flow rate. The pump is constructed of ceramic, Teflon, glass, and perfluoroelastomer O-rings which provide compatibility with a wide variety of fluids used in diagnostics and pharmaceutical processing.

The pump crank is directly driven from the motor, taking advantage of the high torque capability of the high pole-count motors employed in this application. The elimination of a gear-head reduces size, cost, and maintenance of the system. The servo controller also allows extremely slow rotation speeds at full torque producing a turndown ratio of 1:1,200,000 which is un-equaled by other pumps.

This high torque capability is produced by the use of high pole-count servo motors. A synchronous motor mechanically advances one pole pair for each electrical cycle of
Figure 3: Pump volume constant velocity versus corrected.
current applied to its windings—for a two-pole motor, a full revolution; for a four-pole motor, a half revolution; for a 100-pole motor, 7.2 degrees. Given the same magnetic path characteristics within the motor, the torque of the motor increases with the number of poles, just as the speed decreases. This effect is commonly referred to as magnetic gearing. In comparing a four-pole synchronous motor to a 100-pole synchronous motor, everything else being the same, the 100-pole motor will produce 25 times the torque at one twenty-fifth the speed.

The 100-pole servo motors used by QuickSilver are two-phase AC permanent magnet motors, commonly deployed as open-loop steppers. The SilverDust operates these motors as AC servo motors, providing the simplicity and high reliability of the stepper with the smooth and precise control of a servo system. The low speed resonance problems often associated with open loop step-motors are also eliminated (see sidebar). With closed loop control, these motors may be operated up to their full torque, whereas in open loop, they would normally be used at no more than 30% to 50% of their capability to prevent loss of synchronization. This closed loop control methodology both increases performance and minimizes motor heating.

Figure 4: Pump volumetric repeatability.
The SilverDust controller also provides Anti-Hunt capability to prevent motor dither when stopped. These small constant re-adjustments common to servo systems would otherwise cause damage to the sealing surfaces of the pump. Anti-Hunt is performed by selectively switching to open loop operation when stopped and the error is sufficiently low, operating the motor again as a stepper. Larger errors cause the system to switch back to normal closed loop operation so position is never lost.

The unique capabilities of the SilverDust controller support the unique requirements of the Car-May Encynova pump to provide precise flow and delivery in a positive displacement pump system.


Online
For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com.




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