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 firstname.lastname@example.org, and Beard can be reached at 970-330-4575 or email@example.com.
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
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
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.
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.
OnlineFor additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com.