Features

In 2006, engineers from the Thermo Fisher Barrington division in Barrington, IL, were faced with a number of challenges to improve their highly successful Masterflex line of peristaltic pumps. These challenges included:

alt
Fig. 1 – Integrated motor/controller system.
  • simplifying the supply chain,
  • improving manufacturing throughput,
  • reducing touch labor cost,
  • improving pump performance at low speed,
  • improving accuracy of dispensing small volumes of fluid, and
  • improving service life of the pump.

The answer to these challenges came in the form of a fully integrated low cost servo motor/controller system manufactured by Specialty Motors (SMI) of Valencia, CA. The system, shown in Figure 1, includes the motor, controller, power supply, and feedback devices in a single compact package. It uses a high resolution 4096 position sensor integrated with a servo controller and a serial communication interface with the pump control panel. The system is extremely flexible and allowed Thermo Fisher to tailor the operation to meet specific customer requirements including: dispensing, constant flow, timed pumping, variable flow rates over time, pressure control, and level control. The integrated design package allowed Thermo Fisher to purchase all the components of the pump motor and control system from a single supplier rather than from three or four.

Pump Operation

Typical pump systems employ velocity control methods that optimize performance and accuracy at the upper end of their speed range but have poor control and accuracy at low operating speeds. To maintain consistent flow rates over the entire speed range, pump manufacturers must use a variety of gear reductions and pump heads to keep the pump motors running near the upper speed ranges. Problems encountered at low pump speeds become even more apparent when dispensing specific amounts of fluid or when varying amounts of fluid are required to be dispensed by the same pump system. Servo control architecture allows the same pump to accurately control fluid transfer over a wide range of flow rates and delivery volumes.

alt
Fig. 2 – Dose error as a function of motor RPM and volume pumped.

In the operation of a peristaltic pump, the transferred fluid is completely contained in a flexible tube fitted inside the pump head. The fluid does not come into contact with the pump or other parts of the fluid transmission system outside the tube. Fluid is moved through the tube by a series of rollers attached to a rotor that compress a section of the flexible tube and squeeze measured amounts of fluid through the tube as they rotate. Loads can vary significantly over relatively short time periods with changing flexibility of the tube, viscosity of the fluid, and temperature. Although the pump applications discussed here are used specifically for laboratory processes, the benefits of improved accuracy of the servo control architecture can be applied to a wide variety of medical uses such as dialysis, infusion, and drug delivery. The built-in feedback capability is also useful for setting safety limits, providing user alerts, and communicating with a user interface control panel.

The traditional non-servo pumping control approach in this application has a significant error in the lower speed ranges that is increased by large torque variations associated with the peristaltic pump design. When dispensing doses, the system must stop and start for each dose dispensed, requiring the system to operate in the low speed range for every dose. This can lead to significant dispensing errors that vary with dose volume and motor running speed for any particular pump head and gear combination. Additional errors may occur with non-servo control as a result of variations in supply voltage, mechanical wear, ambient temperature, motor temperature, fluid viscosity, and a number of other factors. The servo approach eliminates these sources of error because the closed loop servo control reacts to changing conditions by changing power to the motor and maintaining then commanded revolutions per minute (rpm) with a high degree of accuracy.

As conditions change, the integrated servo package senses changes in speed or load and increases or decreases power to maintain the commanded speed, thereby maintaining precise flow control in spite of changing load conditions. This allows precise performance over the entire speed range, unlike pumps with non-servo velocity control. These advantages apply to any precision pumping application and have the same precision over the entire speed range of the system. Rotor position feedback to the control system also prevents cumulative position and dispensing errors from building over time as can happen with typical motor speed controls.

alt
Fig. 3 – Speed/torque curve for a servo controlled system.

Figure 2 compares flow rate errors of typical servo and nonservo drive pumps as a function of motor speed and volume pumped. The data in Figure 2 are based on motor speeds of 50 rpm to 3,000 rpm and a 5:1 gear box, which produces pump head speeds of 10 rpm to 60 rpm.

Data results for the non-servo control method show that dispensing error is dependent on motor speed as well as volume dispensed. Because the error varies with volume pumped and pumping speed, the non-servo pump drive must vary speed as the pumped volume increases in order to minimize error. This error is compounded by the poor low speed control characteristics of non-servo velocity control systems.

« Start Prev 1 2 Next End»