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:

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.

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.

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.

Servo Control Systems

The servo control system, on the other hand, has essentially zero error over the entire range of rpm and volume pumped. This is because the error for the servo pump is a function of position. Position feedback and servo control keep position error essentially at zero over time and greatly improve precision and control over a wide range of fluid transfer tasks from high rpm pumping to low rpm dispensing. The servo control algorithm controls motor shaft and pump head rotational position within a fraction of a degree over the full speed range for the entire time the pump motor is running in a speed range from 1 rpm to 3,000 rpm.

Precise low speed control characteristics of servo control systems make dispensing functions more accurate and easier to implement. Dispensing entails metering a precise amount of a new fluid into the pumping process. A non-servo pump drive controls dispensed volumes by commanding the pump to run at a fixed speed for a fixed period of time. Achieving high accuracy requires calculation of a velocity/time profile with adjustments made for starting, acceleration, deceleration, and stopping. The errors associated with starting and stopping are variable and increase significantly as pumping time decreases. With the servo system, no calculations or corrections for acceleration and deceleration are necessary. Because the volume pumped is directly proportional to pump head rotational position, the drive simply commands a specific rotation that directly controls dispensed volume. This allows the pump to accurately control dispensed volumes as small as the volume of a teardrop.

In addition to lower cumulative pumping error and more accurate dosing, the servo system also maintains higher accuracy over a wide speed range. As shown in Figure 3, the servo control system can maintain motor speeds from 3,600 rpm down to 0 with full torque and pump head control. This means pump functions can be accomplished at the speed of a clock’s minute hand with no degradation in accuracy or control. It also allows the pump manufacturer to use lower gear ratios for low speed control which expands the high speed end of the operating range and may also reduce gear box complexity and cost. For example, a 5:1 gear box could be used instead of a 15:1 or 30:1. In some applications, it could eliminate the need for a gear box altogether. The system is extremely flexible and allows Thermo Fisher Barrington to tailor performance to meet specific customer requirements including: dispensing, constant flow, timed pumping, variable flow rates over time, pressure control, and level control. For new product development applications, it also has the advantage of common responsibility for integrating the motor, controller, and communication with the user interface control, which reduces development time, complexity, and cost. Feedback features of the servo control architecture also facilitate programming of user alerts, setting safety limits and interface with user-friendly control panels. This is particularly valuable for in-home applications, such as pain management.

Some other advantages of this design approach over systems with individual drive and control components include:

  • reduced parts count with less wiring and fewer interconnect devices;
  • smaller, more compact package;
  • lower weight;
  • less assembly time and cost, fewer manual operations;
  • simplified component testing; and
  • lower RFI and EMI noise.

While this discussion has focused on pumping applications, this technology can also improve control functions in other applications such as dialysis equipment, enteral feeding pumps, infusion pumps, and irrigation pumps.

This article was written by Bradley W. Spahr, President and Chief Executive Officer of Specialty Motors, Inc., Valencia, CA. For more information, Click Here