Multiple sizes of
miniature pumps offer varying degrees of strength and portability.
Unlike piston pumps, miniature diaphragm pumps do not require lubricants in the pump's stroking mechanism. On a similar note, rotary-vane pumps are prone to the vanes wearing and spewing debris in the flow path. By contrast, miniature diaphragm vacuum pumps and compressors provide an oil-free and contaminant-free fluid pathway. This is a critical requirement for many medical device applications (see Table I).
To meet the changing needs of the industry, medical device designers are advancing product technology by specifying fluidic modules that can achieve increased performance capabilities while fitting in ever-smaller envelopes. To lower healthcare costs, industry has pushed for the development of patient medical therapies that are available at home or that are applied with lightweight, portable devices. This dramatic market transition has pushed medical device manufacturers to transform once large and costly systems into portable, cost-efficient ones. The basic goal is to incorporate components that have the same or better performance into a smaller package size while improving reliability and operational life.
The Pump Challenge: Performance and Size Goals
Traditionally, techniques for improving the performance of miniature pressure and vacuum diaphragm pumps included increasing the size and stroke of the diaphragm, increasing the volume of the pump chamber, and increasing the motor size to generate the additional required torque. Improving the performance while shrinking the size of a pump poses several interesting new challenges.
Table I. (click to enlarge) Miniature diaphragm pumps and compressors provide fluid transport,
pressurization, and vacuum draw in many medical device applications.
For example, cramming too many components in a small space has a tendency to substantially increase the operating temperature inside a device's enclosure. Temperatures can sometimes exceed 50°C. Many diaphragm pumps that are in use today should only operate in environments that are under 40°C.
It is also important to decrease maintenance costs and improve system uptime. Therefore, development engineers must incorporate fluidic and pneumatic components that can exceed 20,000 hours of operation. And they must operate under demanding loads, in high-temperature environments, and while taking strict noise levels into account.
In addition, with currently available technology, diaphragms can rip and tear, and motor failures often limit pump life to around 3000 hours. Users accept such limitations as the status quo. In fact, diaphragm failures are so common that getting replacement diaphragm kits from the manufacturer is an accepted practice and cost.
For their new designs, however, device manufacturers often require the pump driving a medical device to last as long as the device without needing repairs or replacement. To improve performance, shrink size, and increase life span, advances in pump technology have included both new materials and new shapes.
New Materials and Shapes
Manufacturers have several ways to improve performance. For example, the choice of pump material can play a role in improving pump life.
To extend diaphragm life under real operating conditions, manufacturers often use an elastomer material that meets the rigorous demands of extended life cycles. Standard ethylene propylene diene monomer (EPDM) elastomers are typically rated up to 40°C and have limited elastic properties to withstand the cyclic stretching required for high-flow-output applications. Because typical operating environments for fluidic modules experience much higher temperatures than 40°C, manufacturers have developed diaphragm materials that withstand 70°C with improved mechanical capabilities.
The dynamic complex modulus, which is referred to as the Tan delta, is the industry standard for determining the flex and stretch of a rubber compound or elastomer. Basically, material testing quantifies the damping modulus versus the elastic modulus. This ratio of the energy dissipated to the energy stored of the material over a wide range of amplitudes, frequencies, and temperatures provides an effective benchmark to qualify an effective elastomer for the demanding requirements placed on the pump's diaphragm.
Standard EPDM materials exhibit a Tan delta of 0.2. This has proven to be unacceptable for current miniature diaphragm pumps that are required to withstand fatigue or failure for longer operating cycles and longer life while performing in higher thermal environments. It has been determined that a Tan delta approaching 0.1 is necessary to meet these challenges.
An advanced EPDM, or AEPDM, has been developed with a Tan delta of approximately 0.1. This elastomer has effectively increased the stroke and surface area potential of diaphragms. Again, the very low Tan delta is indicative of increased flexing durability. Whereas standard EPDM has been found to last less than 5000 hours in typical miniature diaphragm pump applications, AEPDM has been tested to last 20,000 hours or more, depending on the application.
Changing the shape of the diaphragm may also improve vacuum, pressure, and flow performance efficiencies. A typical flat diaphragm is limited by the amount that it can be stretched. High-performance air and gas pumps require an increased pump stroke beyond the stretch limits of a flat diaphragm, which typically sees a stroke up to 0.1 in. or 2.54 mm. High-vacuum or high-flow performance requires the use of either a large, flat diaphragm (which also requires a large pump-head design), or an increased diaphragm surface area.
A shaped diaphragm is molded to the specific pump chamber, which enhances efficiency and maximizes the effective stroke. The ability to engineer an optimized diaphragm-and-chamber configuration maximizes the fluidic output performance. This is useful for devices that need higher flow, vacuum, or pressure performance in a smaller envelope. Shaped diaphragms allow the pump stroke to increase by as much as 80%. As a result, significant increases in output can be achieved by a compact pump.
The motor driving a diaphragm vacuum pump or compressor is an important factor affecting the overall performance and expected operational life. Pump designers can choose brush motors or brushless motors to achieve particular characteristics. However, in some cases, standard models do not meet a manufacturer's requirements. Understanding the benefits and limitations of each motor is critical to choosing a pump for a particular application.
Brush Motors. Dc brush motors are commonly used with diaphragm pressure and vacuum pump applications when operational life is not critical. Iron-core brush motors typically use carbon brushes to conduct the electrical input from the lead wires to the motor's commutator. The constant rubbing of the brushes on the commutator causes the brushes to wear down, much like graphite in a pencil. Brush motors are designed to last from 500 to 6000 hours, depending on the quality of the motor and how it is used.
Motor brushes experience an electrical arcing upon each start-up. Frequent arcing can heat up carbon brushes, causing them to wear out more rapidly. Therefore, brush motors that experience frequent on-off cycles per day wear out more quickly. A top-quality brush motor can be expected to last 3000 hours with frequent on-off cycles. Brush motors used in heavy-duty applications with more-continuous operation can last longer. It must be stated that few applications allow a pump to run continuously. Frequent starts and stops are the norm. Occasional cycling may lead to motor stall because of the buildup of carbon dust between the brush base and commutator. Tapping the outer housing to clear deposits from the brush tips can usually restart the motor. In addition to having a limited life, brush motors can introduce unwanted electrical or radio-frequency interference noise into a system's circuitry.
Brushless Motors. Brushless motors, as the name implies, do not use brushes for commutation. Instead, they are electronically commutated. The stator consists of stacked steel laminations that are axially cut along the inner periphery. Numerous coils are interconnected to form each winding. An even number of magnetic poles is produced from each of these windings that are distributed over the stator periphery. The rotor is a permanent magnet with alternating magnetic poles built in.
Although the operational life of off-the-shelf brushless motors is longer than brush motors, they still do not necessarily meet operational load demands that diaphragm pumps exert on the motor shaft. Brushless motors have a limited bearing life, and their restart capabilities under load are not suited for some medical applications.
Standard brushless motors are typically designed for operational loads in one radial direction—for example, to turn pulleys. For such applications, the bearing cage assembly can afford to have some play (i.e, loose radial or axial motion) because the mechanical loading is sided in one direction.
The goal is smaller components that have the same or better performance with improved reliability.
A miniature diaphragm pump, however, exerts a reciprocating radial load as it drives the pump eccentric to move the diaphragm in and out of the chamber. Depending on the pressure or vacuum load and the play or movement permitted by the bearings, fretting of the bearings supporting the motor shaft will occur. Fretting causes premature bearing wear, increased mechanical noise, and overall motor life degradation.
Enhanced Motors. It is imperative to improve standard motor designs and manufacturing processes to optimize brushless motor technology with regard to performance, reliability, and endurance. For instance, the bearing cage assembly can be designed to accept zero-play tolerances to properly operate under demanding operational loads. Such precision produces a quieter motor than common brushless designs.
Many fluidic system applications require the diaphragm vacuum pump or miniature diaphragm compressor to restart under load. And as designers demand pumps that can fit into smaller packages, smaller motors are required to power the pumps. Hall effect sensors are ideal, but they take up a lot of space. Because of their relatively large size, which prevents them from being located in a small motor envelope, the control boards for Hall effect sensors are typically located outside of the motor.
To reduce the motor envelope, some small brushless motors have incorporated sensorless commutation methods. These sensorless motors, however, are limited in the initial start-up torque they can produce, which hinders their restart capability.
The ideal solution is to create optimized miniature control circuitry for Hall effects sensors that can fit inside the motor enclosure. A compact Hall effect sensor design can achieve high torque to reliably restart under high loads. Optimized control circuitry enables the medical device developer to maintain a small pump or compressor envelope without having to engineer the motor controls.
Brushless dc motors can also be improved by adjusting high-temperature environments, motor efficiencies, and system control. For example, in one design, the commutation circuit operates in a 110°C maximum ambient-temperature environment, allowing the pump to operate in a very broad temperature range. An advanced high-temperature lubricant maintains proper lubrication in bearings at elevated temperatures. In addition to common analog voltage control, three-wire pulse-width modulation controls the motor rotations per minute. A fourth-wire option can also be used to provide a tachometer feedback.
By incorporating advanced elastomers, optimizing the diaphragm geometry, and significantly enhancing brushless dc motors, pump manufacturers can provide device makers with miniature diaphragm pumps that achieve the required performance and life span. One such innovative miniature diaphragm pump, for example, has a free flow of up to 11 L/min while withstanding an operating environment up to 70°C.
Small changes in the components' design can lead to pumps that endure high ambient temperatures and withstand demanding operational loads. In many cases, such pumps can outlast most of the systems that they are integrated into. As a result, device makers can design long-lasting devices and can be confident that the pump component will be easy to integrate and powerful enough for the application. Ultimately, the goal is to enable the device manufacturer to forget about the pump once it is installed.
Dan Schimelman is director of business development at Hargraves Technology Corp. (Mooresville, NC) and can be contacted at [email protected].