A wide range of motor technologies are available that include DC brush, DC coreless brush, DC slotted brushless and DC slotless brushless.
The Hargraves 10 mm Magnum solenoid valves, seen here manifold mounted, can be configured with a large orifice (0.075 in/1.9 mm) providing the pump with a less-restrictive flow path.

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Tiny pumps drive portable medical devices


Dan Schimelman

The performance and cost effectiveness of many portable medical devices can be enhanced by tailoring miniature diaphragm pumps and solenoid valves to meet system requirements. Dan Schimelman of Hargraves Technology examines the issues together with recent advances in fluidic components.

The drive to reduce hospital costs is increasing the administration of medical therapies at home or even on the go. This demand has launched new generations of portable devices. Most of these systems incorporate modules consisting of miniature diaphragm pumps and solenoid valves, and these fluidic components therefore need to be tailored according to the system criteria that best achieve the market demand objectives. Medical device developers benefit from understanding how these components can be optimized to best meet their specific application requirements.

Medical device technology drivers

Both employers and insurers have exerted pressure to contain healthcare costs by shifting hospital care, with its skyrocketing costs, to less costly outpatient procedures. There is an increase in the number of procedures that are being performed as outpatient and ambulatory procedures that were once performed only on an inpatient basis. Advancements in medical technology and the development of non-invasive and minimally invasive surgical procedures have contributed to growth in outpatient and ambulatory care. In many cases, surgeries once requiring several days of postoperative observation and care have become same-day procedures.

The trend for medical practitioners to reduce patients' hospital stays and continue treatments at home has required medical device companies to engineer systems to be more portable, quieter and more cost effective.

The high-technology diagnostic and therapeutic services now available in the home include transfusion therapy, dialysis, oxygen therapy, mechanical ventilation, compression therapy and wound therapy. This changing healthcare landscape is driving explosive growth in the medical device field. The market will continue to accelerate as demographics and market drivers increase their pressure for new product offerings.

To support speed-to-market for innovative products, Hargraves Technology Corp of Mooresville, NC, USA, a manufacturer of miniature, high-efficiency diaphragm pumps and solenoid valves, has been working closely, and earlier in the development process, with medical device companies to identify key system requirements. Miniature diaphragm pumps and solenoid valves have become popular with fluidic system engineers to provide the pressure and vacuum transport of air and gas in a cost-efficient manner. By understanding how key components such as these can be tailored for optimum system performance, medical device developers can speed their time to market by identifying earlier exactly what they need.

Pump optimization criteria

The marketplace for portable medical devices is becoming more segmented and competitive, with each segment having its own distinct requirements for success to meet specific end-user demands. The first step in the development process should be to gain clear definition from the marketing team of the criteria for a successful product release. Then, it is important to prioritize these capabilities and select the components and their respective performance specifications needed to best meet the ranked criteria. Since there are usually trade-offs, this will help to ensure that major product objectives are met and that development timelines don't suffer from subsequent changes to the project scope.

For portable medical device developers, the following criteria and how they are prioritized can make a significant difference to the specific components that should be selected.

Size and weight

For medical devices to be truly portable, they need to be much more compact and lighter than their desk-mounted predecessors. The maximum envelope that the fluidic module can fit into needs to be established. This will have an impact on determining the maximum size of the pump and valves that will govern fluidic performance. Should space be very limited, smaller pumps and fewer valves will be needed, which will either limit performance or increase noise if smaller pumps are run at maximum motor speeds to achieve similar performance to a larger pump that is run more slowly.

Performance

The required flow at pressure/vacuum at the key operational points of the medical therapy and how they need to be controlled (analog controlled, PWM controlled, PWM with tachometer output, etc.) should be determined in order to set baselines for key components. Increasing the performance while shrinking the size of the miniature solenoid valves and diaphragm air pumps has posed several interesting challenges. Advanced designs, materials and motor technologies have resulted in the launch of innovative pumps and valves offering greater performance in a smaller package.

 

Noise

Medical therapies taking place next to a patient while sleeping or while they are in public will need to take noise into account. Diaphragm pump manufacturers can minimize noise by reducing the stroke, optimizing diaphragm shape and durometer, and lowering chamber efficiencies. Depending on the level of these actions, trade-offs may impact pump efficiency and fluidic performance. Another tactic noted earlier is to utilize a larger capacity (e.g. a dual head pump versus a single head pump) running at lower speeds. The performance stays the same but the noise level drops significantly. The negative trade-off, of course, is a larger pump envelope.

Efficiency

If it is a battery-powered device, what is the desired target operational life from the battery? Would there be a market advantage if the portable medical device ran longer from a battery than its competition? As will be discussed later, the proper selection of motor technology for the diaphragm pump will contribute greatly to the efficiency of the system. In addition, properly matching the orifice size of the solenoid valves will ensure that they are not acting as a restriction thus forcing the diaphragm pump or miniature compressor to work harder.

Operational life

Will this device be disposable or requiring the fluidic components to run intermittently, or will it be required to have proven high reliability under demanding cyclic operation that can exceed 10 000 hours of operational life? Operational life requirements are affected by selection of the motor technology, diaphragm elastomer, and fluidic loads and cycling, in addition to the maximum temperature environment that the components will be exposed to.

Cost

The priority placed on cost among the decision criteria for fluidic components will greatly impact the ability to maximize the advantage of each of the preceding factors. It should be noted that too strong an emphasis on cutting costs of the diaphragm pumps and solenoid valves could actually increase the overall costs and reduce the marketability of the device. For example, the selection of advanced, high-efficiency and high-reliability motor technology and solenoid valves will significantly decrease power consumption; sometimes by half. This can result in battery requirements being greatly reduced – giving development engineers the flexibility to design an even lighter and more-compact device or extending battery capacity.

A number of technology drivers greatly affect the above decision criteria for miniature air diaphragm pumps and solenoid valves; these are discussed in the following sections.

DC motor selection

The motor of the miniature diaphragm vacuum pump or compressor is probably the biggest driver affecting the overall performance, efficiency, expected operational life and cost. The motor is the highest cost component of a diaphragm pump and is therefore a major cost driver impacting the overall cost of a fluidic module. Two major motor technology designs, DC brush and DC brushless, can be configured on the pump with their respective advantages and disadvantages.

DC brush motors

DC brush motors have been common with many diaphragm pressure and vacuum pump applications when low cost is critical but operational life is not important. 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 like the lead in a pencil. Brush motors are designed to last from 500 hours to 5000 hours, depending on the quality of the motor and how it is used.

The motor brushes experience an electrical arcing upon each start up. Frequent arcing will heat up the 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 no more than 3000 hours with frequent on/off cycles. Brush motors used in high-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 due to the build-up of carbon dust between the brush base and commutator. Tapping the outer housing to clear these deposits from the brush tips can usually restart the motor. In addition to limited life, brush motors can introduce unwanted electrical or RFI (radio frequency interference) noise into a system's circuitry.

Coreless motor technology differs from the standard brush motor in that the winding is wound onto itself on the rotor. The brushes are made from a highly conductive and efficient precious metal. No iron is on the rotor, making the lighter, coreless (or ironless core) rotor spin at a given performance level with less required input energy. This results in lower current draw required to power the respective diaphragm pump. Due to the precious metal brushes and the complexity of manufacturing the wound rotor, coreless motors come with a price premium. As a result, coreless motors are commonly used in portable, battery-operated systems requiring exceptional efficiencies to achieve longer battery operation.

Brushless DC motors advances

Brushless DC motors eliminate these problems. In a brushless motor, the magnets are on the rotor and the windings are wrapped around poles on the stator. Instead of brushes and a commutator bar, the windings are switched on and off sequentially by solid-state electronics. Brushless motors require less maintenance and are smaller, lighter and more efficient than brush motors with comparable outputs. With motor designs that focus on performance, reliability and endurance, operational life can be expected to exceed 10 000 hours with a high-precision bearing cage design to take out any play that causes bearing fretting. This precision design also can produce a quieter motor as the mechanical noise common with brushless motors is significantly reduced.

Brushless motors do have a limitation though since they incorporate slotted stators. The stator consists of slotted iron laminations that are fused to form a solid, uniform stack. The slots form rows that extend the length of the stack, and the windings are inserted into each row. As the rotor turns, the magnets are more attracted to the stator's teeth than the gaps between them. This uneven magnetic pull, called cogging, reduces the motor's efficiency and makes it difficult to produce smooth motion at low speeds. With typical operating pressure and vacuum loads, current-technology brushless motors can achieve efficiencies in the 50-60% range.

Hargraves has introduced an innovative design variation – a brushless motor that incorporates a slotless stator (one that has no slots to keep the windings in place). Instead, the windings are attached to the inside surface of the stator with adhesive. With no teeth to attract the magnets, cogging is eliminated, and the motor produces smooth, quiet rotation. The absence of teeth also provides room for larger magnets in the rotor and more wire in the windings, which means that slotless motors can generate more torque without a corresponding increase in size. Additionally, the slotless design significantly reduces damping losses.

In both slotted and slotless motors, eddy currents are induced as the magnets pass the stator. However, these currents are weaker in slotless motors, because the distance between the stack and the magnets is greater than in slotted motors. This makes slotless, brushless motors more efficient than slotted motors: miniature diaphragm pressure and vacuum pumps can expect to see improved efficiencies up to 70% coupled with the exceptional life that the brushless design produces.

Innovative diaphragms

The diaphragms in miniature diaphragm pumps and micro compressors are stretching and flexing under load and sometimes at elevated temperature conditions. Due to the limitations of standard EPDM (ethylene-propylene diene monomer) elastomers, many current technology miniature diaphragm pumps and compressors are only rated up to 40°C and have limited elastic properties to endure the rigorous cyclic stretching required for higher output applications. Pumps configured with EPDM and operating in higher ambient environments will typically experience ripped diaphragms before they achieve 3000 hours. To extend diaphragm life past 10 000 hours under the operating conditions that new-generation portable medical devices require, innovative research was conducted by the Hargraves' materials research team to develop an advanced performance elastomer that could withstand up to 70°C with improved mechanical capabilities. This research project resulted in the development of an advanced EPDM, or AEPDM, a proprietary elastomer material configuration that has been found in tests in house to last ten times longer than standard EPDM. Depending on the fluidic loads and ambient temperatures that the miniature diaphragm pump will be operating under, AEPDM diaphragms have been found to exceed 20 000 hours of operational life.

The shape of the diaphragm itself has been evaluated and optimized to improve vacuum, pressure and flow performance efficiencies. Typical flat diaphragms are performance limited by the amount that they can be stretched. High-performance air and gas pumps require increased pump stroke beyond the stretch limits of the flat diaphragm. Higher vacuum or higher flow performance requires that either a larger flat diaphragm be used (which would require a larger pump head design) or an increased diaphragm surface area by using a shaped diaphragm. Shaped diaphragms allow the pump stroke to increase by as much as 80%! By optimizing the pump's diaphragm shape, a significantly increased performance output can be achieved in a much smaller, compact envelope size.

 

 

 

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