As miniature diaphragm pumps get smaller, performance and life increase
System designers have been challenged to advance their product technology by specifying fluidic modules that can achieve increased performance capabilities while fitting in ever-smaller envelopes. Since quality, reliability and high system uptime is demanded by their customers, development engineers have been tasked to incorporate fluidic and pneumatic components that can exceed 20,000 hours of operation under demanding loads, higher temperature environments and strict noise levels.
The Miniature Diaphragm Pump Challenge
Increasing the performance of miniature pressure and vacuum diaphragm pumps has traditionally been accomplished by 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. Increasing the performance while shrinking the size of the miniature diaphragm pump has posed several interesting challenges. Several companies whose development engineers could not find miniature diaphragm pumps to meet their demanding performance and long life requirements challenged Hargraves Technology, www.hargravesfluidics.com, to think out-of-the-box to advance this critical fluidic component technology.
Miniature diaphragm pumps have become popular with system engineers to provide the pressure and vacuum transport of their fluid media in a cost efficient manner. Diaphragm pumps exhibit excellent gas tightness and offer the advantage of the fluid chamber being totally sealed from the pumping mechanisms. An eccentric connecting rod mechanically flexes a diaphragm inside the closed chamber to create a pressure or vacuum. 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 carbon vanes wearing and spewing debris in the flow path. Therefore, miniature diaphragm vacuum pumps and compressors ensure an oil-less, contaminant free fluid pathway. This is critical for many medical device, analytical instrumentation, clinical chemistry, gas analyzer, fuel cell and other advanced technology applications.
The Hargraves product development team set out to clearly define their customers’ application operating parameters while benchmarking existing miniature diaphragm air pumps available in the industry to understand their limiting factors. It was found that the miniature diaphragm pump and compressor technology available were experiencing diaphragm rips and tears and motor failures that limited pump life around 3,000 hours. Diaphragm failures had become so common that replacement diaphragm kits from the manufacturer had become an accepted practice and cost.
Advanced EPDM Elastomers
The shape of the diaphragm itself was evaluated and optimized to improve vacuum, pressure and flow performance efficiencies. Typical flat diaphragms are performance limited by the amount that it 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%! The Hargraves product development team became very innovative with designing optimized diaphragm shapes. As a result, significant increased performance output was achieved in a much smaller, compact envelope size.
To extend diaphragm life under “real world” operating conditions, the Hargraves engineers searched throughout the elastomer industry for a material that would endure these rigorous demands at extended life cycles. Standard EPDM elastomers are typically rated only up to 40°C and have limited elastic properties to endure the rigorous cyclic stretching required for higher output applications. The Hargraves team took the initiative to develop an increased performance diaphragm material. Since typical operating environments for fluidic modules see far higher temperatures, the Hargraves engineers sought after an elastomer that could withstand 70°C with improved mechanical capabilities. This research project resulted in the development of an advanced EPDM, or AEPDM, a proprietary material configuration that has been tested to last ten times longer than those used by other diaphragm pump manufacturers. The life of the Hargraves’ AEPDM diaphragms can exceed 20,000 hours depending on the application.
Limitations of Brush Motor Technology
The motor driving the diaphragm vacuum pump or compressor is an important factor affecting the overall performance and expected operational life. DC brush motors have been common with many 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 like the lead in a pencil. Brush motors are designed to last from 500 hours to 6,000 hours, depending on the quality of the motor and how it is used.
The motor brushes experience an electrical arcing upon each start up. Therefore, brush motors that experience frequent on/off cycles per day wear out more quickly. The frequent arcing heats up the carbon brushes will also cause them to wear more rapidly. A top quality brush motor can be expected to last 3,000 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 carbon dust build up between the brush base and commutator. Tapping the outer housing to clear these deposit from the brush tips can usually restart the motor. In addition to limited life, brush motors can introduce unwanted electrical or RFI noise into a system’s circuitry.
Standard Brushless Motors are Better but still have Limitations
To achieve significant improvement to motor life, the product development team thought that they would be able to simply incorporate off-the-shelf brushless DC 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 are 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 these off-the-shelf brushless motors improved compared to brush motors, they were not adequate for the demanding operational loads that diaphragm pumps exert on the motor shaft. Bearing life was found to be limited and restart capabilities under load were unacceptable.
Standard brushless motors are typically designed for operational loads in one radial direction, for example to turn pulleys. For these applications, the bearing cage assembly could afford to have some “play” since the mechanical loading is sided in one direction. Miniature diaphragm pumps, on the other hand, exert 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” permitted by the bearings, fretting of the bearings supporting the motor shaft will occur. This will cause premature bearing wear, increased mechanical noise and overall motor life degradation.
Enhanced Brushless DC Motor Technology
The Hargraves engineering team took the initiative to improve standard motor designs and manufacturing processes to optimize brushless motor technology with regards to performance, reliability and endurance. The bearing cage assembly was designed to “zero-play” tolerances to properly operate under demanding operational loads. This precision design also produced a quieter motor as the mechanical noise common with brushless motors was significantly reduced.
Many fluidic system module applications require the diaphragm vacuum pump or miniature diaphragm compressor to restart under load. As the requirements for these pumps to fit in even smaller package sizes, the motors powering these pumps have had to also become smaller. To reduce the motor envelope, some smaller brushless motor sizes have incorporated sensorless commutation methods since Hall effect sensors take up more space. These sensorless motors are limited to the initial startup torque they can produce hindering their restart capability. To combat these tradeoffs, the Hargraves motor development team produced an innovative and compact Hall effect sensor design that achieves high torque to reliably re-start under higher loads.
Other areas of improvement to the brushless DC motor were made in the area of high temperature environments, motor efficiencies and system control. The commutation circuit was designed to operate in a 110°C maximum ambient temperature environment, allowing the pump to operate in a broader temperature range. An advanced high temperature lubricant was selected to maintain proper lubrication in bearings at elevated temperatures. In addition to common analog voltage control, three-wire PWM control was added modulate the motor RPM. A fourth wire option was made available to additionally provide a tachometer feedback.
Hargraves BTC-IIS acheives the highest performance/size ratio
Highest Output in the Smallest Package
The result from incorporating advanced elastomers, optimizing the diaphragm geometry, and significantly enhancing the brushless DC motor design led to a miniature diaphragm pump achieving unprecedented performance and life. The BTC-IIS model pump, for example, has a free flow of up to 11 LPM and can withstand an operating environment up to 70°C. The ability for the components to endure much higher ambient temperatures along with a rugged design to withstand the demanding operation loads has enabled this pump technology to outlast most of the systems that it will be integrated into. This compares with similar sized diaphragm pressure and vacuum pumps found on the market today that can only achieve approximately 8 LPM and are limited to a maximum allowed ambient temperatures of 40°C and much shorter operational life.
The Hargraves BTC-IIS configured with AEPDM elastomers and advanced Brushless DC Motor technologies can produce 11 LPM free flow, more than 35% more than comparable sized diaphragm pumps found on the market today