Selecting a miniature motor for analyzers
Written by Udayan Senapati, Ph.D., Director of Operations - Portescap July 05, 2010
Medical analyzers are
the workhorse of the medical diagnostics industry. They are versatile tools with multiple functions; from testing
human bodily fluids such as blood, and urine,
to processing drug-protein interaction studies that deliver key
information for the diagnosis, prevention and
treatment of disease.-
Different types of analyzers perform sample movements for analysis with different motorized solutions (motor, encoder) and transmission mechanisms (pulley, belt, gearing). And in the quest to design medical analyzers to deliver better, safer, more personalized and cost efficient healthcare, the most common criteria for analyzer automation are high quality, low noise and long life, at an attractive cost.
Numerous motors/gearing/encoders are used to transport fluids, vials or assays within medical analyzers. Stepper motors are ideally suited for low rate sampling analyzers such as blood sugar testers that run 1-10 samples an hour, while state of the art brush and brushless coreless motor technologies function well in high throughput applications (on the order of more than 1000 assays an hour), such as immunochemistry or DNA
screening.
In the simplest versions of turn table analyzers where speed is not a primary issue, stepper motors are a reliable, cost effective method of meeting the analyzer’s functional requirements. A primary advantage to analyzers that utilize stepper motors is that they have many stable positions (steps) per revolution while providing a high torque for a given size (as an example, a Portescap 16 mm Disc Magnet Motor can offer up to 5-6 mNm of torque) . The disadvantage of utilizing a stepper motor is that it is not able to run at high speed (>2000 rpm), due to the inductance combined with the commutation frequency, and iron losses.
For high throughput applications - those where over a thousand assays are analyzed in an hour - high efficiency and higher speed motors such as brush DC coreless motors are a suitable choice. Their low rotor inertia (a 22 mm diameter Portescap motor has motor inertia in the range of (10 - 30) x 10-6 kgm2) along with short mechanical time constant makes them ideally suited for such applications. It should be noted that the load characteristics on the motor, depending on the torque required to turn the assay sample table at a certain speed, would determine the actual time the motor takes to ramp up to a certain speed.
Disc magnet stepper motors and brushless DC motors can also work in variants of this application based on speed, acceleration, performance and cost requirements.
In some critical applications where the sample size available for analysis is limited, motor characteristics such as speed, torque, efficiency and positioning accuracy play a significant role. Here again a brush DC coreless motor is highly applicable due to the power density it packs in a small frame size. And as mentioned earlier, the low inertia of a brush DC coreless motors aids in efficient fluid transport, especially in cases where the requirements for sample availability are in the micro liter range. Typically an incremental encoder can be used for feedback with a brush DC coreless gearmotor to gauge motor position and speed. High encoder resolution of >128 lines is typically desired at lower speeds of <1000 rpm, such as during the final stages of suctioning fluids from the vials.
An extension of the application uses pumps with stepper motors to dispense certain reagents into the assays in order to aid the analysis process. Such stepper motors can be controlled using open or closed loop feedback.
If power density, efficiency, speed and value are primarily important criteria, then brush DC coreless might be the technology of choice. If positioning without added electronics and low cost are the primary requirements, as in low rate sampling analyzers, then steppers could be the preferred option. The user has to make a selection based on performance-to-price needs, keeping in perspective the costs associated with control electronics and drives, along with the life span of such analyzers which can run 15 - 20 years, and the application needs the analyzers serve, in the heath care segment.
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Different types of analyzers perform sample movements for analysis with different motorized solutions (motor, encoder) and transmission mechanisms (pulley, belt, gearing). And in the quest to design medical analyzers to deliver better, safer, more personalized and cost efficient healthcare, the most common criteria for analyzer automation are high quality, low noise and long life, at an attractive cost.
Numerous motors/gearing/encoders are used to transport fluids, vials or assays within medical analyzers. Stepper motors are ideally suited for low rate sampling analyzers such as blood sugar testers that run 1-10 samples an hour, while state of the art brush and brushless coreless motor technologies function well in high throughput applications (on the order of more than 1000 assays an hour), such as immunochemistry or DNA
screening.
In the simplest versions of turn table analyzers where speed is not a primary issue, stepper motors are a reliable, cost effective method of meeting the analyzer’s functional requirements. A primary advantage to analyzers that utilize stepper motors is that they have many stable positions (steps) per revolution while providing a high torque for a given size (as an example, a Portescap 16 mm Disc Magnet Motor can offer up to 5-6 mNm of torque) . The disadvantage of utilizing a stepper motor is that it is not able to run at high speed (>2000 rpm), due to the inductance combined with the commutation frequency, and iron losses.
For high throughput applications - those where over a thousand assays are analyzed in an hour - high efficiency and higher speed motors such as brush DC coreless motors are a suitable choice. Their low rotor inertia (a 22 mm diameter Portescap motor has motor inertia in the range of (10 - 30) x 10-6 kgm2) along with short mechanical time constant makes them ideally suited for such applications. It should be noted that the load characteristics on the motor, depending on the torque required to turn the assay sample table at a certain speed, would determine the actual time the motor takes to ramp up to a certain speed.
Disc magnet stepper motors and brushless DC motors can also work in variants of this application based on speed, acceleration, performance and cost requirements.
In some critical applications where the sample size available for analysis is limited, motor characteristics such as speed, torque, efficiency and positioning accuracy play a significant role. Here again a brush DC coreless motor is highly applicable due to the power density it packs in a small frame size. And as mentioned earlier, the low inertia of a brush DC coreless motors aids in efficient fluid transport, especially in cases where the requirements for sample availability are in the micro liter range. Typically an incremental encoder can be used for feedback with a brush DC coreless gearmotor to gauge motor position and speed. High encoder resolution of >128 lines is typically desired at lower speeds of <1000 rpm, such as during the final stages of suctioning fluids from the vials.
An extension of the application uses pumps with stepper motors to dispense certain reagents into the assays in order to aid the analysis process. Such stepper motors can be controlled using open or closed loop feedback.
If power density, efficiency, speed and value are primarily important criteria, then brush DC coreless might be the technology of choice. If positioning without added electronics and low cost are the primary requirements, as in low rate sampling analyzers, then steppers could be the preferred option. The user has to make a selection based on performance-to-price needs, keeping in perspective the costs associated with control electronics and drives, along with the life span of such analyzers which can run 15 - 20 years, and the application needs the analyzers serve, in the heath care segment.
For more DPN News, click here
To subscribe to the DPN eNewsletter, click here
To subscribe to the Print or Digital Edition of DPN, click here
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