Mini Motors Deliver Performance for Analyzers

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. 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, while stepper motors are ideally suited for low rate sampling analyzers such as blood sugar testers that run 1-10 samples an hour. Some medical analyzers use a turn table-based approach (See Figure 1) to stack assays that are identified, marked and serialized to track human fluids, enabling labs to deliver timely and accurate feedback to the health care professionals. (See Figure 2)

Stepper Motors

In the simplest versions of such 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. The current in each phase has to be commuted many times per revolution in a stepper motor, as a stepper motor is essentially a BLDC motor with many poles. For instance, a 2-phase stepper with 100 steps / revolution will need 25 current reversions in each phase to make one full revolution. 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 (current reversed so many times).

That said, a range of steppers - from permanent magnet to hybrid to linear - are available to satisfy analyzer application needs. Permanent magnet can-stack steppers are suitable for analyzers when space and power demands are critical. Hybrid steppers are small, powerful and cost effective enough to be used in analyzers. Offering advantages such as limited maintenance or wear, simplicity of integration and part reduction versus standard rotary systems, linear stepper systems are also ideally suited for many analyzer applications. Rotary systems typically need translation mechanisms to transfer rotary motion to linear motion, thus increasing part count and integration complexity. Linear steppers are ideal for analyzers requiring light loads and open-loop performance, and due to the lower inertia associated with fewer components, they can typically accelerate faster than rotary systems.

Brushless DC Coreless Motors

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. As an example, a Portescap 22 mm motor brush coreless DC motor offers no-load speed of 8,000 rpm and a mechanical time constant of 6.8 milliseconds. The time required for the motor to attain such speeds is governed by the equation:

w = w_o (1- exp (-t/ t_m))

where w_o is the no load speed, t_m is the mechanical time constant of the motor and w is the speed attained after a certain lapse in time t. Based on the motor characteristics, 90% of the no load speed can be attained in the turn table application in about 15 milliseconds as shown in Figure 3. 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.

Another analyzer function that plays a vital role in their output is collecting samples from the vials or assays, and serving them up to measurement systems based on photometry, chromatography or other appropriate schemes.

Open tube or close tube format samples assays are typically presented to a piercing or plunging mechanism via the turn table for suction of the sample from the vial, with disposition to a measurement system. (See Figure 2)

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 (See Figure 4) to gauge motor position and speed.

Such incremental encoders can be optical or magnetic, and produce pulses (See Figure 5) that are proportional to speed and distance. High encoder resolution of >128 lines is typically desired at lower speeds of <1,000 rpm, such as during the final stages of suctioning fluids from the vials.

Hybrid Stepper Motors

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. A hybrid motor such as that shown in Figure 6, can be used in different axes to position the test samples under appropriate reagent dispensers, and a closed loop system, although more expensive, might be justifiable in such a case.

A typical can stack motor has discrete angular positions where the shaft is retained in discrete positions using a holding torque. As an example, a 15 mm can stack motor with an 18 degree step angle can be run in open loop without a feedback sensor, but the positioning would be crude. On the other hand, a hybrid motor with closed loop system can have an encoder for position feedback to the drive electronics, with added encoder costs of $10 - $25, and costs for drive electronics enhancements.

Summary

The performance-to-price consideration of an appropriate motion solution ultimately depends on the complexity of the analyzer, along with the precision, efficiency and environmental conditions required for the operation. A range of different motor technologies are applicable for different motion requirements and axes of operation in a medical analyzer, as shown in the table below.

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.