Smaller, faster motors drive medical analysers

Dave Beckstoffer looks at how advances in miniature motor technology are addressing these requirements for increased power and resolution in ever-smaller packages

With designers of medical analysers under pressure to develop systems that are smaller, faster and even more accurate, the selection of the most appropriate motor to drive the different axes becomes a key consideration.

Medical diagnostics play vital roles in assuring our health, with doctors relying on these analysers to inform us of changes in our bodies or potential issues. 

The accuracy and speed of analysis is critical, and designers aim to reduce the time it takes to complete analysis without compromising the accuracy of the process.

In a typical design, a tray is loaded with samples (either in test tubes or wells), and conveyors or linear stages move pipettes into position above the samples. Then the pipette is driven down into the sample, samples collected and the pipettes goes back up to its starting position. 

Proper timing is required: the pipette must reach the proper depth in the sample to draw the required amount, then wait in position for the sample draw to be completed.

To increase the throughput (or number of samples analysed per hour) designers look to machines that can deal with more samples simultaneously, accelerating the process by having large trays of samples with multiple pipettes moving simultaneously.

To accomplish this, the pipettes need to be closer to each other than in current designs. The requirement for moving the pipettes up and down is unchanged, which means that for multiple pipettes in close proximity the same motion must be accomplished using a smaller diameter motor.

This vertical pipette motion has to be both fast and accurate. A high force can also be required either to perforate a cap on the sample or to clip a needle tip. The motive power is best provided in this case with a motor optimised for high speed operation, maximising the power even it means a gearbox is required.

The maximum continuous torque that a motor can deliver is related to the technology selected but is also dependant on its size. So, to have the smallest motor package size, it is better to create the mechanical power by using speed and not torque. Higher speed motors such as disc magnet steppers or brushless slotless DC motors can be appropriate choices.

Rare Earth magnets

Through the use of the latest rare earth magnet technology developments, which offer power exceeding the energy content previously available, a given motor output can be achieved with a significant reduction in diameter.

Where a typical application might previously have needed a 16mm motor, rare earth magnets mean the same application could be driven by an 8mm diameter motor.

We can take this further by looking at disc magnet technology, which has the added advantage of low inertia. This allows the motor to be stopped more quickly without the risk of overshoot and oscillation. A direct drive solution can be achieved with the disc magnet stepper to fully utilize this benefit.

New encoder technology offers high resolution in a small package. Resolutions of up to 512 lines can be achieved in an 8mm diameter package. This allows the loop to be closed for position feedback, enabling faster movement with higher accuracy.

Closing the loop with stepper motors is a key trend in the development of the latest medical analysers.

Closed loop steppers meet the needs of applications where high torque is needed at speeds up to 2,000-3,000rpm, offering a cost-effective solution.

Adding an encoder to an existing design using open loop steppers could increase performance by as much as 30%, with increased torque over the entire speed range.

The ability to switch between closed loop and stepper mode can bring further performance advantages when holding a position after motion. The stepping mode offers a high stiffness at position; to achieve the same by driving a different motor technology in closed loop mode would require high gain, which could make the system unstable.

It is sometimes tempting to hope as we begin to push the boundaries of what is possible in equipment design that there will be a single, universal motor technology that will meet the needs of each aspect of the application requirement.

As we have seen, that is not the case, and several options are available each with their own benefits.

What this means for the equipment designer is that they have increased freedom to optimise every aspect of the design. By taking advantages of advances in electronic component technologies, new materials and different control technologies, designers have the flexibility to make equipment that is smaller, faster and more accurate.

Image 1: Designers of medical analysers are under pressure to develop systems that are smaller, faster and more accurate. Therefore the selection of the most appropriate motor is a key consideration. Image: Portescap

Dave Beckstoffer is with Portescap.