Valentin Raschke reports on motor selection for electronic pipettes
The production of multiple vaccines against Covid-19 has highlighted the important work done in research laboratories around the world. The accuracy and precision needed in support of reproducible results to enable such rapid drug development is increasingly reliant on automated systems. The motorised electronic pipette is an excellent example of just such a product.
The engineering behind the electronic pipette is sophisticated and, for the design engineer, the choice of motor for the product is a key consideration. Here, we compare the strengths and weaknesses of DC motors and stepper motors in this critical task.
When it comes to measuring and dispensing a specific volume of a liquid in the laboratory environment, pipettes are essential. In the battle against Covid-19, 2020 saw the emergence of a new pipette technology that would go on to play a significant role, with the air displacement micropipette becoming one of the products of choice for sample preparation in pathogen detection.
Piston is heart of pipette
At the heart of the air displacement pipette is a piston that, moving up or down inside the pipette itself, creates negative or positive pressure on the air column. This allows the user to take in or eject the liquid sample through a disposable tip, with the air column in the tip separating the liquid from the non-disposable part of the pipette.
Electronic air displacement pipettes have become increasingly popular, overcoming some key limitations of manual designs, where repeated use over a long period of time – often several hours – can quickly become fatiguing. Electronic pipettes offer an ergonomic alternative, providing a digital interface where the operator adjusts the volume for motorised aspiration or dispensing. The result is an efficient means of increasing sample throughput while maintaining the levels of precision and accuracy that are critical in laboratory sample preparations.
The motor is a key element of the design, greatly influencing the achievable precision and accuracy. The design engineer specifying the motor for the application must consider such factors as motor power, size and weight, and of course the implications of the motor technology for the control electronics. The two most considered options are stepper motors and DC motors; and both have their strengths and weaknesses.
DC motors are simple electric machines that rotate when DC power is applied, and do not require complex electronics for the controller. A key consideration, though, is how the rotary motion of the motor will be translated into the linear motion required to actuate the piston. Most commonly this will be accomplished using a lead screw and gearing system, adding size and weight to the overall drive system. The DC motor solution will also need a feedback mechanism in the form of an optical sensor or encoder to accurately control the linear piston position. Some designers may also add a braking system to improve positioning accuracy, due to the high inertia of its rotor.
Although the use of a DC motor will undoubtedly provide a solution with ergonomic benefits for the user – and may well enhance the overall precision and accuracy of the pipetting system – it can be an expensive option once the additional components required are considered. An alternative is the stepper linear actuator, offering the benefits of ease of integration, improved performance and lower ultimate cost.
A stepper linear actuator comprises a can stack stepper motor with a threaded rotor and an integrated lead screw, providing direct linear motion in a compact package. Because the actuator is built around a stepper motor, the leadscrew moves up and down in discrete step increments when electrical pulses are applied.
A key advantage of the stepper linear actuator is that it can be accurately controlled in an open loop system, meaning no expensive feedback device or braking system is required for positioning. Typically, a small step angle and different lead screw pitches to choose from provide the possibility for high-resolution positioning, which can be further increased by driving the stepper linear actuator in micro-stepping mode.
An important consideration in the specification of the stepper linear actuator is to properly size the stepper motor. If not sized correctly, the motor can lose steps which would lead to inaccuracies during dispensing.
We can see that both stepper and DC motors have advantages and disadvantages for use within electronic pipettes. Although the DC motor concept with built-in feedback mechanism provides improved precision and accuracy, the step motor concept is the most cost-effective solution and can be precisely controlled in the open loop system simply by varying the number of input pulses and their frequency. If sized properly for the application, a step motor provides the reliability needed for precise and accurate dispensing.
It is worth noting that by engaging with a knowledgeable supplier, such as Portescap, at the earliest stages of product development, the supplier’s engineering team can work with the pipette designer to optimise the size, weight, performance and cost of the finished product, further helping in the fight against life-threatening, infectious disease.
Valentin Raschke is application engineer at Portescap