Choosing a Motor for Electronic Pipettes

When it comes to dispensing a specific volume of liquid in a lab environment, pipettes are essential. Electronic air displacement pipettes have taken the lead due to the need for a high level of precision and accuracy required for repeated use over a period of time. With the improvement of a digital interface for adjusting aspiration or dispensing, electronic pipettes offer a huge advantage and relief from ergonomic methods.

The motor is the key element in the design. When design engineers specify a motor    they must consider factors such as motor power, size and weight, and control electronics. Let’s look at the two most considered motor options - stepper motors and DC motors to help determine the fit for your application.

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.

While 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. While 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.

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 optimize the size, weight, performance and cost of the finished product, further helping in the fight against life-threatening, infectious diseases. Read the full white paper to learn more or discuss your needs with an engineer.