Motion solution specification crucial for electronic pipettes

May 04, 2023

Ubiquitous throughout laboratories specializing in life sciences and clinical diagnostics, pipettes are a vital tool for transferring liquid samples. Repeated activation of manual devices can strain lab technicians while electronic pipettes improve ease of use and increase dispensing accuracy and repeatability. To optimize the requirements of liquid dispensing, and meet the needs of ergonomic design, specifying the right motion system is crucial.

The motion system is central to an electronic pipette's function, and its specification has wide-ranging implications for the device's other features built around it. Involving motion system design as early as possible within the development process will lead to a more effective and efficient outcome.

A key requirement of the motion system is accuracy and repeatability. Pipette devices are typically programmable to dispense the precise amount of liquid, every time, which demands linear motion control. To achieve accuracy, the design requires a motor feedback device, which reports the position of the motion system, or alternatively a motor confirming its position in open loop by its mode of operation. In addition to control accuracy, the dispensing cycle should also be achieved as quickly as possible to minimize valuable laboratory time and make the process easier for the lab technician.

The motion system also needs to deliver sufficient linear force to propel the fluid. The force required is affected by the fluid's viscosity. Many pipettes can operate with a range of fluids with varying viscosities, so the greater the force a motor can deliver, the more flexible its use to a laboratory; force requirements are also multiplied if a pipette is intended for multi-channel dispensing. In addition to force to propel the fluid, a pipette's greatest peak torque requirement is for ejection of the disposable pipette tip, typically removed to avoid sample cross-contamination.

To optimize use by the lab technician, the pipette should also be compact and lightweight. Motor type and its control method significantly impact the device's total footprint and weight, meaning motor performance demands, including considerations relating to its power supply, have to be balanced with ergonomic requirements. This wider impact, in addition to piston actuation alone, highlights the need for motion system specification to occur as soon as possible within a pipette's complete design process.

Transposing lab technicians’ requirements into a pipette's motion design, actuation characteristics remain fundamental. Starting with control accuracy, a brush DC motor with an encoder provides precision over piston actuation. Alternatively, a can-stack stepper motor rotates in defined steps for each current pulse, meaning its position, relative to the angle of each step, is always known. While this doesn't provide the same precision as a DC motor with an encoder, a stepper gives high accuracy across most pipette applications. Stepper positioning can also be optimized by designing small step angles and driving the motor in micro-stepping mode. The pitch of the lead screw connecting the motor to the piston can also be customized for fine control.

If the pipette requires higher torque, such as for multi-channel dispensing, a DC brush motor has an advantage. The DC motor can run faster than a stepper, enabling the incorporation of gearing or the use of a narrower lead screw pitch. The pipette can generate greater force yet still maintain the desired dispense rate.

As a stepper doesn't require an encoder to control its position, this helps achieve a more compact, lightweight design. And, as the motor can be designed to include a threaded rotor and an integrated lead screw, this achieves a linear motion solution that connects coaxially with the piston, enabling a thinner pipette profile. From an original equipment manufacturer's (OEM's) design perspective, this makes the integration of a linear stepper motor relatively simple, saving time and development cost. Alternatively, to convert rotary motion to linear motion, a DC motor needs a gear or pulley linking the motor to the lead screw and piston on a separate, parallel axis. This approach increases design complexity and adds size and mass, requiring a wider pipette body to accommodate the design.

To power the motion solution, battery size and weight are important considerations for ergonomics, but these attributes are trade-offs with charge life. A battery needs to last for a day's use with the desired number of dispenses, but to optimize ease of use for the lab technician, the battery needs to be as compact as possible. This impacts motor winding design and torque output, so a balance must be found between performance at the available voltage with the specific current draw, and battery capacity and charge life.

Stepper motors are the common approach for pipette design, not least because they are more cost effective. However, brush DC motors might be considered if higher torque and accuracy are required. Whichever technology is selected, accurately defining its specification, including sizing, is vital. Considering the broad implications, engaging with a motion solution designer ahead of any specification decisions keeps flexibility open for optimized design of the pipette.

Customizing a motion solution to achieve customer requirements in pipette design demands a multitude of considerations, covering areas from the windings and magnets, through to the mechanical integration with the syringe. The pipette has a fundamental reliance on the motion solution, and there's a high level of design integration required. As a result, a collaborative approach between the motion solution and pipette design engineering teams is vital.

Portescap: https://www.portescap.com

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