Its Life Science, Not Rocket Science: How Motion Specialists Contribute to Optimizing Liquid Handling Workstations

Laboratory automation plays a critical role in automating many routine procedures in the life sciences field. Liquid handling workstations offer an excellent example, as they involve a multitude of repetitive tasks such as decapping and recapping test tubes; dispensing liquid samples accurately; and mixing, stirring, and transporting processed test tubes. Automating these tasks allows for higher throughput, eliminates the risk of human error, and improves the overall reliability and consistency of the process. This article will focus on automated liquid handling workstations and the micro-motors best suited for these applications.

ADVANTAGES OF AUTOMATED LIQUID HANDLING PROCESSES

Automated liquid handling processes have revolutionized laboratory operations, offering numerous practical benefits. These include:

Elimination of Human Error

Using an electronic pipette for liquid handling takes a certain level of experience and skill. The pipette can only ensure the required precision and accuracy if used in the right settings, which depends on the viscosity of the liquid to be analyzed. If an operator sets the dispensing speed of the pipette at too high a level for a low-viscosity liquid, the risk of splashing during dispensing is created. The concentration of the operator using the pipette may also diminish when pipetting is completed over a longer timeframe due to complex analyses; this can result in costly mistakes like using the same pipette tip for two different liquids, which should be avoided due to the risk of cross-contamination. Automation provides an edge by eliminating these risks of human error.

Higher Throughput

Precise manual liquid handling is typically completed by using an electronic pipette that aspirates and dispenses a liquid once a specific button is pressed. An electronic pipette can be equipped with any number of tubes, from a single tube up to dozens; in the case of a multichannel pipette, sixty-four parallel tubes can be used. While the pipette itself ensures that a precise dose is dispensed, the speed of the operation – or the dispensing of the liquid sample from one test tube to the next – is largely dependent on the operator’s capabilities and speed. Automating this step allows for a higher throughput, meaning that a higher number of samples may be analyzed than with a single operator.

Higher Reproducibility

Reproducibility is one of the most critical factors for analyses completed in a laboratory environment. It can be described by two factors: precision and accuracy. Precision means that the same volume is dispensed during every dispensing operation, whereas accuracy means that the dispensing volume hits as close to the targeted amount as possible. It’s critical to ensure that you have both precision and accuracy for the outcome of your experiment to be reproducible. If an analysis does not produce consistent results over several iterations, the analysis itself is already a source of variation of the actual test result and therefore prevents the right conclusions from being drawn.

TYPICAL MOTION AXES IN LIQUID HANDLING PROCESSES

A laboratory automation machine for liquid handling can be designed in different ways, depending on the desired throughput and the number of liquid samples to be analyzed in parallel. For example, when analyzing a low number of test tubes, a single robotic arm with several rotary joints and a gripper as an end effector might be sufficient. For higher throughput and more than one liquid sample to be processed in parallel, however, a design based on a cartesian robot with linear motion axes can become much more effective.

A typical design features static test tubes while the pipette is positioned above the test tubes using three linear axes: X, Y, and Z. This article will focus on the Z axis, which is often the most challenging in terms of actuation. The actuation system of the Z axis is responsible for moving the pipetting tip to the liquid for aspiration and raising it again after a successful aspiration. This motion needs to be fast to achieve higher throughput, but also precisely controlled to ensure the pipetting happens with the same precision and accuracy regardless of the viscosity of liquid involved. A typical pipetting procedure can be described as follows:

1.	The pipette head is lowered at high velocity towards the liquid container. The actuation system must provide a high acceleration torque, as well as reach a high velocity, to allow fast movement.
2.	Once the pipette head approaches a possible filling level of the liquid container, the pipette head needs to be slowed down and slowly approach the liquid. A slow and precise movement is required while the liquid surface is detected, e.g. through a capacitive sensor.
3.	Once the liquid surface has been detected, the pipette head is positioned to a defined level below the liquid surface to start aspirating. During aspiration, the pipette head’s position is continuously adjusted depending on the container geometry to maintain the same level of positioning below the liquid surface.
4.	After aspirating the required liquid volume, a small volume of air is aspirated for transport and the pipette head is retracted from the liquid container to dispense the liquid where desired.

In summary, positioning the pipette head requires not only slow and precise movement when precise positioning is required, but high acceleration and velocity as well when lowering or retracting the pipette head.

BRUSHLESS DC MOTORS FOR PIPETTE HEADS

Due to the limited space available and the highly dynamic motion requirements for the pipette head, brushless DC motors are ideal for this application. Their high-power density allows for maximum performance within the available dimensions. Two typical designs allow for a different form factor of the motor: an inner rotor will typically offer a small diameter compensated by a longer motor length, whereas an outer rotor design is an ideal option when a flat solution with a higher motor diameter is required. They can additionally be equipped with encoders in order to achieve precise positioning.

APPLICATION IN FOCUS: ROBOTIC PIPETTE’S Z-AXIS ACTUATION

Higher Reproducibility

Let’s look at an example where a motor is required for the Z-axis movement of a pipetting head. Up to four pipetting heads will be placed next to each other to process up to four test tubes in parallel. Apart from the dynamics, the form factor of the motor is critical, as the limited space available does not allow for a large motor. For precise positioning, an encoder should also be used on the motor.

We will consider the following requirements as an example:

•	Overall diameter < 20mm
•	The maximum load torque during pipetting is up to 2mNm
•	For fast pipetting the acceleration of the motor should be as high as possible

Considering a maximum motor diameter of 20mm, the following micro motors can be considered:

Table 1 – Catalog comparison between 16ECP24 and 20ECF14

Motor Technologies Comparison

Based on the slotless inner rotor 2-pole design, the 16ECP24 can reach considerably higher speeds than the 20ECF14 slotted outer rotor design. This can be an advantage depending on how the linear motion of the Z-axis is implemented in the application. Typical solutions include lead screws or ball screws; however, these are often limited to speeds below 10,000 RPM. Additionally, during pipetting, the linear travel required to move from test tube to test tube is short. Depending on the acceleration, the motor might not have enough time to reach higher speed levels, meaning that taking full advantage of the slotless inner rotor design won’t always be possible.

Motor Performance Comparison

Most important for the dynamics of the drive system is the acceleration capability of the motor, which depends on factors like the rotor inertia of the motor; the maximum motor torque available for acceleration; and the load inertia connected to the motor. Considering a lead or ball screw connected to the motor, a different lead can be chosen depending on the available motor torque and speed. The lead won’t, however, have a significant impact on the inertia of the screw. The acceleration capability will therefore mainly depend on the rotor inertia and the available motor torque for acceleration.

The 20ECF14 is based on an outer rotor design, which means the rotor and therefore the moving mass overall reaches a much higher inertia than the inner rotor design of the 16ECP24. Opposite this is the multipolar outer rotor design of the 20ECF14, which – in combination with its larger diameter – provides a much lower motor regulation factor. This results in a more efficient and powerful motor that can potentially develop a higher peak torque for acceleration. The motor needs to be able to dissipate the heat generated resulting from this peak torque; this means that the thermal resistance, describing the temperature rise of the motor for a given amount of power loss in the motor, matters as well.

Let’s take all three factors – the rotor inertia, the motor regulation factor, and the thermal resistance – into consideration and compare the total merit factor of both motors by multiplying them, as all three factors should be as low as possible:

Table 2 – Performance comparison between 16ECP24 and 20ECF14

When looking at the above data, both motors are actually on a comparable level based on the chosen three comparison factors. The much higher rotor inertia of the 20ECF14 is largely compensated by the much lower motor regulation factor and therefore higher torque capability. The lower thermal resistance and therefore better heat dissipation can provide an additional advantage when high peak torques for acceleration are required.

Motor Integration Comparison

As precise positioning is required for this application, an encoder on the motor is a critical choice to ensure precise positioning and feedback. Here the 16ECP24 provides an edge, as encoder options are readily available and easy to add based on the design of the motor. For a motor design like the 20ECF14, encoder options must typically be integrated into the existing motor PCB, where space is limited, and more engineering effort is required. Depending on the desired development timeline of the pipetting machine, choosing a readily available solution can accelerate the development and allow more design iterations. The smaller diameter of the 16ECP24 will also be an important advantage as the available space in terms of diameter is often much more limited than the acceptable length of the motor.

CONCLUSION

This article has reviewed two possible motor solutions for driving the Z-axis of a robotic pipette: the 16ECP24 brushless slotless DC motor and the 20ECF14 brushless slotted flat motor. Different technologies allow for different form factors and differing performances of the motor, as well as differing individual motor performance parameters. Determining the ideal solution requires however comparing several factors in combination. This is where an experienced motion supplier can add value by providing experience and guidance on selecting the right solution and optimizing it. Only a well-optimized motion solution will allow for maximum throughput and reliability of the liquid handling machine.

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