Portescap Motor Selection for Electronic Pipettes
When it comes to measuring and dispensing a specific volume of any liquid, pipettes are essential in today's laboratory environment. Different types of pipettes are commonly used depending on the scale of the laboratory and the volume to be dispensed:
- AIR DISPLACEMENT
- POSITIVE DISPLACEMENT
In 2020, we began to see air displacement micropipettes play a significant role in the battle against COVID-19, where they are used for sample preparation for pathogen detection (e.g., real time RT-PCR). Typically, two different designs can be used, either manual or electronic air displacement pipettes. (Figure 1)
MANUAL VS. ELECTRONIC PIPETTES
In the example of an air displacement pipette, a piston is either moved up or down inside the pipette, in order to create a negative or positive pressure on the air column. This allows the user to take in or eject the liquid sample with the disposable pipette tip, while the air column in the tip separates the liquid from the non-disposable part of the pipette. (Figure 2)
The movement of the piston can be designed to be done manually by the hand of the operator or as an electronic version, where a motor is moving the piston, controlled by the operator through the use of buttons.
LIMITATIONS OF MANUAL PIPETTES
Using manual pipettes over a longer period of time can provide discomfort or even injuries to the operator. The force required to dispense the liquid and eject the pipette tip in combination with the repetitive movement, often over several hours, can increase the risk of so-called RSI (repetitive strain injuries) of joints, especially thumbs, elbows, wrists and shoulders.
Manual pipettes require depression of the thumb-button in order to release the liquid, while electronic pipettes offer much better ergonomics in this example with an electronic trigger button. (Figure 3)
AN ELECTRONIC ALTERNATIVE
Electronic or motorized pipettes are an ergonomic alternative to manual pipettes and an efficient way to increase sample throughput with precision and accuracy. Instead of the traditional thumb-controlled pipetting button and manual volume adjustment, electronic pipettes feature a digital interface to adjust volume and a motorized piston for aspiration and dispensing. (Figure 4)
Some other advantages of electronic pipettes include:
Less user force required to dispense the liquid during each pipetting step
Fatigue-free operation, reduced risk of injuries
No manual thumb-controlled pipetting button required
More ergonomic design, reduced stress on the thumb of the user
Digital interface e.g. colored touch display
Clear representation of critical parameters, quick access to available pipetting functions
Advanced functions possible like automatic tip ejection, automatic detection of tip volume, sequential multi-dispensing
Comfortable and safe operation, reduced risk of errors, higher efficiency
MOTOR SELECTION FOR ELECTRONIC PIPETTES
Because pipetting is typically the first step of a multi-step process, any inaccuracy or imperfection in measuring this tiny amount of liquid carries through the process and in the end impacts the total accuracy and precision.
WHAT IS PRECISION AND ACCURACY?
Precision is achieved when the pipette dispenses the same volume multiple times. Accuracy occurs when the pipette dispenses the targeted volume exactly without any error. It is possible to achieve one without the other, however, the industries that use pipettes require both precision and accuracy. In fact, this critical requirement is what makes it possible to reproduce the outcome of experiments.
The heart of any electronic pipette is its motor, which greatly influences precision and accuracy, in addition to several other important factors like envelope size, power and weight. Pipette design engineers primarily choose stepper linear actuators or DC motors. Ultimately, stepper motors and DC motors have both their strengths and weaknesses.
DC motors are simple electric machines that rotate when DC power is applied. They do not require complex electronics to run the motor. However, considering the linear motion requirement for the electronic pipette, a DC motor solution needs an additional lead screw and gearing system to convert rotational movement to linear movement and provide the required force. The DC solution would 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. Use of a DC motor may enhance the overall precision and accuracy of the pipetting system, but it can be an expensive solution.
On the other hand, many engineers prefer to use a stepper linear actuator solution due of its ease of integration, performance and low cost. A stepper linear actuator is comprised of a can stack stepper motor with a threaded rotor and an integrated lead screw which provides direct linear motion in a small package. (Figure 5, Figure 6)
Unlike the DC motor, the stepper linear actuator lead screw moves up and down in discrete step increments when electrical pulses are applied. One of the important advantages of using a stepper linear actuator is its ability to 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 of high-resolution positioning, which can be further increased by driving the stepper linear actuator in micro-stepping mode.
If not sized properly, stepper motors can lose steps which would lead to inaccuracies during dispensing. However, this can be easily corrected by ensuring accurate output of the motor to the drive system.
Below are some of the key considerations for motor selection:
- KEEP MINIMUM 50% SAFETY FACTOR FROM THE SPEED/FORCE CURVE
- AVOID LOW FREQUENCY RESONANCE POINTS DURING OPERATION
- AVOID BACK-DRIVING, SUDDEN MOVEMENTS AND EXTERNAL LOAD CHANGES
In summary, both stepper and DC motors have their unique advantages for this application. 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 need for precise and accurate dispensing.
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