The pipette is a basic fluid control tool for transferring tiny amounts of liquids. 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.
Because pipetting is typically the first step of a multi-step process, any inaccuracy or imperfection in measuring this tiny amount of liquid adds up through the process and in the end, the failure rate is much higher. What is precision and accuracy?
Precision means when you pipet multiple times, you get the same volume dispensed every time. Accuracy means that you hit exactly the volume you targeted without any error. You may always be precise, but you might not always hit the right volume. You must make sure you have both precision and accuracy. This is absolutely critical for the outcome of your experiment to be reproducible.
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. 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 spin 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. 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. 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 devise or braking system is required for positioning.
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
Example Stepper linear actuator selection
An application needs 10 N force and need to travel 40 mm at the speed of 5 mm/s.
|Add 50% safety factor to the required force||10 N x 1.5 = 15 N|
|Lead screw pitch||1.22 mm|
|Travel per step||0.0254 mm|
So we need to travel 5.0/0.0254 steps in 1 sec = 197 pps (pulses per second)
The Portescap 26DBM10DXB -L is capable of 25 N force at the required speed of 197 pps, providing additional safety factor for the application. The speed vs force curve is shown below, detailing the ability to increase the speed for quicker dispensing if need.
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.