Aging populations, increased preventative care, and the recent pandemic are driving growth in the healthcare, pharmaceutical, and agriculture industries. These gains have led to increased workloads in laboratories, and motorized tools and equipment are enabling scientists to meet ambitious productivity goals. This article describes the various motors that designers can incorporate into automated machines that prepare, manage, and analyze laboratory samples.
Handheld Electronic Pipettes
Before analyzing a sample, a lab technician must complete a long list of procedures that may include dissolution, extraction, reaction with some chemical species, pulverizing, treatment with a chelating agent, masking, filtering, dilution, and subsampling. Handheld pipettes are used to transfer liquid samples from one test receptacle to another during preparation.
Pipettes typically consist of a system of cams, pistons, and cylinders. When the piston is moved, a partial vacuum is created, and the liquid is drawn into the pipette tip. Pipettes come in a variety of sizes, with volumes ranging from a few microliters to a few hundred milliliters.
Prior to each manipulation, the user must clip on a brand-new tip to ensure that cross-contamination does not occur between samples. At the end of the manipulation, the operator ejects the pipetting tip for disposal or reuse after sterilization. Multi-channel pipettes are also common, allowing lab technicians to perform several fluid transfer operations simultaneously, as shown in Figure 1.
Thanks to low-cost and improved electronics, designers can now integrate a miniature electric motor inside a handheld pipette. Benefits include:
Accuracy and repeatability. An electronic motor allows a pipette to accurately dispense a specific volume of liquid while ensuring repeatable results during each subsequent manipulation.
Cost savings. In addition to improving accuracy and repeatability, miniature electric motors also allow pipettes to handle smaller volumes of liquids than before. That means companies can reduce the amounts of reagents they use during analyses and save money.
Ergonomics. Electronic pipettes require less manual force and hand movement, thus reducing the risk of repetitive strain injuries. Once an electronic pipette is set, the user simply clicks a button to fill, empty and, in some cases, eject the tip.
Productivity. Thanks to built-in automation, electronic pipettes can perform quicker than their mechanical counterparts and users can process more samples.
Flexibility. Capabilities can include a save mode, multi dispensing, and mixing.
Here is an overview of the motor technology options for electronic pipettes.
Stepper motors. Figure 2 shows a linear stepper motor. Also known as a digital linear actuator (DLA), this readymade device comes with a built-in lead screw that interfaces directly with the pipette’s piston to provide linear movement. Stepper motors do not require an additional feedback sensor or encoder for control. Instead, the stepper motor rotates incrementally at each current pulse, making it easier to control. The linear travel accuracy of the motor, along with the possibility for micro-stepping, ensures the very high resolution that electronic pipettes demand. This motor can maintain a position without power, which extends the use of the device.
DC motors. The coreless brush DC motor, shown in Figure 3, is highly efficient. Unlike iron core motors, the rotor is only comprised of a coil and a shaft. As a result, the coreless motor has no iron losses and achieves greater efficiency and acceleration. Because electronic pipettes are battery powered, the increased efficiency allows longer periods of use on a single charge. For pipetting applications, the brush DC motor requires additional components such as belt drives and encoders in order to convert rotational motion into linear motion and control the motor’s motion.
Both the stepper and DC motors come in a small, lightweight package that allows designers to optimize the weight of their devices. However, these motor technologies come with trade-offs. For example, some DC motors are less expensive than linear stepper motors, but the additional components needed for the transmission must be considered. Pipette designers must also consider the potential errors and tolerances associated with adding more links in the chain. Ultimately, motor technology selection depends on the commercial and technical requirements of the electronic pipette manufacturer.
Automated laboratory machinery fulfills a wide range of operations from simple liquid handling to end-to-end sample preparation to final analysis. Typical laboratory automation functions include:
Identification and scanning. Cameras and scanners can identify, track, and record results.
Cap and de-cap. Samples are stored in containers such as test tubes. When samples are processed, the container must be opened and then closed.
Transfer. Tiny samples need to be transferred from one location to the next.
Manipulation. Trays and conveyors need to be moved from one station to the next.
Analysis. Typical diagnostic steps include centrifuge, chemical reaction with reagents, spectrometry, and many more.
Many laboratory automation applications can benefit from integrated miniature electric motors. Advantages include:
Long life. Automated equipment usually works continuously for many years.
Reliability. Because failures or maintenance issues delay diagnostics as well as patient treatment, motor reliability is paramount.
High speed. Fast motors increase productivity and, thus, the number of diagnostics per day.
Precise positioning. Motors control the amount of reagent being placed into the samples. Miniature electric motors provide consistent motion to ensure accurate, repeatable results.
Because some applications have multiple automated axes, designers must choose from a variety of motion technologies, including stepper motors and BLDC motors.
Stepper motors. These motors have an electronic commutation, so there is no mechanical wear. Accordingly, they have a long lifespan and are ideal for lab automation machines. Stepper motors have several magnetized poles, so the current commutates many times during one rotation for stable positions and high torque performance. They are easily driven and deliver high precision without an encoder, making them well suited for applications where positioning is important.
One of the trade-offs with steppers is speed. A high-frequency commutation is needed to work quickly. Due to the inductance effect of the coil, the current needs some time to rise. If the commutation frequency is too high, the current does not have enough time to rise. Therefore, stepper motors are limited in speed and are typically suitable for output speeds below 1,000 rpm. When designing large, automated laboratory equipment, engineers select stepper motors for medium power applications. Stepper motors are also used for moving trays, whereas smaller steppers can be used to control small valves.
BLDC motors. Brushless DC (BLDC) motors combine the advantages of brush DC and stepper motors. As with stepper motors, brushless motors have an electronic commutation for long life. However, BLDC motors have a limited number of magnet poles — generally two or four; so fewer commutations are needed for one motor revolution.
With the same commutation frequency, the brushless DC motor works more quickly thanks to a well-balanced rotor and a strong ball-bearing assembly. In fact, some BLDC motors reach speeds up to 100,000 rpm, making them the preferred choice for high-speed requirements as well as high-power density applications.
BLDC motors are generally controlled via positioning feedback, such as integrated Hall sensors or encoders. Since the rotor position is always known, BLDC motors are reliable and do not lose steps. Figure 6 shows two versions of brushless motors:
Cylindrical motors: These motors typically have longer length versus diameter and are often used in robotic pipettes.
Flat or pancake motors: These motors usually have a larger diameter compared to length. Disc magnets and flat coils are used for this motor design. Flat motors are typically found in robotic arms.
Thanks to their high-power density capabilities, BLDC motors are appropriate for applications requiring high power in a compact package. In laboratory automation machines, brushless DC motors are ideal for reliable and high-volume tasks. They are also typically used in robotic pipettes as well as arms and grippers.
Optimizing Automated Laboratory Equipment
Today’s healthcare, pharmaceutical, and agriculture laboratories rely on increasingly automated and faster equipment to meet challenging productivity demands with high accuracy and repeatability. Miniature electric motors, along with smaller and inexpensive electronic components, are transforming laboratory equipment to give scientists the ability to achieve these goals. Alt hough a single motor technology cannot fit every laboratory application, designers can choose from several motor types — each with their own benefits — to optimize machine performance.
Miniature motor manufacturers like Portescap can work with manufacturers to select the right motor technology to develop unique and customized laboratory devices that will perform with precision and reliability during continuous operations.