maxon Story
Why drive system design is vital for automated liquid dispensing systems
Automated liquid dispensing machines rely on high precision and repeatable performance, and operating within a laboratory setting, they must also occupy a compact footprint. The electric drive system responsible for the motion of the pipetting head is a crucial component to meet these needs. Sandro Walter, maxon’s Business Development Manager for Laboratory Automation, presents the key design considerations.
Automated liquid dispensing systems are vital across fields ranging from drug development through to medical diagnostics, as well as quality assurance in food and beverage testing. Handling small volumes of liquid for purposes such as analysis or sample preparation, these machines rapidly deliver precise microliter (µl) quantities of liquid with a contamination-free approach.
The features of each liquid dispensing system, including its capability to handle specific types of liquid as well as its degree of automation, depend on the specific requirements of the end users of each application. However, essential across all automated liquid dispensing machines is the reliance on a coordinated motion system, responsible for aspirating, moving, and dispensing the media with high throughput and repeatability.
The liquid handling head comprises multiple channels, controlled by a multi-axis drive architecture. Two individual motors control movement of the channels in the X and Y directions via linear stages or belt systems, while individual channels move up and down along the Z axis, each driven by its own drive system. Each motor includes a gearhead and a servo controller per channel, coordinated by a higher-level motion controller or master PLC.
The output of each motor is connected to a mechanical transmission specific to the OEM’s design, such as a timing belt, rack-and-pinion, or spindle drive. To aspirate and dispense the liquid, an additional drive controls the pipetting or syringe pump. This design is typically based on a motor-driven syringe or positive displacement pump, powered by a brushless DC (BLDC) motor, gearbox, and controller, commonly connected to a lead screw or ball screw mechanism.
A low-backlash gearbox, like the maxon strain wave, enhances precision and stability.
The importance of torque density
A general requirement across automated liquid dispensing system development is to design a machine that can cover as many application requirements as possible while still achieving the precision, throughput, and robustness required for target applications. For the OEM, this approach expands the market for each design, while for the end user, a machine with broader capabilities reduces the amount of equipment they need, saving on cost and, crucially, floor space.
Concerning drive system specification, this means that high torque density is important to enable a compact design, allowing a configuration such as eight channels in parallel within a smaller pipetting head. For this reason, to drive the pipetting channel in the Z direction, as well as the syringe pump, brushless DC (BLDC) motors are usually specified.
By eliminating the physical brush–commutator system inherent to a brushed DC (direct current) motor, a BLDC design reduces mechanical losses, friction, and heat generation, achieving greater torque density and enabling a smaller footprint. Motor design techniques such as optimised copper windings, high-quality magnetic materials, and advanced electronic commutation control, contribute to improved torque density and energy efficiency.
An 8mm diameter BLDC motor is commonly used to actuate the pump. This size motor is also used to control channels with a 9mm grid spacing, while a 16mm motor is required for 18mm pitch configurations.
Combined with a BLDC motor, attention to gearbox design can also improve torque density and efficiency with features such as low-friction bearings that reduce resistance under load and increase support for axial and radial forces, as well as using materials that enhance thermal dissipation. To drive the syringe piston pump, the spindle can also be upgraded to ceramic material rather than steel, which minimises friction, increasing efficiency and extending lifetime. Additionally, ceramics don’t require lubrication, reducing the contamination risk — an essential benefit in laboratory environments.
Throughput and precision
Overcoming forces such as friction also enables the motor to accelerate faster for the same current input, maintaining a compact footprint. The quicker an automated liquid dispensing machine can operate, the higher its throughput, which can be vital to reduce patient waiting times for diagnostic results or to increase productivity in industrial laboratories.
As well as offering higher torque, a BLDC motor can operate at significantly higher rotational speeds (rpm) and achieve more dynamic bidirectional motion profiles, compared to its brushed counterpart.
Although throughput is often important, it must be matched with precise motion control, especially considering the dynamic nature of the system’s motion profile. While positional precision is important for controlling the pipetting head, it is critical for the pump, directly determining the dispensed volume. Every degree of variation in the plunger’s linear speed can cause measurable deviation in the delivered volume. The greater the repeatability and reliability of dispensing, the faster the machine can safely operate, up to the limits of the motor–gearbox–lead screw combination.
To control the pump, we usually specify a servo position controller in combination with a high-resolution encoder, which directly determines plunger displacement and the resulting volume of liquid dispensed. With this system, repeatability can reach ±3 µm (micrometres) deviation across a typical operational lifetime of 10,000 hours. Although the motion requirements for the pipetting head are less stringent, a position controller is also specified here for consistent multi-axis coordination.
To ensure long-term precision and repeatability, mechanical design aspects must also be considered. For example, with a linear spindle drive system, axial play can develop over time due to wear. To prevent this and maintain repeatability, a ceramic or preloaded ball screw can be used. Similarly, a gearbox design that minimises backlash, such as a strain wave, further improves precision and system stability.
Customisation and sub-system development
The ability to customise drive components and subassemblies is critical to achieving specific performance goals, and this is a typical requirement when working with automated liquid dispensing OEMs. The foundation of such collaboration is a clear understanding of the design objectives, such as repeatability targets, torque demands, or motion speed requirements. At this stage, it’s essential to fully assess the interaction between mechanical load, drive characteristics, and control dynamics, including feed force and cycle speed. This determines which parameters are fixed and which can be optimised.
Typical areas of customisation for liquid dispensing machine design include motor winding adaptation to match available supply voltage and target speed, gear ratio selection, and lead screw pitch optimisation.
To enhance system integration, it’s advantageous to develop the drive solution as a complete electromechanical sub-system, including the motor, gearbox, encoder, controller, and transmission components such as the lead screw or pulley. maxon can also manufacture a complete pipetting channel assembly based on the OEM’s design, with engineering refinements to make it lighter, stiffer, and easier to integrate.
The influence of the drive system on overall liquid handling performance means that careful co-design and integration as a sub-assembly, or even as a complete cassette, is vital. Incorporating drive system engineering early in the design phase ensures all mechanical and control considerations are addressed, enabling faster time-to-market and improved system efficiency.
Have questions about optimising your dispensing system’s drive design? Our engineers are here to help. Feel free to get in touch for expert guidance 01189 733337 or sales.uk@maxongroup.com.