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Enabling high-precision dispensing with advanced motion control

July 1, 2012
Correctly positioning a dispensing head while coordinating material flow is a daunting task. Sophisticated motion-controller programming — including realtime kinematics and position synchronized output — is rising to the challenge.

The dispensing of both organic and inorganic materials onto various substrates is increasingly employed in a variety of processes and industries, from conformal coatings and printed electronics to biological printing and additive manufacturing. As technologies develop, many of the end products are expected to revolutionize their industries. For example, additive manufacturing is enabling engineers to produce uniquely shaped parts that are stronger and lighter in weight than components made with traditional subtractive machining processes, such as drilling and cutting. This is crucial for industries such as aerospace, where part strength and weight reduction are paramount design objectives.

Other advanced applications involve printing functional and flexible electronic devices, displays, sensors, ,photovoltaic panels, and medical structures. In fact, researchers in the medical industry are now printing organic structures such as bone scaffolds to help heal severe fractures. In the future, functional organs may be printed from individual organic cells. All of these technologies and processes rely on extremely accurate dispensing methods.

In a nutshell, the dispensing of compounds and coatings requires a motion system to move either the printing head or the part being processed. Overall system accuracy and throughput are vitally important to creating complex structures with a commercially viable process. As 3D part geometries become more complicated, programming the motion controller to correctly position the dispensing head and coordinate material flow is daunting. That said, some motion controllers are up to the challenge — more specifically, those with realtime kinematics and Position Synchronized Output (PSO), which significantly reduce development time and improve dispense consistency.

Coordinate space programming

Dispensing fluids or coatings over complex geometries requires a motion system with multiple degrees of freedom to keep the print head in line with the part surface. Programming individual axis motion is anything but intuitive and is often performed offline, using a software package configured to support a specific layout that separates the part geometry into small discrete Cartesian coordinate steps, and then calculates the resulting linear and rotary positions. However, this approach is highly inflexible, as the relationship between the program and part is lost.

A more efficient approach is to calculate the transformation in realtime on the controller itself. Similar to the kinematics used by robotic controllers that transform Cartesian coordinates into angular joint positions, this allows users to program in part coordinate space. The mathematical relationship of the part’s geometry to the individual axis motion is run in a specific task on the controller.

Here’s how it works: The controller converts the commands to individual axis motion in realtime, which moves the dispense head to the correct spatial orientation. The result is a direct correspondence between the lines of the program and part geometry. Additional path modifiers can be easily added, such as specifying an offset at runtime to compensate for part misalignment, or regulating speed to minimize accelerations that occur from processing a small radius on the part.

Dispensing in realtime

An optimized motion profile can also be used to directly trigger the dispense head in realtime. When dispensing droplets or fluids, the design must maintain a consistent volume flow over a given area to maintain quality, because too much or too little material may distort or weaken a structure, or compromise a coating overlay. When dispensing at a fixed rate, constant vector velocity is required to maintain consistent deposition volume. However, as contour geometries become more complex, significant speed and throughput is sacrificed to maintain this constant velocity.

Utilizing PSO eliminates the constant velocity requirement, as it interfaces directly to the print head to trigger or adjust flow in realtime based on actual position feedback in 3D space. When dispensing droplets, PSO can be configured to trigger the print head at fixed preprogrammed intervals. If irregular spacing is required, array-based triggering allows the user to define specific dispensing patterns based on position. During fluid dispensing, PSO controls a high-speed analog output to manipulate flow rate based on actual position and velocity feedback. In each case, because the dispensing rate is tied directly to encoder feedback in 3D space, throughput and flow consistency can be significantly increased regardless of velocity and acceleration.

To fully leverage these advanced control features, the motion controller must be paired with high performance linear and rotary stages capable of traversing complex geometries. For example, contouring often involves frequent direction reversal of the axes. Direct-drive motors are preferred because they exhibit none of the backlash, windup, or stiction associated with screw-based translation systems. What’s more, direct-drive motors are also capable of higher acceleration to reduce process time, and their noncontact design provides maintenance free operation.

For more information, call (412) 963-7470 or visit

Linear actuator streamlines large format printing

The evolution of large-format digital printers continues to reshape the printing and publishing markets. These mammoth machines are used to print everything from banners, posters, signs, and photographs to proofs, drawings, and textiles. As digital printers continue to improve in terms of cost, performance, quality, and speed, they are beginning to replace traditional analog sign and display technologies, such as screen-printing.

One company specializing in the design and manufacture of large-format digital printers is Gandy Digital, based in San Antonio, with manufacturing facilities in Oakville, Ontario. The company’s Pred8tor model is a hybrid ultraviolet (UV) flatbed and roll-to-roll machine that prints high-resolution images on a 4 x 8 ft sheet of flat media at speeds not possible with earlier designs. The machine achieves its high speeds and superior print quality using new print head technology, streamlined software algorithms, an iPad user interface, and advanced motion control components. An automated head-cleaning system eliminates the messy task of cleaning print head nozzles before printing, while a vacuum surface on the UV flatbed keeps rigid or delicate substrates perfectly flat for precise printing.

“Our new machine prints 900 dpi, photographic quality images with inline white or clear, and is the first commercially available, high-speed UV true flatbed grayscale printer,” explains senior software engineer Bryan Hackney. “Pred8tor will print on a large sheet of rigid material up to two inches thick.” Precise motion control is central to achieving the Pred8tor’s high print quality. The two main motion axes include the print table, which moves forward and backward, and the print head, which moves left and right. In earlier printer generations, linear motors were used on both axes. Linear motors on the highspeed print head axis work as expected, but cause problems on the table axis. For one, this approach necessitates a linear encoder — adding several hundred dollars to the bill of materials. The linear encoder is also difficult to install and service on this axis, requiring additional, downtime during maintenance. “In some of our earlier printer designs, the linear motor was not quite able to hold its position, which introduced a small amount of system vibration,” says Hackney. “We knew there had to be a better solution for moving the table axis, and we wanted to build this design into the Pred8tor from the beginning.”

After exploring several options, Gandy’s engineering team turned to motion component supplier Bell- Everman Inc., Goleta, Calif. The company’s engineers worked with Hackney and his team to address cost, design simplicity, and minimizing the inertial mismatch of earlier printer generations. The Bell-Everman team recommended its ServoNut Power Module, which mates a high-performance NEMA 23 motor directly onto a zero-backlash precision ballscrew. In this design, the screw itself remains stationary while the nut is rotated to achieve linear motion, allowing higher speeds and longer strokes than are possible in traditional ballscrew applications. A simple rotary encoder supplies position feedback.

Compared to using a linear motor, the ServoNut’s low inertia, high-force driven nut design offers greater load capacity, acceleration, and speed, in addition to easier installation and lower cost. The ServoNut is driven using a jerk-limiting algorithm, which results in extremely smooth acceleration and deceleration while reducing forces. The algorithm is not required for operation, but results in less internal vibration in the machine, further boosting print quality. For more information visit

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