Indexing systems remain the norm in many assembly applications, and those that involve dispensing a liquid into a product container are no exception. However, in many cases, continuous motion assembly may be a better way to go. This assembly technique offers faster speeds, a smaller footprint, and greater quality. Let's take a closer look at the differences between indexing and continuous motion systems — and which type might be right for your next assembly task.

Continuous motion vs. indexing

The primary differences between continuous motion and indexing assembly involve speed and precision. With indexing motion, products being processed start and stop at each station on the machine. With continuous motion, multiple processes can occur without interruption for every cycle, so that the jobs effectively overlap. The result is a faster process: 400 to 1,000 parts per minute (ppm) are possible with continuous motion versus up to 250 ppm with indexing motion. In addition, the tooling never loses contact with individual components, so part alignment is always maintained.

When to choose indexing motion

In most applications that require a complex assembly and speed of less than 100 parts per minute, an indexing system is likely to be more cost effective than a continuous motion setup. So if the application doesn't require high speeds — or the product can't handle it — an indexing system might be the appropriate choice. However, even for faster setups, many manufacturers don't evaluate the potential benefits of a continuous motion system, and ultimately specify an indexing system — simply because the technology is familiar.

When continuous motion is preferable

In applications where the target total net output exceeds the capability of a single indexing machine, continuous motion is probably a good choice. Why is the speed limit of an indexing system lower than that of a continuous motion system? Simple: It comes down to dwell time. The tooling of an indexing system can only complete its task while a part is stationary, which is approximately two-thirds of the time. The part is in motion the rest of the time. Therefore, an indexing system is subject to the constraints of target output speed. This constraint becomes particularly detrimental when working with viscous fluids that can't be forced out too quickly without encountering accuracy issues. For example: If output is 60 parts per minute, there is one second available for each part, but only two-thirds of a second to complete the task while the part is stationary.

Continuous motion, on the other hand, takes this time element out of the equation by using a platform that can be expanded to whatever the output dictates. It's simply a matter of sizing the continuous motion equipment appropriately. Other advantages of continuous motion include:

Smaller footprint

If an application requires multiple assembly processes with an output of 300 ppm, this can be accomplished by building five indexing machines, each with an output of 60 ppm. In contrast, a single continuous motion machine can execute the same job.

Less labor

In the same scenario, five indexing machines each require their own operator, whereas the continuous motion machine requires only one operator.

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Quality and cleanliness

The start-and-stop indexing motion itself can create issues with spillage of liquids and powders, which not only causes accuracy challenges for the end product, but also can damage machinery. The smoother movement of a continuous motion machine makes such problems far less likely.

Control system considerations

Control systems for indexing machines typically have higher energy requirements, more hazards, and longer design and build times than continuous motion control systems. That said, continuous motion control also poses challenges.

Control systems engineers must pay careful attention to details regarding sensor usage, vision and lighting, machine timing devices, PLC performance, and operator interface design. One method of integrating these components involves using remote I/O (RIO) in conjunction with local I/O. Remote I/O offers several benefits. Compared to local I/O, RIO supports a much wider range of input, output, and specialty devices, making sophisticated continuous motion machinery viable.

RIO also facilitates distributed machinery designs — where a machine is built on more than one frame — by consolidating software and operator interface with diverse mechanics and processes. Systems requiring higher performance may be designed with an industrial PC. Some RIO hardware operates considerably faster than typical local I/O, enabling very high performance continuous motion designs.

For more information, visit haumiller.com or call (847) 695-9111.

Video: Fluid dispensing

In this video clip, stepper motors from Nippon Pulse America Inc., Radford, Va., are configured into a fluid dispensing syringe application. The unit is driven by both linear and hybrid stepper motors, and may be programmed to perform a wide array of motion profiles. To view the application, visit motionsystemdesign.com and click on Videos from the home page.

Product gallery: Components for dispensing applications

Linear actuator suits high-precision dispensing

Haydon Kerk Motion Solutions Inc., Waterbury, Conn., introduces the Haydon RoHS-compliant IDEA External Linear Programmable Actuator. This size-17 stepper motor linear actuator offers full programmability through a graphic user interface and is capable of up to 20 in. of stroke length with a variety of screw pitches allowing fine resolution linear motion. The actuator is suited for precision fluid dispensing and other applications requiring precise linear motion. The IDEA's compact electronic drive and fully programmable control is integrated into the linear-actuator package. Inputs and outputs can be simulated in software before connecting actual I/O hardware. For more information, visit idea-drive.com.

Stepper controller includes complete control system

Techno Inc. Linear Motion Systems, New Hyde Park, N.Y., announces the iMC-P1 multi-axis stepper controller. This single to four-axis controller has drives, a power supply, I/O, enclosure, and software, and is suitable for use in dispensing. In addition to point-to-point dispensing, the new controller can dispense fluid during 2D and 3D contouring by using a loop feature that allows for arrays of points or locations at which to dispense. This is especially useful in applications such as potting of circuit boards and sealing complex 3D surfaces. The system includes eight 24 Vdc inputs and outputs as well as a durable desktop enclosure. Step size can be changed, via dip switches, from full to half to micro-step. Each controller can perform linear and circular interpolation; PalPC motion control software is standard. Free LabVIEW drivers may be downloaded from the company website. For more information, visit techno-isel.com.

Adjustable speed drives feature micro dial

Adjustable speed drives from Zero-Max Inc., Plymouth, Minn., include a new Micro Dial feature that provides accurate and repeatable speed settings. When the drive is used in a seeder cart application, for example, different settings can be made more accurately for dispensing corn, wheat, and oats. The drive can be used in a similar way to distribute fertilizer. In these applications, the drive operates off a chain arbor from the wheel drive, which is set by the ground speed on the seeder cart or fertilizer cart; the seed or fertilizer dispensing rate is set by the adjustable speed drive. Accepting input speeds from 0 to 2,000 rpm, output speed changes can be made quickly and easily, whether the machine is running or not. These drives can be used as a prime mover connected to a motor, or as a secondary drive connected to a shaft in the machine's drive line. When used as a prime mover, the drive acts as a variable speed device. When used as a secondary drive, it becomes a variable ratio device. For more information, visit zero-max.com.