Servo drives expand machine capabilities

Go beyond buzzwords. Augmented reality (AR) and artificial intelligence (AI) are familiar industry terms. Augmented reality aims to use technology to enhance the interaction between humans and machines, while artificial intelligence uses technology to accelerate the ability to create control algorithms.

Both automation methods are eye-catching at trade shows and expos. Manufacturers spend a lot of money and resources to make these technologies work for end users. Behind the bright lights and fancy user equipment, manufacturers have also been working to make machines easier to design and program. With the enhanced user experience, vendors can now make their machines smarter without making user interactions more complex. One of the key elements of these boosted machines is the increased use of servo control equipment.

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Traditional control systems on repetitive motion machines rely on a central drive shaft of cam-driven attachments that perform pattern motion based on the position of the main circulating axis.

The great thing about these early designs was the calculated behavior of the various accessory devices. Depending on the shape of the lobes on the cam, the device will travel the same path each time the drive shaft completes a cycle.

An example of such a machine is a horizontal packaging machine. The machine takes a roll of paper or plastic film, converts it into a pouch, fills the pouch with product, seals the pouch, and ejects the pouch for further processing. A number of stations are defined on the machine that perform specific functions.

Each station has an attachment that uses a cam mounted on a main circulating shaft that runs along the length of the machine. Since each station/attachment moves relative to this central axis, each movement is coordinated in a way that adjacent stations can work in close proximity to each other.

The advantage of this type of system is that the auxiliary device is always in position relative to the main circulation axis. In most cases, the bicycle axle can rotate in either direction and the accessory assembly will remain unchanged in time based on the cam relationship to the main axle.

This is also a disadvantage of this type of system, since all functions are synchronized with the bicycle axle. If a problem occurs during this process, the machine must complete the remainder of the machine cycle before the cam drive returns to its original position.

Another disadvantage of physical cam systems is speed. The cam mechanism relies on springs to hold the mechanism on the cam. At high speeds, the device will tend to move away from the cam surface due to centrifugal force, making the station’s action erratic.

With the advent of servo positioning, it became possible to operate each auxiliary station independently of the main cycle axis through the use of precise feedback and precise motor control. The control principle remains the same as the auxiliary attachment or shaft still follows the position of the main circulating axis, but the cam lobes have no physical contact on the circulating axis. All of this means significant improvements in speed and accuracy.

Additionally, recovery from a sudden stop can be handled in a variety of ways. Before or during restart, the slave axis can be individually restored to match the position of the master axis. By decoupling individual stations on the machine from the spindle, not every station needs to be adjusted during a restart; only the parts on the machine that are not in place do.

The servo system allows the cam profile to be changed without physically changing the cam to a different profile, providing additional functionality. Electronic cam means almost unlimited contouring possibilities.

For example, on a packaging machine, different packaging sizes will require different motion trajectories to complete similar tasks. Large cartons with larger flaps will require the folding unit to move in a different arc and with a slight change in timing relative to the spindle. This can be easily done by loading different cam profiles for each product as part of a recipe change.

All of this is seamless for the end user and happens behind the scenes as they select a product from the list and load the recipe into the machine.

In terms of design, the use of servos usually means there are far fewer moving parts. On more traditional machines, the main circulation axis must be driven by a larger motor because it is responsible for moving all the various components throughout the machine.

By using servos and decoupling the components, not only is the active force significantly reduced, but the various components can be driven by smaller motors that only need to be large enough to drive the additional components.

Drawing many slave axes makes the physical design more compartmentalized, but can also quickly add up to the control side of the equation. Each slave axis requires its own servo drive, and the programmable controller must be sized accordingly. The servo drive communicates through the network, and the number of devices that can be connected to each communication network is limited.

These limitations relate to the amount of information transferred between the master and slave devices and how these constantly transmitted and received chunks of information affect the performance of the device itself. Accurate movement absolutely depends on quickly confirming the relationship between the commanded position and the actual position.

On the software side, each servo drive is added to the hardware tree in the development environment. The software automatically creates a unique label for each drive and notifies the programmer if more axes are added than the processor can handle.

Therefore, it is best to create a software version of the control application before completing the actual control circuit design to ensure that the appropriate processor size is selected early in the design process.

On the hardware side, servos have grown tremendously over the past few years. Not so long ago, every servo motor had a servo driver. Each motor connection requires power, encoder feedback, and sometimes brake cables. Each cable must be routed from the housing to the motor on the machine.

On multi-axis machines, there are a lot of cables to manage, which can be a big reason to use pneumatics instead of servos, even if this results in less accuracy. A recent trend is single-cable technology, where power, feedback and braking are combined into a single cable from the drive to the motor. The advantages of routing a single cable compared to two or three cables are obvious.

Another emerging trend is multiple axes in a single drive package. This technology significantly reduces the impact on the cabinet footprint. Two axes powered by a drive with the same footprint as the single-axis version can be a big help where floor space is an issue. In addition, common input power and protection circuits also reduce this part of the circuit.

An important thing to consider when using this technology is that failure of one axis in the drive package means that both axes will be down as a result. Dual-axis drives also cost a bit more, so while we save panel space, the replacement cost to the end user will be higher.

Another trend in servo technology is placing the drive within the motor. With this approach, servos can be connected via a daisy chain, as a single cable can be used for power and network, with each drive/motor becoming a node on the network. All smart devices are located in the drive/motor device and communicate with the programmable controller in the main panel through the same trunk line. This technology can be likened to devices on a CAN bus, but we’re talking about more than just sensors.

The benefits of these evolving technologies are more compact machine footprints, smaller control packages, greater efficiency, and therefore easier deployment and replication. Machine suppliers are more likely to use servos in their control designs, and the end user is the ultimate winner—a faster process produces higher quality widgets and can be more flexible as trends and desired results change Change machine functions.

Resistance to using new technology has always been related to the learning curve involved in taking new equipment and incorporating it into our designs. It’s human nature to resist change, and we can come up with a million reasons why we can’t or don’t need to accept change. At some point along the way, we either accept the change or are forced to do so when the usual way of doing things is no longer available to us.

The trick is to navigate this path in such a way that you stay ahead of the technology without having to reinvent the wheel. Happily, hardware vendors are making this task easier for us, and the results are well worth pursuing.

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