Mechatronic forces make waves

A: XTS combines the benefits of rotary and linear motors. Traditional transport systems typically have a fixed pitch, and every product is indexed at the same time. With XTS, the movers can move independently at different rates and distances; they can also move in groups to synchronize with external devices. This allows machine designers to re-think their designs.

Station to station pitch can vary. Throughput isn’t dependent upon the slowest process, as you can have multiple up stations and buffers to maximize production.

Two years ago, we released No Cable Technology (NCT), where we created a new motor module that delivers wireless power and data to the movers. On the movers we have an I/O module with dc inputs and outputs along with analog inputs and PWM outputs. This allows the user to add standard devices like grippers and vacuum generators for handling products. Analog inputs allow you to add weighing or pressure sensors and the PWM output allows you to drive dc motors.

Q: With No Cable Technology, you’re essentially turning a passive mover into an active, single- or multi-axis robot. How does the added mass and center-of-gravity shift of a powered end effector, like a gripper or vacuum generator, impact the tuning of the linear transport system surge control or the overall jerk limits of the track?

A: With every application we review the mechanical data for the tooling and product along with the motion profiles and calculate the expected life of the movers. The force required to move and accelerate in the corners is more than on the straights, so we find the balance between the motion and tooling.

One of the key benefits with XTS is the movers move independently, so you can create buffer zones with movers to move slower around the curve and still meet production rates. Keeping the center of mass as close to the mover as possible is best, but we have lots of applications with large overhung tooling.

The same approach is used for NCT, as there will be added weight. To address this, we have expanded our range of movers and magnet sets over the last several years to cover a large range of payloads. We have four, five, seven and 10 pole magnets sets which provide up to 210 N of peak force.

The Beckhoff rails and movers are used for lower payloads up to 1.5 lb; the Beckhoff rail is mounted on top of the motor module for a narrower footprint.

For the HepcoMotion GFX rail system, we have a very large range of movers to meet the customers’ needs. For typical systems we have a range of three and four rollers, which can handle up to 15.0 lb and up to 100 N press forces.

For non-NCT applications, we have new movers with 40 mm bearings that can handle up to 33.0 lb payloads and press forces up to 2,500 N for the strong mover. For even larger payloads, the GFXr has six roller movers that supports 220 lb.

Q: Since No Cable Technology transmits power through the motor modules via hardware-integrated coils, what are the thermal limitations when running high-duty-cycle actuators, such as continuous capping motors, on a mover? How does the TwinCAT software monitor an individual mover's energy usage to prevent overheating the track segment?

A: The NCT module provides up to 35 W continuous and 75 W peak power. The power for this comes from the same 48-V power to the motor module that is used to drive the mover. Each infeed is rated for 16 A nominal and 40 A peak. The length of an infeed segment is dependent upon the number of movers in this section and the NCT power requirements. We simply look at the layout and add infeed motor modules as needed.

Most of the grippers and vacuum generators we have tested have a peak current draw when turned on, and it drops off quickly. We can manage the energy duty cycle via TwinCAT in real time. We also monitor the temperatures in the motor module for diagnostics and shut down if the temperature is too high.

We have an online view of the track that color-codes the temperature of each motor module, so that we can identify warm or hot spots and then adjust motion profiles to keep the system running within limits.

The first step with NCT applications is typically to get prototype tooling and actuators and do some proof-of-concept testing.

Q: EcoLine motor modules are designed to lower the entry price for XTS. Beyond the physical length of the modules, what are the technical deltas in terms of positional repeatability or peak force density compared to the standard XTS modules?

A: We reduced the cost of the feedback, which allows us to keep 95% of the capabilities while lowering the cost by 45%. We still have full rated power but lose 20% of peak power. The repeatability is +/- 70 µm vs. +/- 10 µm, and absolute accuracy mover to mover is 0.7 mm, which is still good for a lot of applications. This allows us to mix and match motor modules based on the requirement. 

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Buffers or return sections can use EcoLine, and high precision sections can use stand modules. For some machines, we can use EcoLine for all of the straight sections.

Q: Traditional robotics often relies on absolute encoders at every joint. Since XTS uses an integrated measuring system in the motor modules, how does the system handle homing" or state-recovery after an emergency stop when NCT end effectors are in the middle of a complex move?

A: The XTS system has absolute feedback. If the total length of the track is 3 m, the positions of the movers are between 0 and 3000; think modulo positioning.

We also have a mover 1 which has the magnets reversed. On power on, we do a simple µm wake-and-shake to see which mover went backward; that is mover 1.

When the machine is e-stopped, we drop the 48 V to put the modules into a safe torque off (STO) mode but keep the communications active. If a mover is pushed to a different location, we know where it is, as the feedback is still active.

The NCT module on the mover will also lose power during an e-stop, but most end-of-arm actuators retain their state when power is lost. If the gripper was open, it stays open. If it was closed, it stays closed. For vacuum generators, they typically have check values that will maintain the vacuum for some amount of time.

Upon resetting the e-stop the customer decides in software how to recover the system as each machine is different.

Q: Given the data-rich nature of the XTS linear transport system, including monitoring current, position and temperature for every mover, how can the TwinCAT Analytics library distinguish between a mechanical failure, such as a worn roller, and a process-induced anomaly, like a jammed capper?

A: From a machine learning (ML) standpoint, we have the ability to capture large amounts of real-time data to feed and run ML models real time to adjust the process as required. We did a show demo several years ago with two XTS tracks, one with fixed motion parameters indexing between stations as fast as possible and one using ML to optimize the motion to reduce speeds and acceleration while still maintaining the same production rate. The ML track had less mechanical wear on the mover rollers.

We have run and continue to run long-term life-cycle testing on what happens when the mover fails. What we are finding is the movers are lasting beyond the expected roller life. What we have found from experience is that, as the bearing wear or the concentric is loose from a mechanical jam, the first thing you notice is high pitch noise, as the mover goes from the straight into the curve. Once you hear this sound, you always remember it.

In most cases the machine is running fine, and the currents look good, but you can hear it. Even with AI and machine learning, the human ears and eyes are sometimes the best tools.

For more information, visit www.beckhoff.com/xts.