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Better Motion Control for Faster, More Economical Surface Treatment

July 5, 2019
Sturdy design and smart components yield affordable, high-performance shot peening

For a certain type of highly engineered components — turbine engine blades, for example — failure is unacceptable. To reduce the possibility of metal fatigue, manufacturers use a surface-treatment technique called shot peening, in which a hail of 0.014-in steel balls creates microscopic dents into metal parts’ surfaces, strengthening the material and extending its performance life.

This process was once performed manually. Now, thanks to companies like Innovative Peening Systems, shot peening is fully automated. IPS combines a proprietary Intelligent Motion Control software with Siemens Industry Inc. motion control technology to create reliable, high-performance machines at an economical price.

It may seem that surface deformation would degrade material strength, not enhance it. In reality, the rounded dents generated by shot peening convert tensile stresses, which tend to compromise material integrity, into compressive stresses that hold the material together. The shot-peening effect extends into the material roughly three times the depth of the dents.

Most automated peening machines leverage computer numerical controlled (CNC) technology. CNC technology is designed for high-precision applications like milling and diamond turning, however, and offers performance somewhat beyond what’s necessary in this, essentially, high-tech sandblasting operation. The IPS designers, working with Siemens, realized that by using alternative intelligent motion they could produce a shot-peening machine that would perform just as effectively, and far more economically.

Robust design — In a shot-peening machine, a reservoir introduces steel shot to a compressed-air stream, passing it through a nozzle directed at the part. The process takes place inside an enclosure that protects both operator and machine — but only to a point. A typical machine contains about 2,000 pounds of shot, which amounts to hundreds of millions of particles flying around at high velocity. The potential for damage to components exposed to the shot makes the punishment inflicted by caustic washdown seem mild.

“The 0.014-inch shot is the perfect size to make its way into the seal of any kind of moving part and lock it up,” pointed out Nick Hart, electrical engineer at IPS. “Metal shot will get inside anything it can, and short it out electrically.”

The IPS team realized early in the design process that the solution was to place the motion-control elements outside of the enclosure. The resultant six-axis nozzle manipulator consists of a three-axis (x-y-z) gantry that rides above the machine and drives a rigid arm tipped with a nozzle. The arm extends into the enclosure through a single hole with a carefully designed seal. Everything else is protected.

The x-y-z motion of the gantry essentially provides coarse positioning for the arm. To ensure that the stream of shot strikes even conformal surfaces at normal incidence, the nozzle itself provides two additional degrees of freedom: rotation about the x-axis (the A-axis) and the y-axis (the B-axis.) The actuators for the A-axis and B-axis are sealed within the solid housing of the arm; the motors and drives remain outside of the enclosure. The final, sixth degree of freedom lies in an axis that rotates around the part itself (the C-axis.)

Rather than redesigning each machine from scratch, IPS started with a standard motion platform, adjusting the enclosure for a particular project. In the case of large parts it might gang together multiple nozzle manipulators. “We used standardized components that have been proven in the industry, but we customize every enclosure to fit the customer’s product,” according to IPS president Dan Dickey. “They get a machine that fits their part, at a good price, but it’s not a new invention for us every time we build a machine.”

Designing for performance — The specifications called for the nozzle manipulator to move at a maximum speed of 500 in./min. On the face of it, such a requirement is more demanding than challenging. The problem is inertia; fully charged with shot, the assembly weighs around 2,000 lbs. It requires a lot of muscle to accurately position that much mass at 500 in./min.

To solve this issue the team turned to Siemens again. The selected system combines Sinamics S120 drives in booksize format and servomotors from the 1FK7 family. Although the S120 drives feature onboard memory and processing power, the IPS team chose to use them in a centralized control architecture, in part because the components were already grouped together on the nozzle manipulator carriage and in part to provide additional protection from contamination.

If Sinamics supplies the muscle, Simotion provides the brains in the form of the D435 drive-based controller. “It allows us the flexibility to create custom software [IMC] for the shot-peening application, whereas some of the available alternatives do not give you a lot of freedom,” Hart said.

The milling and turning applications typically performed by a CNC machine are inherently rotational, while shot peening is raster-based with a different coordinate system. As a result, engineers using CNC technology for shot peening must write additional code to adapt the system. Motion control with the Simotion D435 enabled the IPS team to develop its shot-peening software directly, simplifying engineering and enhancing performance.

“In some of the other applications, you are trying to take a product that was designed for a milling machine and teach it how to run a shot-peening machine,” according to Hart. “This time, we just designed a shot-peening machine from the ground up.”

The Simotion system stores parameters on a removable memory card that can be swapped out to update software instantly. The controller also offers trapezoidal motion profiles, compensating for the inertia of the nozzle manipulator at high speed by providing controlled acceleration and deceleration. This approach improves performance while reducing motor wear and, consequently, points of failure.

Of course, the most common source of failure is cabling. Connecting the components with Siemens Drive-CLiQ helped create a reliable solution. An Ethernet backplane based on a 100 Mb/s serial interface, Drive-CLiQ automatically captures nameplate data and validates components, speeding device integration. In the harsh, shot-peening environment, Drive-CLiQ offers other benefits. Manually terminating cables requires closing the open connectors on the factory floor, always a risk for contamination. Drive-CLiQ connectors are factory pre-sealed, which speeds commissioning and gives engineers one less concern.

Total technology — Ultimately, the machine design adopted Siemens solutions throughout. Working with a single vendor simplified ordering and integration, as well as reduced cost of ownership. “Initially, we only decided to go to Siemens for that one particular machine, but after we saw what it could do, it pretty quickly turned into something that we are going to use in several applications,” said Hart.

In addition to meeting the speed spec, the system positions to 0.001 in., repeatable to within 0.005 in.  And, IPS has multiple machines deployed. “The first machine has 18 months in the field,” said Hart. “I just talked to the customer and they say the machine is repeatable, accurate, and reliable. They have no complaints.” This is noteworthy because the machine is operating 16 hours per day, five days per week. Others are running around the clock, five days per week, and Hart has heard of no problems, thus far.

In addition to providing robust, intelligent, accurate motion control technology, Siemens supplied the engineering assistance that helped springboard IPS to success with their machines. “The technical support was really outstanding. Anytime I had a question or problem, I could pick up the phone and find someone to help me work through it, or I could go to their facility and simulate the problem on some of their equipment,” Hart said.