Automating the Assembly Process
When a major manufacturer wanted to move its assembly of circuit-breaker panel covers from manual operation to an automated process to increase productivity and cut costs, it called Dane Systems, an integrator of automation systems in Stevensville, MI.
The task was to design a system that would position the covers for welding of hinges and other hardware. Not only did the system have to be fast, it had to be highly flexible to accommodate short runs of a variety of panel styles. For a reference cover to test machine performance, the goal was 30 welds in a 33-sec. cycle time. The cycle time included the time required to position the panel, to move welding heads to specific welding points, to perform the welds, and to remove the panel.
The resulting system uses a two-level conveyor: the lower level delivers the panel to the work cell for welding. After the welding is complete, the conveyor carries the completed panel to an elevator, which raises the panel to the higher level for transport to the unloading station.
Two welding assemblies, one at each side of the conveyor, perform the welds. The assembly moves in the X- and Y-axes on Rexroth CKR Compact Modules, while air-over-oil cylinders open and close the welding gun heads.
“The 33-sec. cycle time was aggressive, but not unreasonable,” says Mike Galaske, chief operating officer for Dane. “But the need to accommodate many sizes and styles meant that we also needed motion and control systems that were fast, rugged and capable. We worked with Bosch Rexroth before, so we talked to them about our needs.”
To move the panels, Dane chose the Rexroth TSplus, a non-synchronous conveyor system designed for transporting palleted workpieces. To save space, an over/under configuration was used, with a vertical transfer from the lower part to the upper. “The TSplus conveyor is a flexible system for moving parts,” says Galaske.
Meeting the challenge
The welding assemblies presented a challenge. Using a welding head on each side of the box struck a reasonable balance between productivity and simplicity. “In considering the trade-offs between cost, complexity and productivity, we decided to use two welding heads. It allowed us to reach the cycle time goals while keeping the design simple and clean.”
Each welding assembly is heavy, weighing about 250-lbs. The assembly rides on Rexroth CKR Linear Modules that position the welding head in both the X- and Y- axes. Maximum movement is 60-in. in the
X-axis (side-to-side) and 30-in. in the Y- (in/out) axis. The CKR is a compact module containing two Ball Rail Systems and a belt drive. The use of two rails provides increased stability for heavy loads, with reduced roll, pitch or yaw. The welding heads move smoothly and accurately. In each assembly cycle, the heads are repositioned in short, rapid moves about 15 times, depending on the actual panel. Given the weight of the welding heads, the system provides very fast acceleration and deceleration.
It is critical that the two welding heads never touch, either lightly or, in the worst case, in a catastrophic crash. Using today’s best control systems, machines can now be built with enough intelligence to avoid collisions.
The Dane solution is a perfect example: The CKR modules are driven by high-speed servo motors, in turn controlled by a Rexroth rho 4.1 controller. “The rho controller was a key to delivering machine performance and controlling two welding heads,” explains Galaske. Thierry Dumont, a Bosch Rexroth robotics specialist, explains that the rho controller is built from the ground up for multitasking multiple motion control systems. This ability allows both welding robots to act independently, while still remaining aware of the position of the other.
Finding a solution
“Finding linear actuators that can carry a heavy load quickly and accurately can be difficult. The CKR modules provided the performance we needed,” observes Earle Cooper, vice president of sales and marketing at Dane, “and the Rexroth engineers did a fantastic job of tweaking the servo system to wring every bit of performance out of it.”
For a given payload, the first step in designing any positioning system with linear actuators is to ensure that the linear modules are sized to meet the application’s speed and acceleration requirements. Changes in the weight to be carried can mean the selected system is now undersized or over-sized.
Dumont explains, “We use the acronym LOSTPED to describe the various characteristics that must be evaluated in selecting and sizing a linear motion system. LOSTPED represents load, orientation, speed, travel distance, precision, environment, and duty cycle. The interplay of all the different variables must be considered. Different variables may have greater weight in certain applications, but ignoring any variable can put your design at risk.” Rexroth engineers have an in-house program called FastAxis to help derive the best solution.
Once the proper CKR modules and servo motors were selected, the motors need to be tuned to prevent such things as overshoot during travel. If the linear module has to resolve its proper position through several back and forth movements, efficiency and speed suffer. The rho controller has built-in tuning functions to allow motor characteristics to be adjusted through software.
Bosch Rexroth supplied the complete robot assembly, ready for mounting of the welding head. While much of the development focus was on the conveyor and linear motion systems, Rexroth supplied other important components as well. The welding guns are raised and lowered pneumatically using Rexroth pneumatic components, and Rexroth aluminum framing provided structural elements and machine guarding.
“It’s good to work with a company like Bosch Rexroth,” says Cooper. “The Rexroth engineers apply their expertise with uncommon proficiency, always with the aim of helping us improve our system.”
In the end, the required cycle time was met. The customer was extremely satisfied and had significantly increased its productivity in panel assembly.