Andrew Wilson, Editor, [email protected]
In the manufacture of automotive engines, the assembly of pistons is vitally important. Many engine designs require the tightening of two piston rod bolts to the crank shaft at predetermined torque settings. Typically, an automated mechanism will guide a torque gun to the bolt position and tighten the bolt to the required torque force. The monitoring of the bolt position and torque tightening force is critical to ensuring a high-quality engine assembly process.
Verification of the applied torque is achieved through the torque gun’s on-board transducers’ output signal interfaced to a PLC and the operator display panel. In the event of a cross-threaded bolt, the PLC will write the fault information, including the position of the incorrect bolt, to an RFID tag attached to the engine-block assembly. The assembly is then diverted to a repair bay for operator intervention. The operator manually guides a torque gun to remove the bolt and installs a new bolt to the specified torque setting.
Because errors may occur during the manual process, the operator must loosen and retighten every bolt for each piston. “Monitoring the number of times the gun is used to loosen and tighten these bolts should ensure each bolt is properly tightened,” says Ross Rawlings, president and CEO of Radix Controls (Oldcastle, ON, Canada; www.radixcontrols.com). “Unfortunately, because the operator could theoretically loosen and tighten the same bolt multiple times, it is often difficult to determine that each and every bolt position has been properly tightened.”
To overcome operator error, a system must monitor both the location of the tightening gun and the torque applied. The system that Radix has developed-known as Tool Tracker-uses a combination of Data Matrix-like codes (used as tool tracking targets), PLCs, and machine vision.
During the setup process the system is taught the positions of each bolt that eventually might need repair. As each engine enters the repair bay it is precisely fixtured. The operator reviews the status of the engine’s connecting rod bolts and proceeds with the repair. As the operator is loosening and tightening the bolts, the Tool Tracker is monitoring the location of the torque gun. Each tightening event is recorded together with the x, y, and z position of the gun. Before the engine can be released, this information is compared with the taught-zone information.
To properly guide the operator to the location of any bolts that may need to be loosened and retightened, an overhead image of the top of the engine block is digitized into the system’s host PC with a FireWire camera. Using bolt positioning data read from the engine’s AutoCAD file and the overhead image of the engine block, the user is then presented with a display screen that shows the connecting rods and bolts that must be loosened and retightened superimposed over the image of the engine block.
“To ensure the operator tightens the correct bolts,” says Rawlings, “it is vital to monitor the position of the tightening gun as it moves in 3-D space around the engine assembly.” To do this, an encoded target or Data Matrix code is mounted to the head of the toque gun. By monitoring the tool target using two or more cameras, the exact yaw, pitch, and roll of the torque gun head can be computed in x-y-z space using triangulation software running on the system’s host PC (see figure).
Because the engine assembly’s positional coordinates are fixtured to the part nest, the system then automatically tracks the gun, guiding the operator to the correct engine assembly location in the camera’s field of view. When the correct location is reached, the system prompts the user to perform the correct task. As each bolt is loosened and retightened, the system again measures the torque applied and provides feedback on the system’s display monitor. If the bolt is cross-threaded, it can be immediately removed and reinstalled at the same rework station. The system verifies that the correct bolt sequence, position, and torque force have been applied to the assembly process.
“To increase the throughput of engine-block assembly repair,” says Rawlings, “it is possible for a number of operators using tools with station-specific targets to simultaneously work on individual engine assemblies, and maintain the 3-D tracking position of each operator’s tool.” While the system is currently deployed in the automotive industry, it could equally well be used in similar aerospace or assembly applications.
