A tier one supplier to the automotive industry, ArvinMeritor recently embarked on a project to build an automatic torque wrench for tightening bolts on disc brake calipers used in heavy vehicles. Each caliper has a total of eight bolts, and the machine is required to position a single power screwdriver head over each bolt in turn, following a predetermined sequence, and tighten them to a preset torque. The machine also has to be readily programmable to handle calipers of different sizes, different tightening sequences, and to perform multiple passes over the same caliper so that the bolts could be tightened in stages.
At the heart of this robotic wrench is an X-Y motion system that transports the floating screwdriver head along the required path between the bolts, complemented by a Z-axis slide to allow the screwdriver to be engaged and disengaged with the bolts. Following an assessment of the available options, the design team decided that an X-Y stage from Schneider Electric's Berger Lahr range best met its requirements, prominent among which were robustness and long-term reliability. Driven by ac servomotors, this stage provides travels of 300mm (X-axis) and 200mm (Y-axis). A simple slide system is used to position the free-floating screwdriver head on the Z-axis.
On a typical brake caliper, the automatic torque wrench completes the entire tightening process in less than 50 seconds. This is substantially less than the time required for the previous manual tightening process. In each complete tightening cycle, the head travels a total distance of a little over one metre.
For overall control, the designers selected a Schneider Electric Telemecanique branded Twido programmable controller, a small and simple-to-programme unit that interfaces easily with the Berger Lahr drives. The operator interface for the machine is provided by another Telemecanique product, a Magelis XBTN electronic HMI panel with four-line text display. This gives real time information about the machine's status and its position in the operating cycle, as well as providing facilities for setting, changing between product types, and troubleshooting. Communication between the drives, the PLC and the HMI panel is implemented using the CANopen fieldbus system, which is integrated into all of the devices.
The machine's developers found the equipment from Schneider Electric easy to assemble and integrate with the machine's other mechanical components. And, although there were a few teething problems with the drive programming - mostly because this was the first time the company had used this particular system - these were soon resolved. And, as the machine was delivered fully tested, commissioning on site was both fast and straightforward. ArvinMeritor seems very happy with the outcome, as project lead engineer Gareth Cantle observes:
"Not only is [the machine] fast and efficient, it also offers uniform and precisely controlled tightening torque that can't be achieved by manual tightening methods. We believe that these advantages are very important to our demanding automobile industry customers."