Properly adjusted valve clearances are necessary in IC engines so that the valves will open and close as they should, because clearance distance affects valve timing, and thus engine performance. As it is not cost-effective to achieve the required clearance by machining the mechanical parts, these clearances are adjusted by inserting appropriately sized shims. It is common industrial practice to use feeler gauges to measure these clearances manually.
Bangalore, India based Captronic Systems, has successfully automated this procedure using National Instruments (NI) data acquisition (DAQ) devices, along with NI LabVIEW, a number of LVDT probes for clearance measurement, and four pneumatic systems for mechanical alignment of the cam shaft and cam cylinder with the tappet top.
System configuration
The test system, which Captronic created for four-cylinder engines, consists of two fixture stations, one to measure the cam shaft lobe profile and the other to measure tappet top positions – both with respect to a master, which has the desired cam shaft lobe and cylinder tappet top dimensions. The system contains ten LVDT probes, four for tappet top measurements, another four for cam profile measurements and two for cam base measurements. Four pneumatic systems bring the LVDT probes into contact with the components whose dimensions are to be measured, while light indicators designate machine statuses like ‘no air’ and ‘error’ and indicate initial conditions. Twenty-four digital input feedback signals confirm each action and read the machine status
The system uses the NI PCI-6220 for LVDT probe measurement and the NI PCI-6527 isolated digital I/O device to monitor machine status and to control the mechanical sequence required to bring the LVDT probe over the cam shaft and cylinder tappet top. The entire system was implemented within 23 man-days, an achievement attributed by Captronic to the power of LabVIEW, which helped the company complete the system efficiently, in such a short time span.
An experienced employee takes approximately one minute to measure and select the appropriate shims manually. Error in manual measurements depends on the employee and his fatigue level. With the new test system, the entire measurement and shim selection process takes less than three seconds.
The system software is divided into manual and auto mode of operation. In manual mode, the system carries out all the mechanical actions guided by the user. Every action has its own interlock with past actions, as well as with some proximity and read switch status. In manual mode, all the LVDT probe measurements are referenced against the master, and LVDT probes are adjusted to read within some predefined value in one direction to avoid polarity mismatch.
In auto mode, all operations are carried out automatically after the ‘cycle start’ user acknowledgment. Shim sizes are calculated automatically after LVDT probe measurement and processing. The auto mode includes a provision to stop the cycle at any time using an emergency stop button and reset the current cycle to restart. In auto mode, both the fixtures are operated in parallel. The system guides the user through the message display in the panels and self-diagnostic features indicate failure of any read or proximity switches to the user. The system generates reports for employee productivity monitoring as well as selected shim values at designated times.
Prior to this development, Captronic had employed a PLC-based shim-selection machine, which did not provide the signal processing capabilities and user interface requirements required by the application. The new test system, based on NI data acquisition hardware and LabVIEW software is highly flexible, rugged and cost-effective, increasing assembly line productivity some twenty times and guaranteeing measurement accuracy to within ±10 microns.