Multi-Axis Control

ANA-DIGI pitches in for airscrew shot peener

ANA-DIGI Systems was contracted to convert a manual shot-blasting machine into a fully-automated four-axis system for precision shot peening of aluminium airscrew (propeller) blades.

Multi Axis

The existing framework of the manual system required extensive reinforcement in order to make it rigid and robust enough to withstand the stresses imposed on it by the automated servo controlled system.

System requirements

The basic system requirements to successfully automate this task were:

  • Plot the blade curvature deviation from a straight line in order to enable the shot-blasting head to move upwards or downwards to maintain a constant distance from the blade surface.
  • Plot the screw of the blade to enable the system to rotate the blade during the process to maintain a constant shot-blasting angle to the blade surface.
  • Rotate the blade by 180° to expose the opposite side and reverse the above parameters including any dimensional deviations.
  • Shot-blast inside the blade end at the point where the feathering bearing fits.

Shot peening is important in the maintenance of airscrew blades as a means of reducing air friction losses (in the same way that a golf ball without dimples would not fly as far due to increased air friction) and, importantly, also reducing stress levels in the blade surface.

System solution

The system chosen to achieve the requirements consisted of an LS GM6 PLC with a multi-axis APM card, driving three Mecapion servo drives and servo geared motors, fitted to linear drives. Servos were chosen for these three axes as a fine overlapping accuracy of the shot runs was essential.

The surface for the prop feathering swivel bearing inside the blade end was shot-blasted from a separate head fitted to a screw driven by an LS variable speed drive and standard AC 3 phase geared-motor. This motor was fitted with a Mecapion Rotary encoder, the outputs of which were fed into the counter inputs of the system PLC for depth measurement. This lower cost option was chosen as this parameter was less critical than those relating to the blade surface.

Recipe selection and operator logging were performed via an LS touch-screen HMI. The blade profile is depicted on this and the necessary distance and displacement plot information for the various airscrew blades is entered by means of this HMI.

The results of every shot peened blade are recorded on a remote computer communicating with the PLC via the built-in RS232 port.

Shot peening intensity, settable on the HMI, is controlled from the PLC via a pneumatic regulating valve which controls the airflow over a venturi. The high velocity airflow over the venturi then draws off the required amount of steel shot.

The system has been in operation for 18 months, providing excellent and repeatable results, without any major failures. Indeed the only failures experienced have been those due to errant shot interfering with the regulating valve’s operation.