After one circuit and full stop landing, the pilot taxied again for runway 17 with the intention of a local flight. On the taxiway, near the aircraft parking area, a grader and a road roller were working. The pilot was fully aware of the works in progress. Nearing the grader, the pilot veered the aircraft to the left on to the grass between the taxiway and the runway. There was plenty of room. The aircraft passed the grader and continued north on the grass beside the taxiway. While taxiing at about five knots, the outboard section of the leading edge of the right wing contacted the roller's air filter. The roller was moving slowly south at the time. There were no obstructions to visibility. The pilot had misjudged the clearance between the wing and the roller.