Small swing-radius hydraulic excavators are often used in narrow construction areas, especially in cities. Operators are required to control the movement of the machine very accurately so as not to damage utility wires, trees, advertising signs, gas lines, underground electric wiring, water mains, and so on. The excavators are designed to warn the operator or stop completely when the machine comes close to a known hazard. To perform this function, the machine must be able to accurately monitor the location of the bucket and boom.Angle sensors are put on the joints of the arms and boom to monitor their angles. The bucket position can be calculated with the sensor outputs. Accuracy and rigidity are the most important features for these sensors. Additionally they must be rugged enough to survive in a severe construction environment.The sensor unit has a potentiometer with +/-0.5% independent linearity. It is directly attached to the shaft end which is supported by two pre-loaded precision ball bearings. This arrangement allows little displacement between the brush and resistor of the potentiometer. Because of this feature, this unit can measure the angles accurately and safely under severe operating conditions. The sensor and other components are combined inside a heavy duty cast housing.Many tests have been conducted in order to confirm the performance of this unit. Tests include: endurance test under high or low temperatures, and accuracy under shock load, vibration, dust and water.This paper introduces the unique features of the unit and test results.