provides means for pivotally moving the boom with respect to the frame a second linearly extendable device such as hydraulic cylinder 56 provides means for pivotally moving the stick 34 with respect to the boom a third linearly extendable device such as hydraulic cylinder 58 provides means for pivotally moving the bucket 42 with respect to the stick hydraulic cylinders 54 56 and 58 provide actuation means for moving the cutting edge 50 with respect to frame 22 a first encoder 60 produces a signal or representation proportional to the distance that hydraulic cylinder 54 is extended from its fully retracted position a second encoder 62 produces a signal or representation proportional to the length that second hydraulic cylinder 56 is extended from its fully retracted position a third encoder 64 provides a signal or representation proportional to the distance that third hydraulic cylinder 5 is extended from its fully retracted position the operation of encoders 60 62 and 64 will be explained in detail below a verticality sensor 61 mounted on frame 22 provides a signal or representation proportional to the amount of deviation between the frame 22 and the earth 39 s horizontal plane in other words verticality sensor 61 measures the degree to which frame 22 is not level in its front to back reference plane verticality sensor 61 is typically a rheostat operated by a weighted pendulum and is available from humphrey inc as model no cp17 1101 1 also provided on frame 22 is a motion detector 63 which is a pressure switch.