A diagnostic apparatus determines, at a reference time, whether a device is normal or abnormal, and in which operation state the device is at the time. A reference data creation unit repeats a process of storing sensor values acquired from the device while changing the operation state at the reference time in association with each of the determined operation states until there is no non-corresponding operation state with which the acquired sensor value is not yet associated. At a diagnosis time at which it is not known whether the device is normal or abnormal, an operation state and a sensor value of the device at the time is acquired. The stored sensor value associated with the acquired operation state is read to compare the sensor value acquired at the diagnosis time with the read sensor value to display a result of determination on whether the device is normal or abnormal.

A control system includes an electro-hydraulic servo valve configured to move about a plurality of positions in response to receiving an analog bi-polar current signal. An electronic field programmable gate array (FPGA) generates first and second digital pulse width modulation (PWM) signals according to an adjustable duty cycle. An electronic bridge circuit is in electrical communication with the FPGA to convert the first and second digital PWM signals into the bi-polar current signal. An electronic current sensing circuit is in electrical communication with the bridge circuit, the current sensing circuit configured to generate a digital feedback signal indicating an average current level of the bi-polar signal. The FPGA controls the duty cycle based on at least the average current level indicated by the digital feedback signal.