A volute for use in a centrifugal compressor that includes an impeller. The air mass sensor is connected to the electronic unit which is attached to the housing andor to the air inlet channel or to the air outlet channel. Each of the signal processing blocks processes a signal of reception signal data input from the radio modem or from a preceding stage of one of the signal processing blocks or a signal of transmission signal data input from the CPU or from the preceding stage of signal processing block on the basis of a control signal input together with the transmission signal data or the reception signal data. By rotating the control disk, a guiding portion located on an inner peripheral rim of the control disk can move the conductive element in a biased manner so that the common electrode and the first electrode or the second electrode are electrically connected to generate a leftward or rightward rotational instruction.