Angular rate sensor
Molecular Electronic Transducers
Molecular Electronic Transducers (MET) are a class of inertial sensors (which include accelerometers, gyroscopes, tilt meters, seismometers, and related devices) based on an electrochemical mechanism. METs capture the physical and chemical phenomena that occur at the surface of electrodes in electrochemical cells as the result of hydrodynamic motion. They are a specialized kind of electrolytic cell designed so that motion of the MET, which causes movement (convection) in the liquid electrolyte, can be converted to an electronic signal proportional to acceleration or velocity. MET sensors have inherently low noise and high amplification of signal (on the order of 106).
Principles of Operation
At the heart of a MET device are two (or more) inert electrodes at which a reversible redox reaction occurs, which does not involve either plating of a metal or evolution of a gas. Typically, the aqueous iodide-triiodide couple is used: 3 I- → I3- + 2 e- anode reaction I3- + 2 e- → 3 I- cathode reaction When a voltage in the range of ~ 0.2 to 0.9V is applied across the electrodes, these two reactions occur in a continuous fashion. After a short time, the electrochemical reactions deplete the concentration of triiodide ions [I3-] at the cathode and enrich it at the anode, creating a concentration gradient of [I3-] between the electrodes. When the cell is motionless, the electrochemical reaction is limited by the diffusion of I3- to the cathode (a slow process), and the current dies down to a low steady-state value. Motion of the device causes convection (stirring) in the electrolyte. This brings more I3- to the cathode, which in turn causes an increase in the cell current proportional to the motion. This effect is very sensitive, with extremely small motions causing measurable (and low noise) inertial signals. In practice, the design of the electrodes to create a device with good performance (high linearity, wide dynamic range, low distortion, small settling time) is a complex hydrodynamic problem.
Advantages of MET sensors
The main advantage of MET sensors over competing inertial technologies is its combination of size, performance and cost. MET sensors have performance comparable to fiber optic gyros (FOGs) and ring laser gyros (RLGs) at a size close to that of MEMS sensors, and at potentially low cost (in the tens to hundreds of dollars range, in production). In addition, the fact that they have a liquid inertial mass with no moving parts makes them rugged and shock tolerant (basic survivability has been demonstrated to >20 kG); they are also inherently radiation hard. Applications
Depending on the configuration of the MET device, a variety of inertial sensors can be produced including:
• Linear accelerometers
• Linear velocity meters
• Seismic sensors
• Seismometers
• Angular accelerometers
• Angular rate meters
• Gyroscopes
• Tilt meters
• Pressure transducers
== MET Rate Sensors ==
There are two types of MET sensors that directly measure angular rate (rate sensors or gyroscopes): one that is able to measure constant angular rate (DC sensitive) and the one that is not.
Non DC sensitive angular rate sensor has design of a toroid filled with a special electrolyte that moves when the device is rotates about its sensitive axis. When it happens the electrolyte flows through the sensing element that is rigidly fixed inside the channel and the motion is detected. The output signal is proportional to angular rate due to internal integration that occurs inside of the sensing element. The lower level of measurements could be as low as 0.0005 Hz.
DC sensitive angular rate sensor or gyroscope has more complex design.