Rotational motion sensing is important for the automotive and aviation sectors as well as for GPS navigation and health-monitoring systems. Today, gyroscopes based on microelectromechanical systems (MEMS) technology can be miniaturized to fit into smartphones with minimal power consumption. Such systems monitor the motion of a vibrating proof mass, attached to a mounting frame through a spring-like MEMS structure, in all spatial directions. MEMS gyroscopes are often combined with accelerometers, then providing six-dimensional measurements from a single device.
The proof mass is often forced to vibrate periodically along one direction using drive electrodes (which identify the drive mode). Once the device undergoes a rotational motion, the mass experiences the Coriolis force in its rotating reference plane: this appears as a slight displacement along a direction perpendicular to that defined by the drive mode, as shown in the figure. Sensing the amplitude of this displacement (through what is known as the sense mode) provides the angular rate of rotation. This kind of rotational motion sensing device is often referred to as a Coriolis vibratory gyroscope (CVG).
Accurate rotation measurements with CVGs require a high level of control over both drive and sense modes. For this purpose, additional application-specific integrated circuitry (ASIC) is often developed.
Drive mode control
The first step is to force the mass to vibrate at its resonance along the drive mode using a phase-locked loop (PLL). An additional PID controller ensures that the drive mode has a constant amplitude by achieving automatic-gain-control (AGC). Together, the two feedback loops guarantee that the rotational motion does not affect the drive mode. Carrier modulation in amplitude or frequency can help to reject background noise and further parasitic contributions to gyroscopic measurements. Alternatively, parametric resonance can be performed using multiple demodulators synchronously and a parametric sweeper.
Open-loop sense mode configuration
In the open-loop configuration, a single output component of the demodulator monitors the sense mode; manual adjustment of the sense mode phase is required to retrieve an amplitude directly proportional to the angular rate. However, the amplitude decay time constant determined by the quality factor and the resonance frequency of the sense mode limits the gyroscope's response time to an input rotation.
Closed-loop sense mode configuration
Closed-loop control of the sense mode makes it possible to shorten the device's response time and increase the CVG's bandwidth and dynamic range. The closed-loop scheme produces a feedback force that suppresses motion in the sense mode. This force-to-rebalance method provides direct access to the input angular rate through the magnitude of the applied feedback force. The closed-loop sense mode configuration requires four control loops, three of which rely on PID controllers to build the feedback signal.
The Benefits of Choosing Zurich Instruments
- All the gyroscope control strategies listed above can be seamlessly implemented and tested on your device with Zurich Instruments lock-in amplifiers, eliminating the need for time-consuming and expensive ASIC development. For example, the HF2LI Lock-in Amplifier offers 50 kHz PLL bandwidth and can thus reduce significantly the complexity of operation of a CVG – especially in the closed-loop sense configuration. For sensing applications requiring faster operation and higher control bandwidths, the UHFLI Lock-in Amplifier translates the capabilities of the HF2LI into frequencies up to 600 MHz (with 300 kHz PLL bandwidth).
- You can characterize drive and sense mode behavior, including backbone measurements, thanks to the time- and frequency-domain tools offered by the LabOne software.
- Zurich Instruments' analog electronics give access to differential voltage or current measurements (by combining the HF2LI with the HF2TA Current Amplifier), and offer multiple input stages to minimize the input noise and maximize the signal-to-noise ratio for periodic signals.
- Thanks to fast digital data transfer through USB or GbE connections, no additional digitizer card is required to record your measurement results.