Welcome to the training module on Yaw-rate Gyroscopes.
This training module will introduce basic knowledge of gyroscopes and ST’s single-axis (yaw) gyroscopes.
While accelerometers measure linear acceleration as long as there is no rotation, MEMS Gyroscopes generate an output signal directly proportional to the angular rate applied to the device. To do this, they measure the force generated by the Coriolis effect. When accelerometer is rotated, the projection of gravity acceleration is measured as well and there is no way to distinguish between the two different contributions. By using a gyroscope, angular rotations are measured while linear displacements are not. Accelerometers and Gyroscopes can be combined to create an Inertial Measurement Unit or IMU.
Gyroscopes were once too big, too power hungry and too expensive for consumer applications. Recent MEMS gyroscopes have paved the way to a completely new set of innovative applications. They can be used to implement new advanced user interfaces and give the user an improved gaming experience. For these applications, a full-scale range of thousands of degrees per second, or dps, is required to capture the player’s rapid movements. At the other end of the application spectrum, image stabilization for digital still and video cameras only needs thirty dps, and very low noise and high stability over temperature as key parameters.
A gyroscope can rotate in 3 axes. There are two in-plane aces called pitch and roll – the X and Y axes. The third is an out of plane axis called yaw – the Z axis. Zero rate level refers to the analog or digital sensor output value when no angular rate is applied. Sensitivity always refers to the ratio between the output and input of the sensor.
This chart shows ST’s complete family of single and multi axis gyroscopes. The chart is vertically split in four main different sub-families distinguished by the number and sensed planes. Each of these is further segmented into three main categories of products distinguished by their full scale ranges. In next few pages, we will introduce the three single axis gyroscopes.
ST has a series of high performance low-power single-axis micro-machined gyroscope capable of measuring angular rate along yaw axis. These devices provide excellent temperature stability and high resolution over extended operating temperature range. They have different full scale options and is capable of detecting rates with a -3 dB bandwidth up to 140 Hz. The device includes a sensing element composed of a single driving mass, kept in continuous oscillation and capable of reacting, based on the Coriolis principle, when an angular rate is applied. They provide the measured angular rate to the external world through an analog output voltage.
We see here a block diagram of the ASIC interface for a yaw-rate gyroscope. It contains two main components: the driving circuitry, and sensing chain. The driving circuitry generates the signals needed to drive the capacitive plates that cause the oscillating movement at the resonance frequency of the driving mass. The sensing chain is for the single axis.
The capacitive variation resulting from the up-down wing movement is read by a dedicated charge amplifier block. A similar block is used to measure the movable mass displacement inside accelerometers. The sensing interface detects a signal composed of a voltage proportional to the external angular rate applied multiplied by a sine wave at the driving frequency of the mass.
The schematic shown here provides the basic guidelines to correctly use these single axis gyroscopes. Only a few external components are needed, thus simplifying board design and keeping the design compact. Power supply decoupling capacitors should be placed in combination with an LDO regulator. The device IC includes a PLL (phase-locked loop) circuit to synchronize driving and sensing interfaces. Capacitors and resistor must be added at VCONT pin 3 (see figure above) to implement a low-pass filter.
These gyroscopes include a self-test function. By applying a logic one to a dedicated pin, the mass is moved by an electrostatic actuator. It is then possible to detect a voltage variation of the output and verify that the mechanical mass and ASIC circuitry are working correctly without the need to physically move the device. Another feature is the possibility to power down the sensor. By acting on pin PD, the power down mode is activated and the calibration values remain stored inside internal registers while the rest of the circuit is turned off. The sensor consumers as little at 5 µ A in the power down mode.
Thank you for taking the time to view this presentation on “ Yaw-rate Gyroscopes” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link right beside the play button on the TechCast portal, or simply call our sales hotline. For more technical information you may either visit the STMicroelectronics site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility. You may visit element14 e-community to post your questions.
What are MEMS Gyroscopes? <ul><li>MEMS gyroscopes measure angular movement (pitch, roll, and yaw) using the Coriolis effect. </li></ul>Accelerometers and Newton F = m A Gyroscope and Coriolis F = -2m V x Ω Accelerometer MEMS accelerometers and gyroscopes can be combined into an inertial measurement unit (IMU) X Y Z Roll Pitch Yaw
Key Terms in MEMS Gyroscopes <ul><li>In plane axis (pitch / roll, X / Y) </li></ul><ul><ul><li>The axis along the sensor package surface (roll / pitch) </li></ul></ul><ul><li>Out of plane axis (yaw, z) </li></ul><ul><ul><li>The axis perpendicular to the sensor package surface (yaw) </li></ul></ul><ul><li>Zero rate level </li></ul><ul><ul><li>Sensor output (digital / analog) when no angular rate is applied </li></ul></ul><ul><li>Sensitivity (mV/dps - LSB/dps) </li></ul><ul><ul><li>The ratio between sensor output and angular rate applied (gain of the sensor) </li></ul></ul>
Self-test and Power-down modes <ul><li>Self-test </li></ul><ul><ul><li>Self-test function allows testing of the mechanical and electrical part of the sensor. </li></ul></ul><ul><ul><li>When the ST pin is tied to Vdd, an actuation force is applied to the sensor, emulating a definite Coriolis force. </li></ul></ul><ul><ul><li>The function is off when the ST pin is connected to GND. </li></ul></ul><ul><li>Power down </li></ul><ul><ul><li>The device enables advanced power-saving features. </li></ul></ul><ul><ul><li>The device is put into power-down mode using pin PD. </li></ul></ul><ul><ul><li>The calibration values remain stored inside internal registers while the rest of the circuit is turned off. </li></ul></ul><ul><ul><li>The power consumption is as low as 5 µ A. </li></ul></ul>
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