More and more applications require the collection of data from sensors in high temperature environments. In recent years, great progress has been made in the fields of semiconductors, passive devices, and interconnects, enabling high-precision data acquisition and processing. However, there is an unmet need for sensors that can operate at high temperatures up to 175°C, especially easy-to-use sensors provided by microelectromechanical systems (MEMS). Compared to equivalent discrete sensors, MEMS are generally smaller and consume less power and cost. In addition, they can integrate signal conditioning circuits in the same size semiconductor package.
The ADXL206 high-temperature MEMS accelerometer has been released, which provides high-precision tilt (tilt) measurements. However, more flexibility and freedom are also required to accurately measure system movement in harsh environmental applications where the final product may be subject to shock, vibration and violent movement. This type of abuse can lead to excessive wear and premature failure of the system, resulting in high maintenance or downtime costs.
To meet this need, ADI has developed a new high-temperature MEMS gyroscope with integrated signal conditioning, the ADXRS645. The sensor provides accurate angular rate (rotational speed) measurement even in shock and vibration environments and is rated for operation up to 175°C.
working principle
MEMS gyroscopes use Coriolis acceleration to measure angular rate. For the interpretation of the Coriolis effect, start with Figure 1. Picture yourself standing on a rotating platform, near the center. Your speed relative to the ground is shown by the length of the blue arrow. If you move closer to the outer edge of the platform, your speed relative to the ground increases, indicated by the longer blue arrow. The growth rate of the tangential velocity caused by the radial velocity is the Coriolis acceleration.
If Ω is the angular rate and r is the radius, the tangential velocity is Ωr. So, if r changes while the velocity is v, there will be a tangential acceleration Ωv. Its value is half of the Coriolis acceleration. The other half comes from the change in radial velocity direction, totaling 2Ωv. If you apply a mass (M), then the platform must exert a force—2MΩv—to produce this acceleration, and the mass also experiences a corresponding reaction force. The ADXRS645 exploits this effect by using resonant masses that correspond to when a person moves to the center and to the outer edge on a rotating platform. The mass is made of polysilicon, micromachined, and bonded to the polysilicon frame, so it can only resonate in one direction.
Figure 1. Example of Coriolis acceleration. As the person moves north to the outer edge of the rotating platform, the westward velocity component (blue arrow) must increase,
to keep the course moving north. The required acceleration is the Coriolis acceleration.
Figure 2. Demonstration of the Coriolis effect: responding to resonances of a silicon mass suspended within a frame.
The green arrows indicate the forces on the structure (based on the state of the resonating mass).
Figure 2 shows that as the resonant mass moves toward the outer edge of the rotating platform, it accelerates to the right and applies a reaction force to the frame to the left. As it moves toward the center of rotation, it applies a force to the right, as indicated by the green arrow.
To measure the Coriolis acceleration, we attach the frame containing the resonant mass to the substrate using a spring at 90° to the direction of resonant motion, as shown in Figure 3. This figure also shows the Coriolis detection pointer, which, via capacitive transduction, detects the displacement of the frame when subjected to a force applied to the mass.
Figure 3. Schematic diagram of the mechanical architecture of the gyroscope.
Figure 4 shows the complete structure, from which it can be seen that when the resonant mass moves and the mounting plane where the gyroscope is located rotates, the mass and its frame are affected by Coriolis acceleration and rotate 90° due to vibration. As the rotation speed increases, the position of the mass body and the signal obtained from the corresponding capacitor change. It should be noted that the gyroscope can be placed at any position of the rotating object at any angle, as long as its detection axis is parallel to the rotation axis.
Figure 4. The frame and resonant mass are affected by the Coriolis effect, resulting in lateral displacement.
Capacitance detection
The ADXRS645 measures the displacement of the resonant mass and frame due to the Coriolis effect through a capacitive sensing element attached to the resonator, as shown in Figure 4. These elements are silicon rods, interleaved with two sets of fixed silicon rods connected to the substrate, forming two nominally equal capacitances. Displacement due to angular rate creates differential capacitance in the system.
In practice, the Coriolis acceleration is an extremely small signal that causes beam deflections of fractions of an angstrom and capacitance changes on the order of Zefarads. Therefore, it is extremely important to minimize mutual interference from parasitic sources such as temperature, package stress, external acceleration and electrical noise. Part of this effect is achieved by placing electronics (including amplifiers and filters) and mechanical sensors on the same die. However, it is more important to implement differential measurements as far apart as possible in the signal chain and correlate the signal to the resonator velocity, especially when dealing with the effects of external acceleration.
Vibration suppression
Ideally, the gyroscope is only sensitive to RPM and nothing else. In practice, all gyroscopes have some sensitivity to acceleration due to their asymmetric mechanical design and/or insufficient micromachining accuracy. In fact, acceleration sensitivity can take many forms – the severity of which varies by design. The most severe are usually the sensitivity to linear acceleration (or g-sensitivity) and the sensitivity to vibrational rectification (or g2-sensitivity), severe enough to completely cancel the rated bias stability of the device. Some gyroscopes have rail-to-rail differences in output when the rate input exceeds the rated measurement range. Other gyroscopes tend to lock up when subjected to shocks as low as a few hundred g. These gyroscopes are not damaged by the shock, but also can no longer respond to the rate and require a restart.
The ADXRS645 employs a novel angular rate detection method that enables it to suppress shocks up to 1000 g. It uses four resonators for differential signal detection and rejection of common-mode external accelerations independent of angular movement. The resonators at the top and bottom of Figure 5 are independent of each other and operate out of phase. So, they measure the same amount of rotation, but output in opposite directions. Therefore, the angular rate is measured using the difference between the sensor signals. This eliminates the non-rotating signal that affects both sensors. The signals are combined internally hardwired in front of the preamp. As a result, extreme acceleration overloads are largely prevented from reaching the electronics, allowing signal conditioning to maintain angular rate output during large shocks.
Figure 5. Four-channel differential sensor design.
Sensor installation
Figure 6 shows a simplified schematic of the gyroscope, associated drive and detection circuitry.
Figure 6. Block diagram of an integrated gyroscope.
The resonator circuit senses the velocity of the resonating mass, amplifies it, and drives the resonator while maintaining a well-controlled phase (or delay) relative to the Coriolis signal path. Coriolis circuits are used to detect movement of the accelerometer frame, utilize downstream signal processing to extract the magnitude of the Coriolis acceleration, and generate an output signal consistent with the input rotational speed. In addition, the self-test function checks the integrity of the entire signal chain, including sensors.
Application example
For Electronic equipment, the most demanding environment is the oil and gas downhole drilling industry. These systems utilize a multitude of sensors to better understand how the drill string is operating below the surface to optimize operations and prevent damage. The rotational speed of a rig is measured in RPM and is a key metric that rig operators need to know at all times. Previously, this metric was calculated by a magnetometer. However, magnetometers are susceptible to ferrous materials in the rig casing and surrounding wellbore. They must also have a special non-magnetic drill collar (housing).
Beyond simple RPM measurements, there is a growing interest in understanding drill string movement (or drill string dynamics) to better manage parameters such as magnitude of applied force, rotational speed, and steering. Poorly managed drill string dynamics can result in high drill string vibration and extremely erratic movement, which can lead to extended drilling times in target areas, premature equipment failure, difficult bit steering, and damage to the well itself. In extreme cases, equipment can fracture and remain in the well, where it can be retrieved at a very high cost.
A particularly detrimental movement, stick-slip, can result from poor management of drill string parameters. Stick-slip is the phenomenon in which the drill bit gets stuck, but the top of the drill string continues to rotate. After the bit is jammed, the bottom of the drill string continues to rotate and tighten until enough torque is reached to cause fracture and loosening, which is usually very violent. When this happens, there are large spikes on the drill that spin at RPM. Stick-slip generally occurs periodically and can last for a long time. A typical RPM response to stick-slip is shown in Figure 7. As the drill string at the surface continues to function normally, rig operators are often unaware that this very destructive phenomenon is taking place downhole.
Figure 7. Example of a stick-slip cycle RPM profile.
A key measurement in this application is accurate and frequent measurement of rotational speed near the drill bit. A gyroscope, such as the ADXRS645 with vibration dampening, is ideal for this task because its measurements are not affected by the linear movement of the drill string. In the presence of high levels of vibration and unstable movement, the rotational speed calculated by the magnetometer is susceptible to noise and errors. A gyroscope-based solution can instantly measure rotational speed without the use of zero-crossing or other algorithms susceptible to shock and vibration.
Additionally, gyroscope-based circuits are smaller and require fewer components than flux magnetometer solutions, which require multiple magnetometer axes and additional drive circuitry. Signal conditioning is integrated into the ADXRS645. Housed in a low-power, low-pin-count package, this device enables a high-temperature IC to sample and digitize the gyroscope’s analog output. Using the simplified signal chain shown in Figure 8, a gyroscope circuit that provides a digital output and is rated at 175°C can be implemented. For a complete reference design of the data acquisition circuit, visit www.analog.com/cn0365.
Figure 8. Gyroscope digital output signal chain rated at 175°C.
Summarize
This article introduces the ADXRS645, the first MEMS gyroscope that can be used in a high temperature environment of 175°C. This sensor accurately measures angular rate in harsh environmental applications, preventing the effects of shock and vibration. This gyroscope is powered by a series of high temperature ICs to acquire the signal and process it. For more information on ADI’s high temperature products, visit www.analog.com/hightemp.
About the Author
Jeff Watson is a systems applications engineer in Analog Devices’ Instrumentation, Aerospace and Defense business unit, working on high temperature applications. Before joining ADI, he was a design engineer in the underground oil and gas instrumentation industry and the off-highway vehicle instrumentation/controls industry. He holds bachelor’s and master’s degrees in electrical engineering from Penn State University.
More and more applications require the collection of data from sensors in high temperature environments. In recent years, great progress has been made in the fields of semiconductors, passive devices, and interconnects, enabling high-precision data acquisition and processing. However, there is an unmet need for sensors that can operate at high temperatures up to 175°C, especially easy-to-use sensors provided by microelectromechanical systems (MEMS). Compared to equivalent discrete sensors, MEMS are generally smaller and consume less power and cost. In addition, they can integrate signal conditioning circuits in the same size semiconductor package.
The ADXL206 high-temperature MEMS accelerometer has been released, which provides high-precision tilt (tilt) measurements. However, more flexibility and freedom are also required to accurately measure system movement in harsh environmental applications where the final product may be subject to shock, vibration and violent movement. This type of abuse can lead to excessive wear and premature failure of the system, resulting in high maintenance or downtime costs.
To meet this need, ADI has developed a new high-temperature MEMS gyroscope with integrated signal conditioning, the ADXRS645. The sensor provides accurate angular rate (rotational speed) measurement even in shock and vibration environments and is rated for operation up to 175°C.
working principle
MEMS gyroscopes use Coriolis acceleration to measure angular rate. For the interpretation of the Coriolis effect, start with Figure 1. Picture yourself standing on a rotating platform, near the center. Your speed relative to the ground is shown by the length of the blue arrow. If you move closer to the outer edge of the platform, your speed relative to the ground increases, indicated by the longer blue arrow. The growth rate of the tangential velocity caused by the radial velocity is the Coriolis acceleration.
If Ω is the angular rate and r is the radius, the tangential velocity is Ωr. So, if r changes while the velocity is v, there will be a tangential acceleration Ωv. Its value is half of the Coriolis acceleration. The other half comes from the change in radial velocity direction, totaling 2Ωv. If you apply a mass (M), then the platform must exert a force—2MΩv—to produce this acceleration, and the mass also experiences a corresponding reaction force. The ADXRS645 exploits this effect by using resonant masses that correspond to when a person moves to the center and to the outer edge on a rotating platform. The mass is made of polysilicon, micromachined, and bonded to the polysilicon frame, so it can only resonate in one direction.
Figure 1. Example of Coriolis acceleration. As the person moves north to the outer edge of the rotating platform, the westward velocity component (blue arrow) must increase,
to keep the course moving north. The required acceleration is the Coriolis acceleration.
Figure 2. Demonstration of the Coriolis effect: responding to resonances of a silicon mass suspended within a frame.
The green arrows indicate the forces on the structure (based on the state of the resonating mass).
Figure 2 shows that as the resonant mass moves toward the outer edge of the rotating platform, it accelerates to the right and applies a reaction force to the frame to the left. As it moves toward the center of rotation, it applies a force to the right, as indicated by the green arrow.
To measure the Coriolis acceleration, we attach the frame containing the resonant mass to the substrate using a spring at 90° to the direction of resonant motion, as shown in Figure 3. This figure also shows the Coriolis detection pointer, which, via capacitive transduction, detects the displacement of the frame when subjected to a force applied to the mass.
Figure 3. Schematic diagram of the mechanical architecture of the gyroscope.
Figure 4 shows the complete structure, from which it can be seen that when the resonant mass moves and the mounting plane where the gyroscope is located rotates, the mass and its frame are affected by Coriolis acceleration and rotate 90° due to vibration. As the rotation speed increases, the position of the mass body and the signal obtained from the corresponding capacitor change. It should be noted that the gyroscope can be placed at any position of the rotating object at any angle, as long as its detection axis is parallel to the rotation axis.
Figure 4. The frame and resonant mass are affected by the Coriolis effect, resulting in lateral displacement.
Capacitance detection
The ADXRS645 measures the displacement of the resonant mass and frame due to the Coriolis effect through a capacitive sensing element attached to the resonator, as shown in Figure 4. These elements are silicon rods, interleaved with two sets of fixed silicon rods connected to the substrate, forming two nominally equal capacitances. Displacement due to angular rate creates differential capacitance in the system.
In practice, the Coriolis acceleration is an extremely small signal that causes beam deflections of fractions of an angstrom and capacitance changes on the order of Zefarads. Therefore, it is extremely important to minimize mutual interference from parasitic sources such as temperature, package stress, external acceleration and electrical noise. Part of this effect is achieved by placing electronics (including amplifiers and filters) and mechanical sensors on the same die. However, it is more important to implement differential measurements as far apart as possible in the signal chain and correlate the signal to the resonator velocity, especially when dealing with the effects of external acceleration.
Vibration suppression
Ideally, the gyroscope is only sensitive to RPM and nothing else. In practice, all gyroscopes have some sensitivity to acceleration due to their asymmetric mechanical design and/or insufficient micromachining accuracy. In fact, acceleration sensitivity can take many forms – the severity of which varies by design. The most severe are usually the sensitivity to linear acceleration (or g-sensitivity) and the sensitivity to vibrational rectification (or g2-sensitivity), severe enough to completely cancel the rated bias stability of the device. Some gyroscopes have rail-to-rail differences in output when the rate input exceeds the rated measurement range. Other gyroscopes tend to lock up when subjected to shocks as low as a few hundred g. These gyroscopes are not damaged by the shock, but also can no longer respond to the rate and require a restart.
The ADXRS645 employs a novel angular rate detection method that enables it to suppress shocks up to 1000 g. It uses four resonators for differential signal detection and rejection of common-mode external accelerations independent of angular movement. The resonators at the top and bottom of Figure 5 are independent of each other and operate out of phase. So, they measure the same amount of rotation, but output in opposite directions. Therefore, the angular rate is measured using the difference between the sensor signals. This eliminates the non-rotating signal that affects both sensors. The signals are combined internally hardwired in front of the preamp. As a result, extreme acceleration overloads are largely prevented from reaching the electronics, allowing signal conditioning to maintain angular rate output during large shocks.
Figure 5. Four-channel differential sensor design.
Sensor installation
Figure 6 shows a simplified schematic of the gyroscope, associated drive and detection circuitry.
Figure 6. Block diagram of an integrated gyroscope.
The resonator circuit senses the velocity of the resonating mass, amplifies it, and drives the resonator while maintaining a well-controlled phase (or delay) relative to the Coriolis signal path. Coriolis circuits are used to detect movement of the accelerometer frame, utilize downstream signal processing to extract the magnitude of the Coriolis acceleration, and generate an output signal consistent with the input rotational speed. In addition, the self-test function checks the integrity of the entire signal chain, including sensors.
Application example
For Electronic equipment, the most demanding environment is the oil and gas downhole drilling industry. These systems utilize a multitude of sensors to better understand how the drill string is operating below the surface to optimize operations and prevent damage. The rotational speed of a rig is measured in RPM and is a key metric that rig operators need to know at all times. Previously, this metric was calculated by a magnetometer. However, magnetometers are susceptible to ferrous materials in the rig casing and surrounding wellbore. They must also have a special non-magnetic drill collar (housing).
Beyond simple RPM measurements, there is a growing interest in understanding drill string movement (or drill string dynamics) to better manage parameters such as magnitude of applied force, rotational speed, and steering. Poorly managed drill string dynamics can result in high drill string vibration and extremely erratic movement, which can lead to extended drilling times in target areas, premature equipment failure, difficult bit steering, and damage to the well itself. In extreme cases, equipment can fracture and remain in the well, where it can be retrieved at a very high cost.
A particularly detrimental movement, stick-slip, can result from poor management of drill string parameters. Stick-slip is the phenomenon in which the drill bit gets stuck, but the top of the drill string continues to rotate. After the bit is jammed, the bottom of the drill string continues to rotate and tighten until enough torque is reached to cause fracture and loosening, which is usually very violent. When this happens, there are large spikes on the drill that spin at RPM. Stick-slip generally occurs periodically and can last for a long time. A typical RPM response to stick-slip is shown in Figure 7. As the drill string at the surface continues to function normally, rig operators are often unaware that this very destructive phenomenon is taking place downhole.
Figure 7. Example of a stick-slip cycle RPM profile.
A key measurement in this application is accurate and frequent measurement of rotational speed near the drill bit. A gyroscope, such as the ADXRS645 with vibration dampening, is ideal for this task because its measurements are not affected by the linear movement of the drill string. In the presence of high levels of vibration and unstable movement, the rotational speed calculated by the magnetometer is susceptible to noise and errors. A gyroscope-based solution can instantly measure rotational speed without the use of zero-crossing or other algorithms susceptible to shock and vibration.
Additionally, gyroscope-based circuits are smaller and require fewer components than flux magnetometer solutions, which require multiple magnetometer axes and additional drive circuitry. Signal conditioning is integrated into the ADXRS645. Housed in a low-power, low-pin-count package, this device enables a high-temperature IC to sample and digitize the gyroscope’s analog output. Using the simplified signal chain shown in Figure 8, a gyroscope circuit that provides a digital output and is rated at 175°C can be implemented. For a complete reference design of the data acquisition circuit, visit www.analog.com/cn0365.
Figure 8. Gyroscope digital output signal chain rated at 175°C.
Summarize
This article introduces the ADXRS645, the first MEMS gyroscope that can be used in a high temperature environment of 175°C. This sensor accurately measures angular rate in harsh environmental applications, preventing the effects of shock and vibration. This gyroscope is powered by a series of high temperature ICs to acquire the signal and process it. For more information on ADI’s high temperature products, visit www.analog.com/hightemp.
About the Author
Jeff Watson is a systems applications engineer in Analog Devices’ Instrumentation, Aerospace and Defense business unit, working on high temperature applications. Before joining ADI, he was a design engineer in the underground oil and gas instrumentation industry and the off-highway vehicle instrumentation/controls industry. He holds bachelor’s and master’s degrees in electrical engineering from Penn State University.
The Links: SKM145GB123D 6MBI35S-120
