Force-Balance Sensor Technology
At the heart of every force balance sensor is the torquer. A torquer is a miniature DC motor with a mass and arm attached to the coil to act as a pendulum.
Accelerometer manufacturers began adapting the d’Arsonval meter mechanisms concept for use as torquers in the early 1960s and Jewell was the leading supplier of meter mechanisms. Jewell quickly began supplying torquers to the industry and shortly after also started producing its force balance accelerometers making it the leading supplier and the industry expert in force-balance sensors for more than 60 years. Jewell produces many different types of torquer mechanism. Most of the sensing mechanisms that Jewell produces utilize one of two moving system types, which are Torsional Flexure Suspension (FS) and Pivot and Bearing (PB).
Both the Flexure Suspension (FS) and Pivot and Bearing (PB) torquers are d’Arsonval mechanisms, which are moving coil, stationary coil miniaturized motors. The coil of the flexure suspension mechanism is suspended about the magnet with platinum nickel bands and allowed to rotate. The pivot and bearing mechanisms use pivots that are attached to the coil and fit into bearings on the frame that allow the coil to rotate about the magnet. High-quality mechanisms often utilize jewels such as sapphire, ruby, and even industrial grade diamonds for pivot and bearings. Both types utilize a mass and arm attached to the coil making a pendulum. Jewell’s sensors utilize a position detector to determine the location of the mass. The torque motor is utilized to hold the mass in one position and therefore the force required to hold the position is proportional to the force acting on the mass.
Jewell accelerometers and inclinometers are precision inertial instruments. They utilize closed loop technology to produce a highly accurate output to < 1μrad resolution. The inertial sensor output is an analog voltage, current, or digital signal proportional to applied acceleration and tilt from DC through a specified frequency.
Jewell has produced inertial instrument torquers and complete acceleration sensing assemblies for decades. Hundreds of thousands of acceleration sensors have been manufactured. Jewell sensors are used throughout the world and have models that can detect acceleration and tilt from less than one µG (one µRadian) to more than 20G.
How Does It Work?
The torquer mechanism (Torque Motor + pendulum) is the fundamental subassembly in a servo sensor. An accelerometer torquer is intentionally unbalanced (Mass Unbalance) in its plane of allowable angular motion. When acceleration or tilt is present, a torque proportional to the mechanism unbalance and the physical input is developed. The torque results in an angular motion of the pendulum detected by a Position Sensor. The Position Sensor output is compared to a reference voltage in the Electronics Module, and the difference is an error signal that is the input to a servo amplifier (Servo Amp). The servo amplifier output current is applied to the Torque Motor in opposition to the acceleration or tilt torque. At a constant inertial input, the Mass Unbalance angular position is minutely different from the zero g position. The Servo Amp output current is directly proportional to the applied acceleration or sine of the input tilt angle. An analog voltage is produced by measuring the servo current with a Sense Resistor.
As shown on the above diagram, as the sensor accelerates/tilts, paddle “A” tries to move in the direction of gravity (either earth’s gravity or acceleration). Any resultant motion is converted by sensor “B” to a signal input to the electronic amplifier whose current output is applied to the torque motor “C”. This develops a torque equal and opposed to the original so the pendulous mass no longer moves, but assumes a position minutely different from it’s original to provide the required error signal. The torque motor current is directly and accurately proportional to acceleration/tilt, and by allowing it to flow through stable resistor “R0”, an equally accurate output voltage is developed. Stops are provided on both sides of the paddle to limit its travel when not powered. When powered, the paddle automatically moves to its null position.
Note that an accelerometer and an inclinometer are the same device. The distinction is one of application, not operation. Accelerometer users typically sense changes in velocity and characterize outputs and errors in g. Inclinometer users sense changes in angular position and think of outputs and errors in units of angular measurement. An inertial instrument responds to both earth’s gravity and acceleration. The inclinometer output is sensing the angle with the respect to the gravity vector. The output follows the relationship of g x sin Ꙩ, where g is acceleration of gravity; sin is trigonometric sine function, Ꙩ is reference angle with respect to the gravity vector. The output can be converted to degrees by the arcsine function.
Torquer Mechanism Options
1) Torsional Flexure suspension (TS):
- Coil of the flexure suspension mechanism is suspended about the magnet with platinum-nickel bands and allowed to rotate.
- Very low input and relatively low frequencies capability
- Provide superior repeatability.
- Ideal for tilt sensing applications as well as low g acceleration sensing
- Some models without physical damping for cost-sensitive applications that require the highest level of resolution and repeatability.
- Most utilize fluid damping (FD), which increases durability by several orders of magnitude. They are the best choice for high accuracy measurements of low inputs in harsh environments or when they must survive exposure to extreme events.
- Used to measure inclination.
2) Pivot and bearing (PB):
- Mechanisms use pivots that are attached to the coil and fit into bearings on the frame that allow the coil to rotate about the magnet.
- Have higher input range and frequency response capability than flexure suspension mechanisms.
- Usually used to measure acceleration over 2g.
FD = Fluid Damped
FS = Flexure Suspension
PB = Pivot & Bearing
|Emerald SMI Series||1-axis||FS|
|Emerald RMI Series||1-axis||FS|
Accelerometers, Analog Angular
Accelerometer, Analog Linear
|Emerald SMA Series||1-axis||FS|
Accelerometer, Digital Linear
The Right Force-Balance Sensor For Your Application:
The torquer mechanism is typically the most expensive subassembly in a servo sensor. Torquers are sophisticated meter movements, and JEWELL as a meter manufacturer can produce torquers very efficiently. We, therefore, often have a cost advantage when compared to other traditional technology inertial instrument producers. The torquer has a system that allows the pendulum to move and there are different types of moving systems. The moving system is important to the performance of the accelerometer/inclinometer and it’s typically chosen based on its characteristics for what is best suited for the application.
Sensors utilizing the flexure suspension moving system are characterized for having very low input and relatively low frequencies capability and provide superior repeatability making them the ideal for tilt sensing applications as well as low g acceleration sensing. Jewell offers some models that utilize the flexure suspension mechanism without physical damping for cost-sensitive applications that require the highest level of resolution and repeatability, but most utilize physical damping, which increases durability by several orders of magnitude. Jewell’s fluid damped, flexure suspension force balance accelerometers and inclinometers are extremely robust and have been tested to pyrotechnic shocks. They’re the best choice for high accuracy measurements of low inputs in harsh environments or when they must survive exposure to extreme events. These types of force balance inertial sensors are ideal for Aerospace, Military, as well as Rail and Rail Transportation applications.
Pivot and bearing torquer mechanisms have higher input range and frequency response capability than flexure suspension mechanisms. These sensors are usually used to measure acceleration as opposed to inclination. Jewell’s proprietary high-performance pivot and bearing torquer design provide superior performance in demanding environments and has been used in many Military and Aerospace applications for decades.
The characteristics of flexure suspension sensors make them well suited for tilt sensing so they’re often labeled as inclinometers/tilt sensors and used to measure tilt, and pivot and bearing sensors have characteristics that fit well with acceleration sensing and are usually regarded as accelerometers and used to measure tilt. Flexure Suspension type Force balance inertia sensors are ideal for Bridge Monitoring, and a wide array of Industrial Applications.
Although the LSOX, LSOC, LSRP, LSMP, LCA, LCF and LCI models are equivalent in concept and operation, there are important mechanical differences. LSOX, LSOC, LSRP, LCF and LCI Series moving systems are suspended by torsion (taut band) flexures. LSMP and LCA Series moving systems are suspended by pivot and jewel bearings. Pivot and jewel suspension units can have wider bandwidths and can be used for higher range accelerations. The torsion flexure has superior repeatability and is most often used for tilt sensing and low range accelerations. LSOX, LSOC, LSRP and LCF units have the torsion flexure surrounded by silicone fluid and can withstand higher shock and vibration than the LSMP, LCA or LCI . The LSOX, LSOC, LSRP, and LCF can also provide accurate low frequency output information during the time that the shock and vibration are present.
Force balanced technology offers great advantages that makes it ideal for numerous high-precision measurements. It has a solid dynamic response up to 200 Hz. Some sensors can be fluid-damped in order to attenuate vibration & shock interferences in the reading. Its wide measuring range (up to ±90º for inclinometers, ±20G for accelerometers) opens up the playbook for applications within many markets such as aerospace, rail, industrial, marine, military and geotechnical. There is also very low thermal drift and high shock and vibration resistance, which is an advantage for the tough conditions of industrial projects. Some of the projects where our force-balanced sensors have been used include: metro & light rail control systems, pavement grade control, UAV guidance, high-speed rail control and track monitoring, self-leveling vehicles and mobile antenna measurements, attack helicopter rotation sensing, railroad maintenance of way, off-highway vehicles, semi-automated gun turrets, infrastructure and bridge monitoring, telescope/antenna leveling, underwater drilling, and many more. With over 60 years of accurate inertial solutions, numerous customization capabilities, excellent long-term sensor stability & durability, we are experts in precision instruments. Contact us today to discuss your particular project requirements!