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. It’s composed of a rotating unbalanced coil around a fixed permanent magnet as in a meter movement. Its function is to respond to tilt or acceleration much the way a hinged door in a house would respond. An accelerometer torquer is intentionally unbalanced (Mass Unbalance) in its plane of allowable angular motion. The coil mass unbalance also serves as a position detector which feeds a servo amplifier driving the torquer coil. The servo amplifier output current restores the coil position as a spring would restore the position of a purely mechanical system.
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 opposite direction of the applied force (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.
Full Range Tilt Full Range Equivalent G
±90° Sine 90° = 1 ±1g
±30° Sine 30° = 0.5 ±0.5g
±14.5° Sine 14.5° = 0.25 ±0.25g
±3° Sine 3° = 0.05 ±0.05g
±1° Sine 1° = 0.017 ±0.02g
A stationary accelerometer on the earth whose sensitive axis is parallel to the direction of the gravitational vector will measure 1G. If oriented perpendicular to the gravity vector it will measure 0G. This change in tilt angle will determine the accelerometer output which may be described as being proportional to the trigonometric sine of the tilt angle. The inclinometer is essentially a low range servo accelerometer which senses the earth’s gravitational acceleration to measure tilt. An accelerometer designed to produce full scale output at 1g acceleration (9.8 m/s2) will produce full scale output if the unit’s sensitive axis is directed toward the center of the earth. The device may be called a ±90° inclinometer.
Torquer Mechanism Options
1) Torsional Flexure suspension (TS), also called “Taut-Band” design:
- 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), also known as “Pivot & Jewel” design:
- 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.
The largest contributor of errors for the force-balance sensor is its non-linearity. Typical non-linearity for the Jewell Torsional Flexure Suspension device is 0.05%. Pivot-Jewel devices are rated at 0.1%. Non-Linearity may be viewed as accuracy over the range of the instrument with temperature and other effects such as supply voltage, pressure, shock, etc., all held constant. The Pivot-Jewel device is inferior to the Torsional Suspension in this regard due to the granular (bumpy) nature of the bearing surfaces. Since the effects of non-linearity are unpredictable for pivot-jewel devices, it may be argued that the fixed, predictable nonlinearity of the Torsional Flexure Suspension type may be compensated for by the computer.
Another source of unpredictable (variable) error for the Pivot-Jewel device, however, a predictable (constant) error for the Torsional Suspension types, is Null Bias. Null Bias, often called ‘Zero Offset’, ‘Bias Offset’, ‘Zero Bias’, is the output of the inclinometer/accelerometer at either 0° tilt or 0g acceleration.
Torsional Suspension may be used for linear accelerometer requirements in the range from ±0.02g to about ±10g. As compared to the standard Pivot-Jewel accelerometers, a comparison of performance data is outlined in Table below.
Advantages and Limitations of Each Technology
| TAUT-BAND | PIVOT-AND-JEWEL |
| Little or no turn-on repeatability errors (<10µg) | Higher turn-on errors (~2000 µg) |
| Extremely long life, no wear-out when subjected to vibration long pivotal and cross axis | Susceptible to damage at high shock levels, particularly in sensitive axis |
| Design of frequency response to exact specification of the viscosity of the damping fluid, trimmed by electrical feedback | Electrically damped, relatively high bandwidth (~200-500 Hz) |
| High shock capability in all three axes without performance degradation | Vibration causes wear on Pivot & Jewel, degrading performance parameters |
Performance Comparison
Parameter Pivot-Jewel Taut-Band |
Range ±0.25g to ±50g ±0.02g to ±10g
Shock Survival 100g 1500g
Nonlinearity 0.1% 0.05%
Hysteresis 0.02% 0.0005%
Resolution 0.0005° 0.0001°
Scale Factor Temp. 0.02%/°C 0.005%/°C
Null Temperature 0.002%/°C 0.0005%/°C
Repeatability 0.2% 0.002%
Cross-Axis 0.002 g/g 0.002 g/g
Natural Frequency 50Hz to 200Hz 0.5Hz to 60Hz
Operating Temperature -55° to +95°C -18° to +71°C#
Storage Temperature -65° to +105°C -40° to +75°C
Damping 0.6 Typical 0.6 Typical
Noise 0-10Hz 20µVrms 10µVrms
Full Scale Output ±5Vdc ±5Vdc
Supply Voltage ±15Vdc ±15Vdc
Supply Current <20mA <20mA
*Accuracy 0.14% 0.05%
* Accuracy is defined as the sum of errors caused by Nonlinearity, Hysteresis, Resolution, and Repeatability.
# On special request, temperature ranges from -55° to +175°C are available at extra cost
The data indicates the superiority of the Taut-Band compared to the Pivot-Jewell system in the operational parameters focusing on overall accuracy and high-shock ranges. However, it’s clear that the Pivot-Jewell is currently available in smaller sizes and higher g ranges therefore, it’s sometimes preferred over the Torsional Suspension in some applications.
For majority of applications up to 10g, the Torsional Suspension device is the most acceptable choice for linear accelerometers. Some additional advantages of the Torsional Suspension device not apparent from the performance table are as follows:
- Pivot-Jewel wear during normal usage causes a gradual deterioration of performance. Not such effect is observable in the Torsional Suspension type.
- Shipping and handling shocks may cause failure or additional deterioration.
- Static friction effects in the Pivot-Jewel type cause tap repeatability and null bias to be erratic. Again, no such effect is observable in the Torsional Suspension device.
- The granularity of the Pivot-Jewel bearing causes large variations in in the scale factor and null characteristics between individual units of the same model number. This non-uniformity between units is much less apparent in the Torsional Suspension type.
- A Pivot-Jewel accelerometer will not provide identical outputs when its orientation includes upside-down or on its side operation. This is due to the change in bearing loading as a result of the directional change of gravity and although no immune to this effect, the Torsional Suspension type is much less affected in this regard.
- Reliability and MTBF are better for the Torsional Suspension. It must be emphasized that Pivot-Jewel types are often selected, despite its disadvantages, whenever small size and/or higher frequency response are necessary.
The cost of the Torsional Suspension device is a bit higher than a comparable Pivot-Jewel type. In very high-volume cost factor situations particularly in one-shot devices like missiles or torpedoes, the Pivot-Jewel device may offer cost benefits. The Pivot-Jewel technique has been employed in watch movement design for centuries. While its frictional characteristics are good, the Torquer Suspension is better.
The materials used for the Torsional Suspension provide high fatigue strength and heat-corrosion resistant. The torquer mechanism is fluid-damped and its fluid is extremely heat stable, resistant to oxidation and gumming and useable in a temperature range from -100°F to +527°F. It is also water repellent and highly resistant to permanent shear-breakdown and effects of most gases, salts, dilute alkalis and acids. As a working medium, its fluid shows little change in damping forces at temperatures down to -66°F. This characteristic plus excellent retention of dielectric properties over a wide temperature and frequency range, makes it an ideal damping medium. The inclinometer/accelerometer is backfilled with the fluid and degassed under a 5-micron vacuum for 24 hours and then sealed. Over thousands of fluid-damped force-balanced sensors are built by Jewell Instruments every year for many different applications including the military, rail and industrial markets. This figure has steadily increased as the ruggedness of this technology becomes more widely known.
FD = Fluid Damped
FS = Flexure Suspension
PB = Pivot & Bearing
Inclinometers, Analog
| LSO Series | 1-axis | FD |
| LSRP Series | 1-axis | FD |
| LSOX Series | 1-axis | FD |
| Emerald SMI Series | 1-axis | FS |
| Emerald RMI Series | 1-axis | FS |
| LCI Series | 1-axis | FS |
| LCF-100/101 Series | 1-axis | FD |
| LCF-300 Series | 1-axis | FD |
| LCF-2330 Series | 2-axis | FD |
Inclinometers, Digital
| DXI-100/200 Series | 1&2 Axis | FD |
| DXI-100/200-R Series | 1&2 Axis | FD |
| eDXI-100/200 Series | 1&2 Axis | FD |
Accelerometers, Analog Angular
| ASBC Series | PB | |
| ASMP Series | PB | |
| ASXC Series | PB, FD | |
Accelerometer, Analog Linear
| Emerald SMA Series | 1-axis | FS |
| LCA-100 Series | 1-axis | FS |
| LCF-200 Series | 1-axis | FD |
| LSMP Series | 1-axis | PB |
| LSBC Series | 1-axis | PB |
| LSBC-R Series | 1-axis | PB |
| LCF-2530 Series | 2-axis | FD |
| LCF-3500 Series | 3-axis | FD |
Accelerometer, Digital Linear
| DXA-100/200 Series | 1&2 Axis | FD |
| DXA-100/200-R Series | 1&2 Axis | FD |
One Milivolt Rule of Thumb:
In selecting an accelerometer/inclinometer, it has been observed over long hours of laboratory experience, that average output resolutions of about a millivolt can be achieved. This assumes an average human operator using average equipment in an average seismic environment.
Actual results may be improved by more than an order of magnitude if above average conditions are met. What this implies is that if we assume on millivolt to be the overall resolution of the readout system, we may directly convert this to both tilt angle or micro g acceleration data as the table indicates.
It must be emphasized that this data is conservative. A National Laboratory has successfully achieved results nearly ten times better. However, this may be accounted for by doctorate level physicists using superior instruments and procedures.
| Jewell Inclinometers | |
| Instrument Range | *Resolution (Accuracy) |
| ±1° | 0.72 arcseconds |
| ±3° | 2.2 arcseconds |
| ±14.5° | 10.4 arcseconds |
| ±30° | 21.6 arcseconds |
| ±90° | 64.8 arcseconds |
| Jewell Accelerometers (Fluid-Damped) | |
| Instrument Range | *Resolution (Accuracy) |
| ±0.25G | 50 µg |
| ±0.5G | 100 µg |
| ±1G | 0.2 mG |
| ±2G | 0.4 mG |
| ±5G | 1.0 mG |
| ±10G | 2.0 mG |
*NOTE: This includes all error sources with temperature held constant. If analog to digital converters are used, one may assume these resolution steps per bit, provided enough bits are available to cover the full range. 16-bit converters are recommended.
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-balance sensors 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!
