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Inclinometer Specifications

MEMS

Introduction

Inclinometers, also called tilt sensor, clinometers or slope sensors, are designed to measure the angle of an object with respect to the force of gravity. These tilt or level meters determine the pitch and/or roll angle and output these values via the appropriate electrical interface.

Measurement Principle of MEMS Inclinometers

Inclinometers measure the orientation angle of an object with respect to the force of gravity. This is done by means of an accelerometer, which monitors the effect of gravity on a tiny mass suspended in an elastic support structure. When the device tilts, this mass will move slightly, causing a change of capacitance between the mass and the supporting structure. The tilt angle is calculated from the measured capacitances.

MEMS Principle

Fig. 1: Principle of a MEMS sensor

icon_1 proof mass with
icon_2 electrodes
3 springs
4 fixed electrodes

The function can be illustrated with a simplified model with two electrodes: one fixed, and the other (the proof mass) is movable, suspended by spring elements (see Fig. 1). When the inclinometer is in a horizontal position (Fig. 2.1), the capacitance between the electrodes is measured. If the sensor is tilted (Fig. 2.2), the movable mass and its electrode will change position relative to the fixed electrode. This resulting change of capacitance between the two electrodes is measured by the sensor cell and used to calculate the new inclination value.

Position of a MEMS sensor

Fig. 2: Position of a MEMS sensor

icon_1 MEMS sensor in horizontal position
icon_2 MEMS sensor in tilted position

This working principle has been proven in many industrial and commercial applications, such as mobile phone motion sensors and car airbags. These consumer applications usually require low grade accelerometer cells which typically deliver accuracies of less than 1 degree. Unlike low consumer grade inclinometer cells, the MEMS sensors used in TILTIX inclinometers contain of an array of precise electrodes to improve the resolution and accuracy of the measurement. For TILTIX inclinometers that are designed for static or near-static measurements, the moving mass in the MEMS is physically damped to reduce the sensitivity of these sensors to frequencies above 29 Hz.

Limitations of Static Inclinometers

In case of strong shock and vibration, the physical damping in static inclinometers might not be sufficient to suppress disturbances. Software filters can only help to a limited extent in reducing the impact of such disturbances. For static TILTIX inclinometers, ”moving average” or “exponential” filters can be activated and configured to smoothen the signal, but with the downside that the fast reaction time of the MEMS inclinometer is lost and the response of sensor becomes slower.

For dynamic movements with strong accelerations, POSITAL’s Dynamic TILTIX inclinometers should be used. They are based on a different technology without physical damping so that there is no tradeoff between stability and response time.

Dynamic Inclinometer

For applications where sudden movements, shocks and vibrations are likely to be encountered, it is important to have sensors with fast response and a clean signal output. POSITAL’s dynamic inclinometers combine two measurement principles using two different MEMS sensors: a 3D acceleration sensor and a 3D gyroscope. The 3D acceleration sensor is not damped (unlike the units used in static inclinometers) and can follow rapid dynamic motions. At the same time, the 3D gyroscope measures rotational speeds, based on inertia principles. Signals from the accelerometers and gyroscopes are combined to produce an inclination measurement that fully compensates the effects of accelerations. As a result, dynamic TILTIX inclinometers can be used reliably on mobile equipment such as construction machinery, mining equipment, cranes or in robotic applications.

The diagram below compares the performance of a dynamic inclinometer with integrated gyroscope with the output from a conventional static inclinometer when both instruments are subjected to dynamic movements that involved heavy shocks and vibrations.

Tilt Measurement on a Moving Excavator

tilt_measurement

Innovative Algorithm for Reliable Results

The accelerometer measures the tilt position, while the gyroscope determines the rate of rotation. Accelerations have a huge impact on the accelerometer, but a limited effect on the measured rotation rates of the gyroscope. An innovative algorithm combines both signals, to get the best value out of each sensor. This way the sensor is able to separate the actual position value from the errors introduced by external accelerations.

accelerometer_gyroscope

Range and Mounting Option

The TILTIX series of inclinometers is available in two variants.

1. A dual axis sensor that is used for horizontal mounting. This version got two outputs, one for the X-Axis and one for the Y-Axis. Each of the axes shows the tilt angle with respect to the field of gravity.

dual_axis_sensor

2. A single axis tilt measurement version intended for vertical mounting with one axis output.

single_axis_sensor

Additional Functionalities of the Dynamic Inclinometer

The main purpose of dynamic inclinometers is to provide stabilized data for tilt angles without the need for configuring any sensor parameters. However, for the dynamic inclinometer with CANopen interface, it is also possible to output the acceleration forces (accelerometer) and rotation speed (gyroscope) separately for each of the three axes. These measurements are stored in mappable CANopen objects.

Monitoring the acceleration force along one or more axes can be used, to implement additional functions or safety features on the controller side. The controller could stop the machine when a certain acceleration threshold is exceeded. With the additional information about the rotation speed in x axis, it is possible to measure and monitor the horizontal (yaw-) rotation of the machine. It is up to the machine builder or system integrator to decide how this additional information might be used.

Data Processing

A high performance microcontroller is used to evaluate the sensor signals in real time and calculate the corrected slope angle. Temperatures are also measured and used by the compensation algorithms to correct for unwanted effects. Intelligent digital filter algorithms reduce ambient noise and vibration to provide a precise and stable signal under all environmental conditions.

Non-linearities in the MEMS sensors are identified through a series of reference measurements made during the production process. These non-linearities are stored as a set of calibration data within the sensor. During operation, the calibration data are used to correct the MEMS sensor raw values and output an accurate linearized tilt angle. Customized parameters like an offset correction (Preset) and a scaling function (in case of analog output signals) can be added by the customer.

Manufacturing of MEMS Sensor Chips

Due to the advances in manufacte if Micro-Electro-Mechanical Systems (MEMS) devices, these kind of sensors have become mass market products with an excellent performance/cost ratio. The basic measuring component in TILTIXinclinometers is a MEMS sensor cell that is embedded in a fully enscapuslated ASIC.

Common Specs

Sensor cycle time: This is the internal cycle time of the base sensor. A cycle time of 5ms means that the position value is updated every 5ms.

Interface cycle time: This is the cycle time the position value is transmitted via the communication interface. In contrast to the sensor cycle time (which is a fixed value), the interface cycle time can be easily adjusted by the customer at the interface level.

Absolute accuracy: The absolute accuracy is the worst case deviation between measured position and the actual position within the defined range.

Offset: When the inclinometer is positioned at the zero level, the output will show a small deviation. This error at the zero level is referred to as offset error.

Dynamic accuracy: This accuracy is determined the same way as the absolute accuracy, only that the device is exposed to external vibrations and accelerations. The dynamic accuracy was determined during lab tests on different equipment that simulate the moving environment of mobile machines. The stated dynamic accuracy serves as reference value; we recommend evaluating the dynamic behavior on your own machine, as vibrations and shocks differ from machine to machine. Lab tests were done with the following equipment:

  • Linear accelerations: The sensor is accelerated in one axis with 10 m/s² over 1s
  • Vibrations: Different vibration frequencies between 1-1000 Hz with a force of 1g

Resolution: This is the smallest possible step

Hysteresis: The definition of a hysteresis is that the output value of a system is not only dependent from the actual input, but also from past inputs. For inclinometers this means that the measured tilt angle is also dependent on the past position. There will be a small difference whether the inclinometer is tilted from 0° to 10° or from 20° to 10°. This difference is described by the hysteresis.

Temperature gradient: This value describes the change of the measured tilt angle for a change in temperature. If the inclinometer is in a static position and the temperature decreases or increases, the output value will also change according to the temperature gradient.

Settling time: This is a value that describes the dynamic behavior of a system. The settling time defines the time the inclinometer signal needs to reach and stay within 5% of the final position.

settling_time