Accelerometer sensor for linear position

Accelerometers measures acceleration using a mass suspended on a force sensor, as shown in figure below. When the sensor accelerates, The inertial resistance of mass will cause the force sensor to deflect. By measuring the deflection, the acceleration can be determined. In this the mass cantilevered on the force sensor. A base and housing enclose the sensor. A small mounting stud ( A threaded shaft) is used to mount the accelerometer.

                                                     Fig:  A cross section of accelerometer

Accelerometers are dynamic sensors, typically used for measuring vibrations between 10Hz to 10KHz. Temperature variations will affect the accuracy of the sensors. Standard accelerometers can be linear up to 100,000 m/s²  and high shock design can be used up to 1,000,000 m/s²   .  There is often a tradeoff between wide frequency range and device sensitivity. Higher sensitivity requires a larger mass. Below table shows the sensitivity of two accelerometers with different resonant frequencies. A smaller resonant frequency limits the maximum frequency for the reading. The small frequency results in a smaller sensitivity. The unit for sensitivity is charge per  m/s²   .

Resonant frequency (Hz)	Sensitivity (pC/ (m/s2))
22KHz	4. 5
180KHz	0.004

The force sensor is often a small piece of piezoelectric material. The piezoelectric material can be used to measure the force in shear or compression. Piezoelectric based accelerometers typically have parameters such as,

-100 to 250⁰ C operating range

1mV/g  to 30 V/g sensitivity

Operate well below one fourth of natural frequency.

The accelerometer is mounted on vibration source as shown in figure below. Accelerometer is electrically isolated from the vibration source so that sensor may be grounded at the amplifier so it can reduce electrical noise. Cables are fixed to the surface of the vibration source, close to the accelerometer, and are fixed to the surface as often as possible to prevent noise from the cable striking the surface. Background vibrations can be detected by attaching control electrodes to non vibrating surfaces. Each accelerometer is different, But some general application guidelines are;

·         The control vibration should be less than one third of the signal for the error to be less than 12%.

·         Mass of accelerometer should be less than a tenth of the measurement mass.

·         These devices can be calibrated with shakers, for eg : A 1gm shaker will hit a peak velocity of 9.81 m/s²

                                                        Fig: Mounting an accelerometer

Equipment normally used when doing vibration testing as shown in figure below. The sensor needs to be mounted on the equipment to be tested. A pre-amplifier normally converts the charge generated by the accelerometer to a voltage. The voltage can then be analyzed to determine the vibration frequencies.

                                         Fig: Typical Connection for Accelerometer

Accelerometer is commonly used for control systems that adjust speeds to reduce vibration and noise. Computer controlled milling (CCM) machines now use these sensors to actively eliminate chatter, and detect tool failure. The signal from accelerometer can be integrated to find velocity and acceleration.

Currently accelerometer cost hundreds or thousands per channel. But, advances in micromachining are already beginning to provide integrated circuit accelerometer at a low cost. Their current use is for airbag deployment systems in automobile.