ACCELEROMETERS

An accelerometer is a device that gives an indication, usually in the form of a voltage, that is proportional to the acceleration to which it is subjected. The operation of an accelerometer is based on the property of INERTIA (Newton's First Law of Motion). A simple demonstration of inertia happens to us almost every day. You know that if your automobile is subjected to acceleration in a forward direction, you are forced back in the seat. If your auto comes to a sudden stop, you are drown forward. When your auto goes into a turn, you tend to be forced away from the direction of the turn-that is, if your auto turns left, you are forced to the right, and vice versa.

If we replace the human in an auto with a mass suspended in an elastic mounting system, as shown in figure 3-19, any acceleration of the auto will cause movement of the mass relative to the auto. The amount of displacement is proportional to the force causing the acceleration. The direction the mass moves is always opposite to the direction of the auto's acceleration.

Figure 3-19. - Auto with spring-suspended now.

The mass moves according to Newton's second Law of Motion which states: when a body is acted on by force, its resulting acceleration is directly proportional to the force and inversely proportional to the mass of the body.

When no acceleration is present, the mass will be at rest. When acceleration is present, the mass will lag in proportion to the acceleration force. In other words, the car moves but the mass wants to remain at rest.

Accelerometers are used principally in inertial navigation systems. They are used in aircraft and missile navigation systems as well as aboard ship. Some common types of accelerometers are described briefly in the following paragraphs.

THE BASIC ACCELEROMETER

Figure 3-20 is a simplified drawing of a basic accelerometer. It consists of a mass that is free to slide along the sensitive axis within the case. The movement of the mass is limited by the springs. When the case is accelerated, the mass, because of its inertia, tends to remain stationary. This results in a relative movement of the mass with respect to the case. When the stretch of the springs overcomes the inertia of the mass, the springs cause the mass to stop moving with respect to the case. The displacement of the mass with respect to the case is directly proportional to the acceleration of the case. When the case stops accelerating, the springs return the mass to its zero position (the reference position). To keep the springs from causing the mass to overshoot and oscillate about the reference position, some form of damping is needed. This is usually provided by an oil-filled case with vanes for oil to bypass the mass.

Figure 3-20. - The basic accelerometer.

An accelerometer is sensitive to gravity when its sensing axis is positioned so gravity can move or attempt to move the mass. This is useful in that we can use gravity as a reference for testing purposes, but it can be a serious problem because of the errors it may cause in acceleration measurements. If the unit is placed with the sensing axis vertical, the mass will be displaced such that the output is one "G," or one unit of gravity. This is done during testing. Then when the sensitive axis is turned so it is horizontal to the Earth, the springs center the mass, and the output of the unit is zero.

E-TRANSFORMER ACCELEROMETER

The E-transformer accelerometer (fig. 3-21) consists of a mass suspended from a calibrated leaf spring in a manner similar to a pendulum. The mass is effectively the armature of an E-transformer of the type used as an error detector in a servo system. The mass of the accelerometer is enclosed within a case that is filled with a damping liquid, which helps keep the pendulum from oscillating. The accelerometer is mounted so that acceleration in only the desired geometrical plane is detected.

Figure 3-21. - E-transformer accelerometer.

Refer to figure 3-21. Notice that when there is no acceleration, the pendulum remains centered and the accelerometer output is zero. However, when there is acceleration the mass or pendulum swings in the direction opposite to that of the acceleration, causing an output from the E-transformer. Since the amplitude of the pendulum's swing is proportional to the amplitude of the acceleration to which it is subjected, the output of the device indicates both the direction and amplitude of the acceleration. This output is within the limits of the equipment and is limited by physical stops.

PULSE-COUNTING ACCELEROMETER

The outputs of the accelerometers discussed so far are voltages, which are proportional to acceleration. These voltages are assigned scale factors (such as units per volt). The voltage represents the quantity. In many applications there is need for accelerometer output signals to be in digital form, which means that the signal consists of a series of pulses that indicates an actual number. Pulse counting accelerometers satisfy this need. Their pulse output can be supplied directly to computer circuits and other digital logic equipment. A schematic and a pictorial diagram of a pulse counting accelerometer is shown in figure 3-22, view (A) and view (B) .

Figure 3-22A. - Pulse counting accelerometer.

Figure 3-22B. - Pulse counting accelerometer.

When velocity remains constant, the brush spring holds the mass at null, and the brush rests on the reset contact of the switch. As acceleration occurs, the tendency of the mass to remain at a constant velocity causes the spring to compress. As the spring compresses, it allows the brush to move off the reset contact. If the acceleration is great enough, the brush will pass over the switch contacts for acceleration levels 1, 2, 3, and 4. These levels are determined by the stiffness of the spring.

As the brush passes over each contact (in a positive direction), an output pulse from each contact is coupled to one of four counters. This advances the counter one-half count. The accelerometer is designed so that as acceleration decreases, the mass tends to assume the new velocity. The counters will not advance the remaining half count until the brush once again touches the reset contact. With this type of pulse output, it is possible to record each time gravity forces have reached a predetermined level.

Q.21 Operation of an accelerometer is based on what physical property?
Q.22 What type of systems primarily use accelerometers?
Q.23 What special requirement is the pulse counting accelerometer designed for?