GB1362121A - Accelerometers - Google Patents

Abstract

1362121 Accelerometers FERRANTI Ltd 4 June 1973 [6 June 1972] 26478/72 Heading G1N A force balance accelerometer comprises an arm 20, Fig. 1, attached to a coil 17 which is pivotally mounted in a uniform magnetic field provided by permanent magnet 11. When an acceleration occurs the arm and coil assembly pivots and the arm makes an electric connection with contact 22 or 23 depending on the direction of acceleration; this causes a current to flow through coil 17 in a direction which moves the arm away from the contact to break the circuit. This process is repeated as long as the acceleration continues, and the average value of the current is proportional to the acceleration. The use of two contacts 22, 23 makes the device sensitive to accelerations in the two directions 30, 32, and also allows a cyclic acceleration (such as arises when a constant acceleration occurs but the device is vibrating) to be measured. The sensitivity of the device can be altered by nut 24 which affects the balance of arm 20. For horizontal use the arm may be centred by hair springs, while for vertical use gravity may be relied on or a D.C. current may be passed through the coil. The sensitivity may also be altered by an adjustable air gap 35 in the magnetic circuit, or by using an electromagnet to vary the permanent magnet field. The contacts 22, 23 may be replaced by an optical arrangement such as a light emitting diode and a light detector, arm 20 interrupting the path between them. When contact is made in Fig. 1, one of a pair of transistors may be activated as in Fig. 2 (not shown) to pass a constant current through the coil in an appropriate direction. An alternative arrangement which passes a linearly increasing current through the coil is shown in Fig. 4 (a), where switch 49 is formed by arm 20 and contact 22 of Fig. 1. When switch 49 is open, i.e. no acceleration is occurring, there is no potential at point 46 and consequently transistor switch 40 is closed. This prevents any voltage occurring across capacitor 38. If switch 49 closes, the rise of potential at 46 opens switch 40 and the capacitor charges at constant rate due to current flowing through transistor 39. As the capacitor voltage rises an increasing current is fed to coil 17 by Darlington pair 36 until the switch 49 is opened. This closes switch 40 to short out capacitor 38 and the process then repeats, the mean coil current being measured as mean voltage across resistor 35. Diode 47 compensates temperature changes in the switch 40. A circuit similar to Fig. 4 (a) is also provided for contact 23 of Fig. 1.