From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The principle of operation and usage instructions for the Wimperis Accelerometer.
| Identifier | ExFiles\Box 113\2\ scan0021 | |
| Date | 14th November 1922 guessed | |
| 6 WIMPERIS ACCELEROMETER PRINCIPLE OF OPERATION The general nature of the internal mechanism may be described as a lop-sided copper disc, mounted on a vertical axis and controlled in its rotation by a coiled spring. Any acceleration causes the heavier side of the disc to lag behind, and so partially wind up the spring. The degree to which the spring is wound up measures the acceleration. Any tendency of the disc to oscillate is checked by a magnetic field at right angles to the plane of the disc. Besides these parts there is what is called “the compensating balance,” which causes the instrument to record absolutely correctly, even when travelling around railway curves, or when on a road heavily cambered to one side or the other. In Fig. 2 the copper disc is shown at D; it has a hole cut in it near the circumference, which has the effect of throwing the centre of gravity slightly out of the centre of figure. On the pivot of the disc is fastened a spur wheel which gears in with another equal spur wheel mounted on a parallel axis and carrying the pointer N. It will be seen that there is a small permanent magnet placed so as to “damp” the motions of the copper disc without having any of the “sticking” qualities which accompany frictional damping. The spring which is coiled up by the rotation of the disc is not shown, but it lies in the horizontal plane just above the disc. In later forms of the instrument the arrangement described has been varied slightly, but not so as to affect in any way the general principle of working. The forces acting at the centre of gravity of the disc are directly proportional to the impressed acceleration, but as the disc rotates through angles of 30 deg., 45 deg., 60 deg., &c., the “arm” gets less, and the couple twisting the spring does not rise so rapidly as the acceleration producing it. The scale is graduated in accordance with this fact, and the divisions close up when the angles get considerable. The law which governs this is that of the change of the ratio 1/cos θ. In the Recording form the angle θ is kept small and the scale is sensibly linear. It is now necessary to describe the mode of action of the compensating device. If, whilst the disc is deflected by an acceleration, a second acceleration (or slope) should be acting at right angles to the first, there will be an additional couple tending to wind (or unwind) the spring. The needle must then give false indications of the acceleration which it is desired to measure. This would be a very serious fault, since such transverse accelerations are very common in practice and often of considerable amount, sometimes as much as 10 ft. per second per second. (This type of error is noticeable in the pendulum forms of instrument, where the complicating vertical acceleration often far exceeds that of gravity itself.) To get over this difficulty the two gear wheels, already mentioned, were added, and it was so arranged that the moments of mass about the two respective axes should be equal. This equality of mass moments makes the system equivalent to two equal copper discs geared together at their circumferences, and each having their centres of gravity excentric to the same extent. The effect of this is illustrated in the diagram to the right-hand side in Fig. 2. Inspection of this diagram will show that forces in the direction P will cause the two discs to roll together, whilst forces in the direction Q cause no rotation whatever. Forces perpendicular to the paper can, of course, produce no rotation of the disc. The instrument records the acceleration in one of the three directions of space only, and is not affected by whatever may be happening in the other two. Thus the dial may even be tilted until it is vertical, so that the whole force of gravity acts across it without the readings being affected at all. An important feature of the instrument is that it singles out the acceleration produced by the driving force operating the vehicle. The reading is independent of the effect of any gravitational forces. Thus, if a car be fitted with electric motors powerful enough to give on a level track a starting acceleration of say 2'2 feet per second per second, the instrument will still read 2'2, even if the start be made up a slope or down it. The instrument will still read 2'2 even if the hill be so steep that the current is unable to increase the speed. In that case the accelerative effect of the motive power is balanced by the retardation due to the hill slope, but tractive effort corresponding to an acceleration of 2'2 is still being exerted, and this the needle will show. ELLIOTT BROTHERS, (LONDON) LIMITED, Established 1800. CENTRAL BUILDINGS, WESTMINSTER, LONDON, S.W., AND CENTURY WORKS, LEWISHAM, LONDON, S.E. 11 WIMPERIS ACCELEROMETER HINTS FOR USING THE RECORDING ACCELEROMETER This instrument is designed and calibrated for use horizontally. When preparing for test the pointer should first be released from the side, so that it is free to swing to the centre of the scale. The case should be quite level, when the pen will come to rest on the zero line of paper when paper is running. Should the pen not do so, the same can be readily adjusted by means of the zero adjusting device, which will be found attached to the brass bridge piece underneath the removable glass cover. The pen should now be washed in methylated spirits and then filled with ink, using that which has been supplied with the instrument. The methylated spirits washes off any dirt or grease that may be on the pen and enables the ink to flow freely, and we strongly recommend this treatment. The pen should now be carefully set to bear very gently on the paper, no more pressure being put on than just sufficient to cause the pen to mark properly across the scale. This adjustment can be made very simply by means of the brass screw, which will be found at the end of the pointer. DO NOT ON ANY ACCOUNT TRY TO BEND THE POINTER To fit a roll of paper first pull the hinged spring lever, on left-hand side of clock, over to an angle of 90°, where it will rest on a stop. The roll of paper can then be placed on the pin on the opposite side of the bracket and the lever put back again into original position. The paper should then be threaded under the three blued steel springs on paper table and made to engage with teeth on pin-wheel, when it will be ready for use. The paper will pay out through the slot in the bottom of the case. When the test is finished the pen should be carefully taken off the pointer and washed in methylated spirits till all sign of ink has disappeared; it will then be in good condition to start marking when required again. If ink is allowed to dry in the fine tube of the pen it will probably cause trouble in getting the ink to flow again. A pneumatic marking pen will be found fitted at the side for marking on margin of paper, and will be found useful for a variety of purposes. The clock is arranged to give correct timing for about half-an-hour, it will then require re-winding. The paper pays out at the rate of 6" per minute in some instruments and at the rate of 3" per minute in others. The lever for winding the clock will be noticed on the side of the instrument, and a “stop and start” lever on the other side of the case. Care should be taken to prevent dirt getting into the covered-in compartment containing the movement. ELLIOTT BROTHERS, (LONDON) LIMITED, Established 1800. CENTRAL BUILDINGS, WESTMINSTER, LONDON, S.W., AND CENTURY WORKS, LEWISHAM, LONDON, S.E. | ||