Rotational movement transducer

f. agric.

Engng

Res.

(1968) 13 (3) 292-294

Rotational

Movement

Transducer

C. A. CARLOW* 1. Introduction In an investigation into the factors affecting a tractor-trailer combination, the measurement and recording on magnetic tape of the amount of rotation of both tractor and trailer wheels was required. Cams and micro-switches proved inadequate mainly because of the limited number of pulses which could be generated during one revolution of the wheel but also because of the physical difficulty of attachment. A smooth analogue output proportional to wheel speed was one of the main requirements and for the type of recording system available, a variable pulse frequency output was necessary. A photo-electric device using a phototransistor and radial optical grating was developed to give a relatively large number of pulses for each revolution of the wheel.

Fig. 1. Lamp andphototransistor

unit straddling Perspex disc transducer method of attachment to wheel

with cover removed und

2. Construction The original grating was made from a drawing of a circle having alternate black and white peripheral areas of 1” arc. Reduced photographs of the drawing on high contrast film were *N.I.A.E. Scottish Station, Penicuik, Midlothian 292

C.

A.

293

CARLOW

sandwiched between two Perspex discs. The resulting grating was not entirely satisfactory, the double thickness of Perspex was clumsy and accurate centring of the completed disc was difficult. It did, however, give the desired result. Later a grating was made by engraving shallow radial lines around the periphery of a thin Perspex disc and filling these with black paint (Fig. 1). The practical limit of this type of construction was found to consist of a disc of 8 in dia with 720 lines equally spaced around the periphery. In each revolution of the transducer 720 pulses were produced and this output was further adapted electronically to give double this number of pulses, if required (Fig. 2). O 01

720pulse output

R 24

nov

Fig. 2. Transducer circuit

RI RZ R, R, R, R, R, R* R, RI0 RI,

ki2 kQ kfl kfi kn kCI kf1 1 kSZ 22 kQ 22 kR 2.2 kQ

15 2.2 220 82 10 I.8 2.2

R 12 R 13 R R:: RI, RI, RI, R 19 RWI RX R 22

33 100 4.1 4.1 82 8.2 15 4.3 2.2 1 22

kbl kn kR kCI kR kfi kR kQ kQ kR kR

C7 C, co

R*3 RN L

C;”

50 32 50 1000 200 0.022

pF BF HF pF pF pF

Cl, Tr, Tr, Tr, Tr,

0.022 1000 25 1000 500

bF pF pF pF IF

Tr, Tr, Tr, Tr,

OCP 71 GET 885

D, D, D3

GET 885 GET 885

L

GET GET GET GET

885 885 885 885

OA 81 OA 81 OA 81 I2 V. 0.1 A

The mechanical form of the device was determined by the need to have standardization of mounting and easy accessibility. A totally enclosed unit was designed comprising a shallow drum cover and a backplate in light alloy. The lamphouse and phototransistor assembly is carried at the periphery of a disc of printed circuit board on which the electronic components are mounted. For use on a wheel the circuit disc is mounted on a hub carried on the central shaft of the transducer. The shaft runs on ball bearings set in the backplate and the lamphouse and phototransistor, assembly is arranged to straddle the radial lines on the Perspex disc, the latter being supported on a flange formed integrally with the backplate. The central shaft is prevented from rotating with the body of the transducer by an arm fixed to it and extending to a suitable fixed mounting point on the tractor or trailer body. For use as a static tachometer, the positions of the circuit disc and the grating are interchanged and it is then possible to drive the shaft while the body of the transducer is held stationary. Electrical connections to the circuit board are made by way of a

294

ROTATIONAL

MOVEMENT

TRANSDUCER

central bore in the shaft for wheel use and through an opening in the backplate as a tachometer. The backplate and cover, when bolted together, form a sealed unit. 3. Circuit design The output from the phototransistor Tr, is amplified by Tr, and shaped by the Schmitt trigger circuit formed by Tr, and Tr,. External output connections are taken from the collectors of both Tr, and Tr, for use with a magnetic tape recorder or, alternatively, the output from Tr, can be used to drive an indicating ratemeter. The output from Tr, is coupled through C, and the bias chain R,, and R,, to form a differentiating network which drives Tr,. This stage operates as a phase splitter, equal and opposite polarity signals being available at the collector and emitter of the transistor and the negative-going spikes from each are selected by diodes D1 and Dz and fed to the inverting stage Tr,. The amplified output triggers the emitter-coupled monostable output stage formed by Tr, and Tr, which produces a pulse of fixed width each time it is triggered. Both the collectors of Tr, and Tr, are provided with output connections in the same manner as Tr, and Tr, to allow the output to be recorded. Two output arrangements are thus provided, (i) that derived from Tr, and Tr, where the pulses correspond to the passage of the dark lines on the grating through the light beam providing 720 pulses for each revolution; and (ii) the output from Tr, and Tr, where the pulses correspond to both leading and trailing edges of the pulses from the first output and provide 1440 for each revolution. The unit can record changes in wheel speed over distances corresponding to under t” of circular movement. It has also been found useful in the control of servo-actuated mechanisms.