J. crgric. Erlgng Res. (1975) 20, 347-352 An Extended Octagonal Ring Transducer for Use in Tillage Studies R. J. GODWIN* The design and the related devetopment problems of an extended octagonal ring transducer are discussed. The transducer was primarily designed to monitor the two force components and the moemnt in the plane of the two forces of the soil reaction to simple times. The performance figures of the transducer are given, the sensitivities of the strain gauge outputs are significantly higher than those expected from the prediction equations used to design the transducer. 1. Introduction The force system in tillage studies where the soil failure is symmetrical consists of two mutually perpendicular force components, a horizontal force F, and a vertical force F, and the moment M,, in the plane of these two forces. Because of the symmetrical soil failure the lateral force F,, and moments M, and M, are zero. Force measurement by suspending the soil engaging implement from a series of single dynamo- meters has been used extensively1-3 but there is little detailed information about the accuracy of such systems. A report by Wilkinson4 suggests that friction in locating bushes could cause substantial errors in static and quasi-static force systems. An alternative technique to the multi-dynamometer system is the extended ring transducer; this offers the following advantages: (a) alleviation of the frictional problems arising from the dynamometer suspension bushes; (b) single component construction, compactness and simple mounting; which reduces the difficulties with precision alignment necessary for accuracy when a multi-dynamometer suspension system is used; (c) simultaneous monitoring of the two force components and the moment in the plane of these two forces. 2. Principle of operation The Extended Octagonal Ring Transducer consists of a machined block, normally of steel or aluminium, the basic shape of which is illustrated in Fig. 1. Strain gauges mounted at strain nodes, i.e. positions where there is no contribution to the strain from the other force component, are used to monitor the horizontal and vertical force components and the resulting moment. The accurate location of the strain nodes and subsequent positioning of the strain gauges is critical in producing a transducer with low cross sensitivity between the force component channels. The strain gauge bridge circuits, shown in Fig . 2 (right), theoretically enable the outputs from the force bridges F, and F, to be independent of the position of th.e load and the output from the moment bridge M, to be proportional to the applied moment. 3. Transducer design The dimensions of the extended octagonal ring transducer given in Fig. I were designed to withstand the applied moment, since in normal use this would cause the maximum strain. The moment sensitivity expression Eqn (I), was used for a design moment of 1260 Nm, with a load factor of 3 to ensure linearity within the working range. *Field Engineering Division, National College of Agricultural Engineering, Silsoe, Bedford Received 5 March 1975; accepted in revised form 2 April 1975 347