Apparatus and method for measuring both suspension displacement and spring force in vehicle

A method for measuring both a suspension displacement and a spring force in a vehicle including first to fourth strain gages whose resistance values are varied when a strain is applied thereto, a suspension spring for varying resistance of the strain gages, and a Wheatstone bridge circuit, comprising the steps of:

• attaching the first to fourth strain gages whose resistance values are varied according to a variation of the suspension spring, to the suspension spring;
• calculating an output voltage about a resistance variation of each strain gage on the basis of an input voltage by using the Wheatstone bridge circuit and following equation (1);
• calculating a tension-compression stress from the following equation (2) using the output voltage calculated from equation (1);
• calculating a suspension displacement and a suspension spring force from following equations (3) and (4) using the tension-compression stress calculated from equation (2)
• Eo=(1+.mu.)/2*.delta.R/R*Ei (1)
• .sigma.=.tau.Eo/F*.delta.R/R (2)
• P=.pi..tau.d'RD (3)
• .delta.=.tau..pi.NaD/Gd (4)
• wherein Eo is output voltage, .mu. is a poisson ratio when reducing the length of the suspension spring, .delta. is suspension displacement, R is an equivalent resistor of the resistors of the strain gages, Ei is input voltage, .sigma. is tension-compression stress of the suspension spiring, .tau. is shearing stress of the suspension spring, F is a strain gage ratio, P is a suspension siring force, d is a diameter of coil of the suspension spring, D is an average diameter of coil of the suspension spring, Na is effective turn ratio of the suspension spring, and G is a shearing modulus of coil.