Feedback and servo control for electric power steering system with hydraulic transmission

A power steering system (10) for a vehicle having dirigible wheels (13), a power cylinder (18) having left and right ports (56 and 58) and adapted to supply a powered assist to steering the dirigible wheels (13) of the vehicle upon the supply of a pressurized fluid to one of the left and right ports (56 and 58), a reversible fluid pump (24) having first and second ports (78 and 80), an electric motor (26) operatively connected to and capable of reversibly driving the fluid pump (24), a first fluid line (20) connecting the first port (78) of the fluid pump (24) to the left port (56) of the power cylinder (18), a second fluid line (22) connecting the second port (80) of the fluid pump (24) to the right port (58) of the power cylinder (18), a pair of check valves (52) disposed in a suction line (49) connected to a fluid reservoir (50) and connected one each to the first and second fluid lines (20 and 22), and a primary applied steering torque sensor (14) which generates a signal (V.sub.TT) in response to at least an applied steering torque (T.sub.s);

• the improvements wherein the system (10) further comprises:
• a first pressure transducer (54) sensing the fluid pressure (LP.sub.p) in the first fluid line (20);
• a second pressure transducer (55) sensing the fluid pressure (RP.sub.p) in the second fluid line (22);
• an electronic control means (32) to which the first and second pressure transducers (54 and 55) and the primary applied steering torque sensor (14) are operatively connected, and which controls actuation of the electric motor (26);
• a redundant applied steering torque sensor (15) which generates a redundant signal in response to the applied steering torque (T.sub.s) and which is connected to the electronic control means (32);
• an actuation speed measuring means (33) operatively connected to the electronic control means (32) for providing a tachometer signal (V.sub.t) representative of steering movement of the dirigible wheels (13);
• and
• means (35) for providing a redundant measure of the fluid pressures (LP.sub.p and RP.sub.p) in the first and second fluid lines (20 and 22), wherein the electronic control means (32) establishes servo control (322, 342) over the powered assist to steering supplied by the power cylinder (18) in dependence upon the fluid pressure (LP.sub.p or RP.sub.p) sensed by one of the first and second pressure transducers (54 or 55);
• wherein the electronic control means (32) establishes closed loop servo control (322, 342) over the electric motor (26) in dependence upon the fluid pressure (LP.sub.p or RP.sub.p) sensed by the one of the first and second pressure transducers (54 or 55) and achieves a substantially linear relationship between the applied steering torque (T.sub.s) and the powered assist to steering supplied by the power cylinder (18);
• wherein the electronic control means (32) generates an error signal (V.sub.c) in response to (at least the generation of the signal (V.sub.TT) by the primary applied steering torque sensor (14), the error signal (V.sub.e) being dependent upon the fluid pressure (LP.sub.p or RP.sub.p) sensed by one of the first and second pressure transducers (54 or 55);
• wherein the electronic control means (32) determines an internal control loop gain value (ICG) as a function of at least vehicle speed, the internal control loop gain value (ICG) being further related to the signal (V.sub.TT) generated by the primary applied steering torque sensor (14), and wherein the electronic control means multiplies the error signal (V.sub.e) by the internal control loop gain value (ICG) and causes the electric motor (26) to drive the fluid pump (24) in a manner which generates the fluid pressure (LP.sub.p or RP.sub.p) sensed by the one of the first and second pressure transducers (54 or 55) in accordance with a product of the error signal (V.sub.e) and the internal control loop gain value (ICG);
• wherein the electronic control means (32) generates the error signal (V.sub.e) by compensating the signal (V.sub.TT) generated by the primary applied steering torque sensor to yield a compensated steering torque signal (V.sub.c);
• subtracting from the compensated steering torque signal (V.sub.c) a tachometer feedback signal (V.sub.t), representative of steering movement of the dirigible wheels (13), to yield an input signal (V.sub.i);
• obtaining a pressure-dependent loop feedback signal (V.sub.p) and subtracting the feedback signal (V.sub.f) from the input signal (V.sub.i) to yield the error signal (V.sub.e);
• wherein the electronic control means (32) terminates the powered assist provided by the system (10) when the redundant signal from the redundant applied steering torque sensor (15) fails to correlate with the signal (V.sub.TT) from the primary applied steering torque sensor (14);
• wherein the vehicle includes a steering wheel (12) to which the primary applied steering torque sensor (14) is operatively connected, and wherein the actuation speed measuring means (33) comprises a tachometer (62) for measuring the rotational speed of at least one of a steering wheel (12) and the electric motor (26);
• wherein the electronic control means (32) terminates the powered assist provided by the system (10) when the redundant measure of the fluid pressures (LP.sub.p and RP.sub.p) provided by the means (35) for providing the same fails to correlate with the fluid pressure (LP.sub.p or RP.sub.p) sensed by one of the first and second pressure transducers (54 or 55);
• wherein the means (35) for providing a redundant measure of the fluid pressures (LP.sub.p and RP.sub.p) in the first and second fluid lines (20 and 22) comprises a third pressure transducer (113) selectively connected to that one of the first and second fluid lines (20 or 22) having a higher pressure than the other;
• wherein the vehicle presents a steering load stiffness to the power steering system (10);
• wherein the electronic control means (32) employs the fluid pressures measured by the first and second pressure transducers (54 and 55) to counteract a hydromechanical resonance occurring predominantly between a moment of inertia of the electric motor (26) and a system spring rate presented by the steering load stiffness;
• wherein the electronic control means (32) treats a hydromechanical resonance as a resonance between a mass element predominated by the moment of inertia of the electric motor (26) and a theoretical spring element dominated by tire loading characteristics associated with the tires (21);
• and wherein the electronic control means (32) employs the fluid pressures measured by the first and second pressure transducers (54 and 55) to control the electric motor (26) so that the fluid pump (24) is not driven in harmony with the hydromechanical resonance;
• wherein the electronic control means provides compensation of at least one of a magnitude of an applied steering torque (308 or 328), and operation (315 or 335) of the electric motor (26) or the fluid pressure (LP.sub.p or RP.sub.p) measured (320 or 340) by one of the first and second pressure transducers (54 or 55), and wherein the compensation (308 or 328) of the magnitude of the applied steering torque predominantly includes at least one pole and compensation (315 or 335) of operation of the electric motor (26) or the fluid pressure (LP.sub.p or RP.sub.p) is predominated by a low-frequency zero;
• wherein the vehicle includes a steering wheel (12), such that the application of an applied steering torque (T.sub.s) to the steering wheel (12) results in application by the system (10) of a steering force assist to the dirigible wheels (13);
• and wherein the electronic control means (32) employs the fluid pressures (LP.sub.p and RP.sub.p) measured by the first and second pressure transducers (54 and 55) to establish an internal servo control loop (322 or 342) between an input signal (V.sub.i) derived from application of an applied steering torque (T.sub.s) to the steering wheel (12) and the steering force assist to the dirigible wheels (13);
• wherein when the fluid pressure (LP.sub.p or RP.sub.p) measured by one of the first and second pressure transducers (54 or 55) is greater than the fluid pressure (RP.sub.p or LP.sub.p) measured by the other of the second and first pressure transducers (55 or 54) and exceeds a predetermined threshold value, the electronic control means (32) calibrates the other of the second and first pressure transducers (55 or 54) by assigning a zero value to an internal signal representative of the fluid pressure (RP.sub.p or LP.sub.p) measured by the other of the second and first pressure transducers (55 or 54);
• and
• wherein both of the first and second fluid lines (20 and 22) are placed in direct fluid communication with the reservoir (50) should the powered assist to steering provided by the system (10) fail.

A power steering system (10) for a vehicle having dirigible wheels (13), a power cylinder (18) having left and right ports (56 and 58) and adapted to supply a powered assist to steering the dirigible wheels (13) of the vehicle upon the supply of a pressurized fluid to one of the left and right ports (56 and 58), a reversible fluid pump (24) having first and second ports (78 and 80), an electric motor (26) operatively connected to and capable of reversibly driving the fluid pump (24), a first fluid line (20) connecting the first port (78) of the fluid pump (24) to the left port (56) of the power cylinder (18), a second fluid line (22) connecting the second port (80) of the fluid pump (24) to the right port (58) of the power cylinder (18), a pair of check valves (52) disposed in a suction line (49) connected to a fluid reservoir (50) and connected one each to the first and second fluid lines (20 and 22), and a primary applied steering torque sensor (14) which generates a signal (V.sub.TT) in response to at least an applied steering torque (T.sub.s);

• the improvement wherein the system (10) further comprises,
• a two-position, three-way valve (39) fluidly connecting the first fluid line (20) and the second fluid line (22) to the fluid reservoir (50), the three-way valve (39) being operable to connect the second fluid line (22) to the fluid reservoir (50) when a fluid pressure (LP.sub.p) in the first fluid line (20) is greater than a fluid pressure (RP.sub.p) in the second fluid line (22), and to connect the first fluid line (20) to the fluid reservoir (50) when the fluid pressure (RP.sub.p) in the second fluid line (22) is greater than the fluid pressure (LP.sub.p) in the first fluid line (20);
• and
• the pair of check valves (52) disposed in the suction line (49) being directly connected one each to the left and right ports (56 and 58) of the power cylinder (18), oriented so as to permit a flow of fluid from the fluid reservoir (50) through the suction line (49) and to the connected left or right port (56 or 58) of the power cylinder (18) when a pressure (LP.sub.p or RP.sub.p) at that left or right port (56 or 58) is less than a pressure in the reservoir (50);
• wherein the suction line (49) has a lesser flow impedance than the first and second fluid lines (20 and 22), thereby causing a flow of fluid from the fluid reservoir (50) through the suction line (49) to be returned to the fluid reservoir (50) through the power cylinder (18), the one of the first and second fluid lines (20 and 22) having higher pressure, the fluid pump (24), the other of the first and second fluid lines (20 or 22) and the three-way valve (39).

A power steering system (10) for a vehicle having dirigible wheels (13), a power cylinder (18) having left and right ports (56 and 58) and adapted to supply a powered assist to steering the dirigible wheels (13) of the vehicle upon a supply of a pressurized fluid to one of the left and right ports (56 and 58), a reversible fluid pump (24) having first and second ports (78 and 80), an electric motor (26) operatively connected to and capable of reversibly driving the fluid pump (24), a first fluid line (20) connecting the first port (78) of the fluid pump (24) to the left port (56) of the power cylinder (18), a second fluid line (22) connecting the second port (80) of the fluid pump (24) to the right port (58) of the power cylinder (18), a pair of check valves (52) disposed in a suction line (49) connected to a fluid reservoir (50) and connected one each to the first and second fluid lines (20 and 22), and a primary applied steering torque sensor (14) which generates a signal (V.sub.TT) in response to at least an applied steering torque (T.sub.s);

• the improvement wherein the system (10) further comprises:
• electronic control means (32) establishing a desired fluid pressure for one of the first fluid line (20) and the second fluid line (22) in response to at least the generation by the primary applied steering torque sensor (14) of the signal (V.sub.TT) corresponding to an applied steering torque (T.sub.s);
• a first pressure transducer (54) operatively connected to the electronic control means (32) and sensing a fluid pressure (LP.sub.p) in the first fluid line (20);
• a second pressure transducer (55) operatively connected to the electronic control means (32) and sensing a fluid pressure (RP.sub.p) in the second fluid line (22);
• and
• the electric motor (26) being controlled by the electronic control means (32) so as to drive the fluid pump (24) in a manner which urges the fluid pressure (LP.sub.p or RP.sub.p) in one of the first and second fluid lines (20 and 22) towards the desired fluid pressure established by the electronic control means (32).