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

• US 6,152,254 A
• Filed: 10/26/1998
• Issued: 11/28/2000
• Est. Priority Date: 06/23/1998
• Status: Expired due to Fees

• Current Assignee: Techco Corp.
• Original Assignee: Techco Corp.
• Inventors: Phillips, Edward H.
• Primary Examiner(s): Swann, J. J.
• Assistant Examiner(s): Lum, L.
• Application Number:
  US09/179,152
• Time in Patent Office: 764 Days
• Field of Search: 180/421, 180/422, 180/423, 701/41, 701/42
• US Class Current: 180/422
• CPC Class Codes: B62D 5/0463 calculating assisting torque from the motor based on driver input
• CPC Class Codes: B62D 5/06 fluid, i.e. using a pressurised fluid for most or all the force required for steering a vehicle
• CPC Class Codes: B62D 5/065 characterised by specially adapted means for varying pressurised fluid supply based on need, e.g. on-demand, variable assist

Abstract

A vehicle power steering system 10 achieves a substantially linear control relationship between an applied steering torque input T_s and a resulting steering force output, that is, a powered assist to vehicle steering. The system 10 includes a reversible fluid pump 24 driven by an electric motor 26. A pair of fluid lines 20 and 22 supply pressurized fluid from the pump 24 and to the left and right ports 56 and 58 of a power cylinder 18. A pair of pressure transducers 54 and 55 sense the fluid pressures LP_p and RP_p in the lines 20 and 22. The pressure transducers 54 and 55 are connected to an electronic control 32 which establishes an internal closed servo control loop 322 or 342 over the powered assist to vehicle steering provided by the power cylinder 18, for example, over actuation of the motor 26. The system 10 is preferably regenerative and includes a variety of safety features which prevent system runaway and allow unencumbered manual steering in case of failure of the system. The system 10 is also arranged for continuous rejuvenation of its hydraulic fluid, and the structure providing such rejuvenation can be employed in a wide variety of other systems. The electronic control 32 employs the measured fluid pressures LP_p and RP_p to counteract a hydromechanical resonance occurring predominantly between the moment of inertia of the motor 26 and a system spring rate consisting primarily of the vehicle steering load stiffness. The system 10 substantially eliminates stability problems in EPS systems, for example, a well-recognized low frequency stability control problem which has typically not been overcome in prior EPS systems. The system 10 provides full time independent verification of instant values of steering boost, substantially eliminating prior concerns relating to auto-steer. The system 10 also eliminates concerns in prior EPS systems such as mechanical over constraint, Coulomb friction, wear and backlash.

Claims

1. 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_TT) in response to at least an applied steering torque (T_s);

the improvement wherein the system (10) further comprises;

a first pressure transducer (54) sensing a left fluid pressure (LP_p) in the first fluid line (20);

a second pressure transducer (55) sensing a right fluid pressure (RP_p) in the second fluid line (22); and

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);

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_p or RP_p) sensed by one of the first and second pressure transducers (54 or

     55).

2. The power steering system (10) according to claim 1, 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_p or RP_p) sensed by the one of the first and second pressure transducers (54 or 55).

3. The power steering system (10) according to claim 1, wherein the electronic control means (32) achieves a substantially linear relationship between the applied steering torque (T_s) and the powered assist to steering supplied by the power cylinder (18).

4. The power steering system (10) according to claim 1, wherein the electronic control means (32) generates an error signal (V_e) in response to at least the signal (V_TT) by the primary applied steering torque sensor (14), an error signal (V_e) being dependent upon the fluid pressure (LP_p or RP_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_TT) generated by the primary applied steering torque sensor (14); and

wherein the electronic control means multiplies the error signal (V_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_p or RP_p) sensed by the one of the first and second pressure transducers (54 or

     55) in accordance with the product of the error signal (V_e) and the internal control loop gain value (ICG).

5. The power steering system (10) according to claim 4, wherein the electronic control means (32) generates the error signal (V_e) by compensating the signal (V_TT) generated by the primary applied steering torque sensor to yield a compensated steering torque signal (V_c);

subtracting from the compensated steering torque signal (V_c) a tachometer feedback signal (V_t), representative of steering movement of the dirigible wheels (13), to yield an input signal (V_i);

obtaining a pressure-dependent loop feedback signal (V_f) and subtracting the feedback signal (V_f) from the input signal (V_i) to yield the error signal (V_e).

6. The power steering system (10) according to claim 1, further comprising a fluid reservoir (50) and a suction line (49) selectively connecting one of the left and right ports (56 and 58) of the power cylinder (18) to the fluid reservoir (50).

7. The power steering system (10) according to claim 1, further comprising a fluid reservoir (50) and 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 the fluid pressure in the first fluid line (20) is greater than the fluid pressure in the second fluid line (22), and to connect the first fluid line (20) to the fluid reservoir (50) when the fluid pressure in the second fluid line (22) is greater than the fluid pressure in the first fluid line (20).

8. The power steering system (10) according to claim 7, further comprising 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 a power cylinder (18), oriented so as to permit the 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 the fluid pressure at that left or right port (56 or 58) is less than the pressure in the reservoir (50);

wherein the suction line (49) has lesser flow impedance than the first and second fluid lines (20 and

     22), thereby causing the flow of fluid from the fluid reservoir (50) through the suction line (49) and one of the check valves (52) 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).

9. The power steering system (10) according to claim 1, further comprising an actuation speed measuring means (33) operatively connected to the electronic control means (32) for providing a tachometer signal (V_t) representative of steering movement of the dirigible wheels (13), and wherein the electronic control means (32) subtracts the tachometer feedback signal (V_t) from the steering torque signal (V_TT) during control of actuation of the electric motor (26).

10. The power steering system (10) according to claim 9, 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 a rotational speed of at least one of the steering wheel (12) and the electric motor (26).

11. The power steering system (10) according to claim 1, further comprising a redundant applied steering torque sensor (15) which generates a redundant signal in response to the applied steering torque (T_s) and which is connected to the electronic control means (32);

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_TT) from the primary applied steering torque sensor (14).

12. The power steering system (10) according to claim 11, wherein the electronic control means (32) calculates an expected fluid pressure as a function of the signal (V_TT) generated by the primary applied steering torque sensor (14);

compares the higher of the fluid pressures (LP_p or RP_p) in the first and second fluid lines (20 or

     22), to the expected fluid pressure, and terminates the powered assist provided by the system (10) when the higher of the fluid pressures (LP_p or RP_p) in the first and second fluid lines (20 or

     22) fails to correlate with the expected fluid pressure calculated by the electronic control means (32).

13. The power steering system (10) according to claim 11, further comprising means (35) for providing a redundant measure of the higher of the fluid pressures (LP_p and RP_p) in the first and second fluid lines (20 and 22);

wherein the electronic control means (32) terminates the powered assist provided by the system (10) when the redundant measure of the fluid pressures (LP_p and RP_p) provided by the means (35) for providing the same fails to correlate with the higher of the fluid pressures (LP_p or RP_p) sensed by one of the first and second pressure transducers (54 or

     55).

14. The power steering system (10) according to claim 13, wherein the means (35) for providing a redundant measure of the fluid pressures (LP_p and RP_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.

15. The power steering system (10) according to claim 13, wherein the electronic control means (32) calculates an expected fluid pressure as a function of the redundant signal generated by the redundant applied steering torque sensor (15);

compares the redundant measure of the fluid pressures (LP_p and RP_p) provided by the means (35) for providing the same, to the expected fluid pressure, and terminates the powered assist provided by the system (10) when the redundant measure of the fluid pressures (LP_p and RP_p) fails to correlate with the expected fluid pressure calculated by the electronic control means (32).

16. The power steering system (10) according to claim 1, wherein the electronic control means (32) calculates an expected fluid pressure as a function of the signal (V_TT) generated by the primary applied steering torque sensor (14);

compares the higher of the fluid pressures (LP_p or RP_p) in the first and second fluid lines (20 or

     22), to the expected fluid pressure, and terminates the powered assist provided by the system (10) when the higher of the fluid pressures (LP_p or RP_p) in the first and second fluid lines (20 or

     22) fails to correlate with the expected fluid pressure calculated by the electronic control means (32).

17. The power steering system (10) according to claim 1, wherein the vehicle presents a steering load stiffness to the power steering system (10), and 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.

18. The power steering system (10) according to claim 17, wherein the electronic control means (32) treats the 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 LP_p and RP_p 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.

19. The power steering system according to claim 1, wherein a electronic control means provides compensation of at least one of the magnitude of the applied steering torque (308 or 328), and the operation (315 or 335) of the electric motor (26) or the fluid pressure (LP_p or RP_p) measured (320 or 340) by one of the first and second pressure transducers.

20. The power steering system according to claim 19, wherein the compensation (308 or 328) of the magnitude of the applied steering torque predominantly includes at least one pole and a compensation (315 or 335) of the operation of the electric motor (26) or the fluid pressure (LP_p or RP_p) is predominated by a low-frequency zero.

21. The power steering system (10) according to claim 1, wherein the vehicle includes a steering wheel (12), such that the application of an applied steering torque (TQ) to the steering wheel (12) results in a 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_p and RP_p) measured by the first and second pressure transducers (54 and

     55) to establish an internal closed servo control loop (322 or

     342) between an input signal (V_i) derived from the application of an applied steering torque (T_s) to the steering wheel (12) and the steering force assist to the dirigible wheels (13).

22. The power steering system (10) according to claim 21, further comprising a tachometer (62) operatively connected to the electronic control means (32) which generates a signal (V_t) equal to (q_m s/K_c), representative of a rotational speed of the electric motor (26), q_m being a rotational position of the electric motor (26), K_c being a tachometer feedback damping factor and s being a Laplace variable;

wherein the primary applied steering torque sensor (14) and the electronic control means (32) together generate a compensated signal (V_c); and

wherein the electronic control means (32) subtracts the signal (V_t) representative of the rotational speed of the electric motor (26) from the compensated signal (V_c) representative of the applied steering torque and employs a resulting difference in forming an input signal (V_i) to the internal closed servo control loop (322 or

     342).

23. The power steering system (10) according to claim 22, wherein the electronic control means (32) generates (320 or 340) a signal representative of the fluid pressure (LP_p or RP_p) measured by the first or second pressure transducer (54 or 55) so as to give an internal loop feedback signal (V_f), and subtracts the internal loop feedback signal (V.sub.^f) from the input signal (V_i) to the internal closed servo control loop (322 or 342) so as to yield an error signal (V_e) from which an internal drive signal (V_d) for operating the electric motor (26) is derived.

24. The power steering system (10) according to claim 23, wherein the electronic control means (32) determines an internal control loop gain value (ICG) as a function of vehicle speed and the compensated signal (V_c) representative of the applied steering torque, and operates the electric motor (26) so as to generate a measured fluid pressure (LP_p or RP_p) at the one of a first or second pressure transducers (54 or 55) in accordance with a product of an error signal (V_e) and the internal control loop gain value (ICG).

25. The power steering system (10) according to claim 1, wherein when the fluid pressure (LP_p or RP_p) measured by one of the first and second pressure transducers (54 or 55) is greater than the fluid pressure (RP_p or LP_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_p or LP_p) measured by the other of the second and first pressure transducers (55 or 54).

26. The power steering system (10) according to claim 1, further comprising a fluid reservoir (50), 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.

27. The power steering system (10) according to claim 26, further comprising a two-position, compound two-way relief valve (37) fluidly connected between the first and second fluid lines (20 and 22) and the reservoir (50), disposed in parallel with the power cylinder (18), for placing the first and second fluid lines (20 and 22) in direct fluid communication with the reservoir (50) should the powered assist to steering provided by the system (10) fail.

28. 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_TT) in response to at least an applied steering torque (T_s);

the improvements wherein the system (10) further comprises;

a first pressure transducer (54) sensing the fluid pressure (LP_p) in the first fluid line (20);

a second pressure transducer (55) sensing the fluid pressure (RP_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_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_t) representative of steering movement of the dirigible wheels (13); and

means (35) for providing a redundant measure of the fluid pressures (LP_p and RP_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_p or RP_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_p or RP_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_s) and the powered assist to steering supplied by the power cylinder (18);

wherein the electronic control means (32) generates an error signal (V_c) in response to (at least the generation of the signal (V_TT) by the primary applied steering torque sensor (14), the error signal (V_e) being dependent upon the fluid pressure (LP_p or RP_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_TT) generated by the primary applied steering torque sensor (14), and wherein the electronic control means multiplies the error signal (V_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_p or RP_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_e) and the internal control loop gain value (ICG);

wherein the electronic control means (32) generates the error signal (V_e) by compensating the signal (V_TT) generated by the primary applied steering torque sensor to yield a compensated steering torque signal (V_c);

subtracting from the compensated steering torque signal (V_c) a tachometer feedback signal (V_t), representative of steering movement of the dirigible wheels (13), to yield an input signal (V_i);

obtaining a pressure-dependent loop feedback signal (V_p) and subtracting the feedback signal (V_f) from the input signal (V_i) to yield the error signal (V_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_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_p and RP_p) provided by the means (35) for providing the same fails to correlate with the fluid pressure (LP_p or RP_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_p and RP_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_p or RP_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_p or RP_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_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_p and RP_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_i) derived from application of an applied steering torque (T_s) to the steering wheel (12) and the steering force assist to the dirigible wheels (13);

wherein when the fluid pressure (LP_p or RP_p) measured by one of the first and second pressure transducers (54 or

     55) is greater than the fluid pressure (RP_p or LP_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_p or LP_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.

29. 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_TT) in response to at least an applied steering torque (T_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_p) in the first fluid line (20) is greater than a fluid pressure (RP_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_p) in the second fluid line (22) is greater than the fluid pressure (LP_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_p or RP_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).

30. 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_TT) in response to at least an applied steering torque (T_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_TT) corresponding to an applied steering torque (T_s);

a first pressure transducer (54) operatively connected to the electronic control means (32) and sensing a fluid pressure (LP_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_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_p or RP_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).

31. In a power steering system (10) for a vehicle having dirigible wheels (13), a primary applied steering torque sensor (14) which generates a signal (V_TT) in response to an applied steering torque (T_s), an electric motor (26) for developing a powered assist to steering the dirigible wheels (13) and an electronic control means (32) to which the primary applied steering torque sensor (14) and the electric motor (26) are operatively connected, the electronic control means (32) controlling actuation of the electric motor (26) in dependence upon at least the signal (V_TT) from the primary applied steering torque sensor;

the improvement wherein the system (10) further comprises a redundant applied steering torque sensor (15) which generates a redundant signal in response to the applied steering torque (T_s) and which is connected to the electronic control means (32), and 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_TT) from the primary applied steering torque sensor (14) or with any other parameter of the system (10), but does not use the redundant signal from the redundant applied steering torque sensor (15) for otherwise controlling any other element or parameter of the system (10).