Electrohydraulic valve servomechanism with adaptive resistance estimator

• US 20050211936A1
• Filed: 03/18/2005
• Published: 09/29/2005
• Est. Priority Date: 03/25/2004
• Status: Active Grant

• Current Assignee: Husco International Incorporated
• Original Assignee: Husco International Incorporated
• Inventors: Pfaff, Joseph L., Tabor, Keith A.
• Granted Patent: US 7,377,480 B2
• Time in Patent Office: Days
• US Class Current: 251/129.40
• CPC Class Codes: E02F 9/22 Hydraulic or pneumatic drives for dipper or bucket arm position control E02F3/43, for blade position control for graders E02F3/844; for turntables E02F9/121; for fork-lift trucks B66F9/22
• CPC Class Codes: F15B 11/006 Hydraulic "Wheatstone bridge" circuits, i.e. with four nodes, P-A-T-B, and on-off or proportional valves in each link
• CPC Class Codes: F15B 19/00 Testing; Calibrating; Fault detection or monitoring; Simulation or modelling of fluid-pressure systems or apparatus not otherwise provided for
• CPC Class Codes: F15B 21/087 Control strategy, e.g. with block diagram
• CPC Class Codes: F15B 2211/30575 in a Wheatstone Bridge arrangement (also half bridges)
• CPC Class Codes: F15B 2211/328 with signal modulation, e.g. pulse width modulation [PWM]
• CPC Class Codes: F15B 2211/875 Control measures for coping with failures
• CPC Class Codes: F16K 37/0083 by measuring valve parameters

Abstract

A servomechanism includes a controller which dynamically estimates the resistance of the solenoid coil in an electrohydraulic valve as part of determining a level of electric voltage to apply to open the valve. The servomechanism receives a current setpoint designating a desired electric current level and senses the actual level of current flowing through the coil. A proportional term is derived from the current setpoint and the actual level of current. Creation of a derivative term is based on the difference between the current setpoint and the actual level of current. A feedforward term is produced by estimating the resistance of the electrohydraulic valve and limiting the feedforward term to a predefined range of acceptable values. The proportional term, derivative term, and the feedforward term are summed to define a desired voltage level, and a PWM signal for driving the electrohydraulic valve is generated based on the desired voltage level.

Claims

1. A method by which a servomechanism operates an electrohydraulic valve, the method comprising:

receiving a current setpoint which designates a desired level of electric current to be applied to a coil of the electrohydraulic valve;

sensing an actual level of electric current flowing through the coil to produce a coil current value;

producing a current error value based on the current setpoint and the coil current value;

deriving a coil resistance value from the coil current value;

deriving a desired voltage level for the coil in response to the coil resistance value and the current error value; and

generating a PWM signal for driving the coil based on the desired voltage level.

2. The method as recited in claim 1 wherein deriving a coil resistance value comprises steps of:

dividing a previous value of the desired voltage level by the coil current value to produce an estimated resistance value;

subtracting a previous coil resistance value from the estimated resistance value to produce a resistance error value;

multiplying the resistance error value by an observer gain value; and

adding the previous coil resistance value and the result of the multiplying to produce the coil resistance value.

3. The method as recited in claim 2 further comprising limiting the resistance error value to a predefined range of values to produce a limited resistance error value;

and employing the limited resistance error value as the resistance error value in the multiplying step.

4. The method as recited in claim 3 wherein limiting the resistance error value employs limits derived from a nominal resistance value defined for the coil.

5. The method as recited in claim 2 further comprising limiting the coil resistance value to a predefined range of values to produce a limited coil resistance value;

and using the limited coil resistance value in deriving a desired voltage level.

6. The method as recited in claim 5 wherein limiting the coil resistance value employs limits derived from a nominal resistance value defined for the coil.

7. The method as recited in claim 1 wherein deriving a coil resistance value also employs a previous value of the desired voltage level.

8. The method as recited in claim 1 wherein deriving a desired voltage level further comprises multiplying the coil resistance value by the current setpoint and adding the result to the current error value.

9. The method as recited in claim 1 wherein producing the current error value comprises calculating a current error representing a difference between the current setpoint and the coil current value;

and multiplying the current error by a proportional gain.

10. The method as recited in claim 1 wherein:

producing the current error value comprises calculating a current error representing a difference between the current setpoint and the coil current value; and

deriving a desired voltage level further comprises deriving a derivative of the current error and multiplying the derivative of the current error by a derivative gain value.

11. The method as recited in claim 1 further comprising sensing an actual voltage level from a voltage source and dividing the desired voltage level by the actual voltage level to produce a value that is used to generate the PWM signal.

12. The method as recited in claim 1 wherein the coil current value is produced by acquiring a plurality of samples of the electric current flowing through the coil during a give period of the PWM signal;

and averaging the plurality of samples.

13. A method by which a servomechanism operates an electrohydraulic valve, the method comprising:

receiving a current setpoint which designates a desired level of electric current to be applied to a coil of the electrohydraulic valve;

sensing an actual level of electric current flowing through the coil;

producing a proportional term by calculating difference between the current setpoint and the actual level of electric current to produce a current error; and

multiplying the current error by a constant proportional gain value;

producing an feedforward term by;

(a) dividing a previous value of a desired voltage level for the coil by the actual level of electric current to produce an estimated resistance value,

(b) subtracting a previous coil resistance value from the estimated resistance value to produce a resistance error value,

(c) multiplying the resistance error value by an observer gain value,

(d) adding the previous coil resistance value and the result of the multiplying to produce a coil resistance value, and

(e) multiplying the coil resistance value by the current setpoint to produce the feedforward term;

producing a derivative term by determining a derivative of the current error and multiplying the derivative of the current error by a derivative gain value;

deriving a desired voltage level for the coil by combining the proportional term, the feedforward term, and the derivative term; and

generating a PWM signal for driving the coil based on the desired voltage level.

14. The method as recited in claim 13 further comprising limiting the resistance error value to a predefined range of values.

15. The method as recited in claim 13 further comprising limiting the coil resistance value to a predefined range of values.

16. The method as recited in claim 13 wherein generating a PWM signal comprises producing a duty cycle value based on the desired voltage level, adding a PWM offset to the duty cycle value, and limiting the result to a predetermined range of values.

17. A servomechanism for operating an electrohydraulic valve that has a coil with a resistance, the servomechanism comprising:

a node which receives a current setpoint which designates a desired level of electric current to be applied to a coil of the electrohydraulic valve;

a sensor of an actual level of electric current flowing through the coil;

a first processing stage connected to the node and the sensor, and producing a proportional term based on the current setpoint and the actual level of electric current;

a second processing stage connected to the sensor and deriving a coil resistance value from the actual level of electric current,

an output stage connected to the first processing stage and second processing stage, and deriving a desired voltage level for the coil in response to the coil resistance value and the proportional term; and

a signal generator connected to the output stage and producing a PWM signal in response to the desired voltage level.

18. The servomechanism as recited in claim 17 wherein first processing stage produces the proportional term by calculating difference between the current setpoint and the actual level of electric current to produce a current error;

and multiplying the current error by a proportional gain value.

19. The servomechanism as recited in claim 17 wherein the second processing stage produces a feedforward term by:

(a) dividing a previous value of a desired voltage level for the coil by the actual level of electric current to produce an estimated resistance value;

(b) subtracting a previous coil resistance value from the estimated resistance value to produce a resistance error value;

(c) multiplying the resistance error value by an observer gain value;

(d) adding the previous coil resistance value and the result of the multiplying to produce a coil resistance value; and

(e) multiplying the coil resistance value by the current setpoint to produce the feedforward term which is supplied to the output stage.

20. The servomechanism as recited in claim 17 wherein the first processing stage produces a proportional term;

and further comprising a third processing stage which produces a derivative term by determining a derivative of the current error and multiplying the derivative by a derivative gain value, and the derivative term is supplied to the output stage.

21. The servomechanism as recited in claim 20 wherein the output stage produces the desired voltage level for the coil by summing the proportional term, the feedforward term and the derivative term.