Electrohydraulic valve servomechanism with adaptive resistance estimator

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;

using the coil current value to produce an error value denoting a change in the resistance of the coil;

deriving a coil resistance value by using the coil current value to produce a resistance error value denoting a change in the resistance of the coil, and using the resistance error value to produce the coil resistance 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.