Device and method for monitoring an electrical insulation in a vehicle electrical system

1. A device for monitoring an electrical insulation in an on-board system, the device comprising:

a voltage source for generating a first DC voltage with a first voltage value and a second DC voltage with a second voltage value;

a voltage-measuring unit between a positive power supply line of the on-board system and a negative power supply line of the on-board system for the measurement of a voltage between the positive power supply line and the negative power supply line;

a current path between a positive power connection of the voltage source and the positive or the negative power supply line;

a current-measuring unit in the current path for the measurement of a current flowing in the current path; and

an evaluation unit electrically connected via a first signal input to a signal output from the voltage-measuring unit and via a second signal input to a signal output from the current-measuring unit, the evaluation unit designed to determine, from a first measured voltage value measured in response to the first DC voltage and from a second measured voltage value measured in response to the second DC voltage on the voltage-measuring unit, from a first measured current value measured in response to the first DC voltage and a second measured current value measured in response to the second DC voltage on the current-measuring unit, and from the first voltage value and the second voltage value, an insulation resistance between the on-board system and electrical ground.

2. The device of claim 1 , further comprising a resistor in the current path for limiting the current.

3. The device of claim 1 , further comprising a controllable switch in the current path for the interruption or formation of an electrical connection in the current path.

4. The device of claim 1 , wherein the evaluation unit is designed to determine the insulation resistance as a quotient of the difference between the first voltage value and the second voltage value, corrected respectively by voltage values observed respectively on a resistor in the current path, and the voltage values of the respective voltages between the positive power supply line and the negative power supply line, and the difference between the first measured current value and the second measured current value.

5. The device of claim 1 , wherein the evaluation unit is designed for the iterative determination of the insulation resistance using the following equation:

R_ISO

m

=

(

Uq

⁢

⁢

1

-

I

⁢

⁢

1

·

R

)

-

(

Uq

⁢

⁢

2

-

I

⁢

⁢

2

·

R

)

-

k

m

·

(

Ub

⁢

⁢

1

-

Ub

⁢

⁢

2

)

(

I

⁢

⁢

1

-

I

⁢

⁢

2

)

where R ISO m is the insulation resistance, Uq 1 is the first voltage value of the first DC voltage, Uq 2 is the second voltage value of the second DC voltage, I 1 is the first measured current value, I 2 is the second measured current value, Ub 1 is the first measured voltage value, Ub 2 is the second measured voltage value, R is an electrical resistance of the resistor, and k m is a corrective value.

6. The device of claim 5 , wherein the evaluation unit is designed for the iterative determination of the corrective value k m using the following equation:

k

m

=

Uq

-

I

·

R

ISO

m

-

1

-

I

·

R

Ub

where Uq, I and Ub respectively are the first voltage value, the first measured current value and the first measured voltage value, or respectively the second voltage value, the second measured current value and the second measured voltage value, and R ISOm-1 is an insulation resistance value, which is determined in the (m−1) th iteration, where m≥1.

7. The device of claim 1 , wherein the first voltage value is 50 to 60V and the second voltage value is −60 to −50V.

8. An on-board system for a vehicle, wherein the on-board system incorporates a device as claimed in claim 1 for the monitoring of electrical insulation in the on-board system.

9. A method for monitoring electrical insulation in an on-board system of a vehicle, the method comprising:

switching a voltage source between an electrical ground and a power terminal on a positive power supply line of the on-board system or a negative power supply line of the on-board system;

generating a first DC voltage with a first voltage value, using the voltage source;

measuring a first measured voltage value of a voltage between the positive power supply and the negative power supply line and a first measured current value of a current flowing between the voltage source and the power terminal, as the first DC voltage is generated;

generating a second DC voltage with a second voltage value, using the voltage source;

measuring a second measured voltage value of the voltage and a second measured current value of the current, as the second DC voltage is generated;

determining an insulation resistance between the on-board system and electrical ground from the first and the second voltage values, the first and the second measured current values and the first and the second measured voltage values.

10. The method of claim 9 , wherein generating the first DC voltage, measuring the first measured voltage value, generating the second DC voltage, measuring the second measured voltage, and determining the insulation resistance are executed while the vehicle is in service.