Mobile device tester for precise inductive power measurement and calibration unit therefor and method therefor

1. A test unit for testing inductive power charging of a mobile device, with a precisely calibrated power output, comprising

an electrical circuit being powered by a DC power source, said circuit being capable of generating electrical power at each of a number of frequencies f, the electrical circuit comprising

a primary transmitter coil for inductively transmitting power to a secondary receiver coil of the mobile device in use, and

a capacitor connected in series with the primary transmitter coil in order to get a resonance circuit with resonance at a certain frequency, and

a control unit configured to operate in a power calibration mode and a testing mode, wherein the control unit in the power calibration mode is configured to:

receive a measurement of a voltage U supplied over the primary transmitter coil;

receive a measurement of a current I flowing through the primary transmitter coil;

and for at least one power frequency f:

derive in a state where no secondary receiver coil is inductively connected to the primary transmitter coil:

a time compensation parameter Δt(f) compensating for phase shift in the signal paths in the electrical circuit between the measured voltage U and the measured current I, such that the inductive power transmitted from the primary transmitter coil being a product of U and I integrated over a period equals zero by time compensating either of the measured U or measured I with the time compensation parameter, and store the time compensation parameter Δt(f) in a memory, and

a voltage amplification constant K U-MDT (f) using an external voltmeter measuring a reference voltage over the primary transmitter coil, such that the voltage amplification constant K U-MDT (f) multiplied by the measured voltage U equals the reference voltage for each power frequency,

receive, in a state where a secondary receiver coil of a calibrator unit is inductively connected to the primary transmitter coil, a signal containing data comprising information of actual inductive power transmitted from the primary transmitter coil P TX-ACTUAL (f) from the calibrator unit; and

derive a current amplification constant K I-MDT (f) required for a calculated inductive power transmitted from the primary transmitter coil P TX (f) to be equal to that of the actual inductive power transmitted P TX-ACTUAL (f) for each power frequency, and store the amplification constant in a memory,

whereby the test unit can calculate inductive power transmitted from the primary coil in the testing mode using the time compensation parameter, the voltage amplification constant, and the current amplification constant.

2. The test unit according to claim 1 , wherein the inductive power transmitted from the primary coil P TX (f) is calculated by using the following formula:

P TX ( f )=∫ U*K U-MDT ( f )* IΔT ( f )* K I-MDT ( f )− R ESR ( f )* I 2 *K I-MDT ( f ) 2 ,

wherein K U-MDT denotes the derived voltage amplification constant, IΔt(f) denotes the time compensated measured current I, U denotes the measured voltage, K I-MDT (f) denotes the derived current amplification, and R ESR (f) denotes a known transmitter coil equivalent series resistance for power frequency f, and wherein the integration is made over a period.

3. The test unit according to claim 1 , wherein the inductive power transmitted from the primary coil P TX (f) is calculated by using the following formula:

P TX ( f )=∫ UΔt ( f )* K U-MDT ( f )* I*K I-MDT ( f )− R ESR ( f )* I 2 *K I-MDT ( f ) 2 ,

wherein K U-MDT denotes the derived voltage amplification constant, UΔt(f) denotes the time compensated measured voltage U, I denotes the measured current, K I-MDT (f) denotes the derived current amplification, and R ESR (f) denotes a known transmitter coil equivalent series resistance for power frequency f, and wherein the integration is made over a period.

4. The test unit according to claim 2 , wherein in the testing mode is configured to:

for each relevant power frequency:

receive a signal containing data comprising information of a measure of received inductive power P RX (f) from a mobile device comprising a secondary receiver coil being inductively coupled to the primary transmitter coil in use;

receive a measurement of a voltage U supplied over the primary transmitter coil;

receive a measurement of a current I flowing through the primary transmitter coil;

calculate the inductive power transmitted from the primary transmitter coil P TX (f), and

compare the received power P RX (f) with the calculated inductive power transmitted from the primary transmitter coil P TX (f) to determine a quality assessment of the inductive power charging capability of the mobile device for each power frequency, and

transmit a signal comprising information of the quality assessment of the inductive power charging capability of the mobile device.

5. The test unit according to claim 2 , wherein an inductive power loss P LOSS (f) in the primary transmitter coil is defined by the following formula:

P LOSS ( f )= R ESR ( f )* I 2 *K I-MDT ( f ) 2 .

6. The test unit according to claim 1 , wherein each voltage measurement is based on a number of voltage samples over a predefined time period.

7. The test unit according to claim 1 , wherein each current measurement is based on a number of current samples over a predefined time period.

8. The test unit according to claim 1 , wherein the control unit is further configured to:

generate a first polynomial function describing the relation between the time compensation parameters Δt(f) and the power frequency f.

9. The test unit according to claim 1 , wherein the control unit is further configured to:

generate a second polynomial function describing the relation between current amplification constant K I-MDT (f) and the power frequency f.

10. The test unit according to claim 2 , wherein the control unit is further configured to:

generate a first polynomial function describing the relation between the time compensation parameters Δt(f) and the power frequency f

and

wherein the time compensation parameter Δt(f) is obtained from the first polynomial function for the purpose of calculating the inductive power transmitted from the primary coil P TX (f).

11. The test unit according to claim 2 , wherein the control unit is further configured to:

generate a second polynomial function describing the relation between current amplification constant K I-MDT (f) and the power frequency f

and

wherein the current amplification constant K I-MDT (f) is obtained from the second polynomial function for the purpose of calculating the inductive power transmitted from the primary coil P TX (f).

12. A calibrator unit for calibrating a test unit, the calibrator comprising

an electrical circuit comprising:

a secondary receiver coil for receiving electrical power of a certain frequency from a primary transmitter coil of the test unit in use,

a precision resistor connected in series with the secondary receiver coil and having a well defined resistance R CAL ,

a calibrator control unit configured to:

for at least one power frequency

receive a voltage measurement U CAL (I) over the resistor,

calculate the actual inductive power received by the secondary receiver coil P TX-ACTUAL (f) based solely on the measured voltage and resistance of the resistor by using Ohm's Law, and

transmit a signal, containing data comprising information of the calculated actual electrical power P TX-ACTUAL received by the secondary receiver coil, to the test unit in use.

13. The calibrator unit according to claim 9 , wherein the actual electrical power received by the secondary receiver P TX-ACTUAL (f) is calculated using the following formula:

P TX-ACTUAL ( f )= U CAL ( f ) 2 *K U-CAL ( f ) 2 /R CAL ,

wherein K U-CAL (f) denotes a voltage amplification constant derived for the calibrator unit.

14. A method for testing inductive power charging of a mobile device, using a test unit having a precisely calibrated power output and operating in a calibration mode, comprising:

measuring a voltage U supplied over a primary transmitter coil of the test unit;

measuring a current I flowing through the primary transmitter coil;

and for at least one power frequency f:

in a state where no secondary receiver coil is inductively connected to the primary transmitter coil:

deriving a time compensation parameter Δt(f) compensating for phase shift in the signal paths of the test unit between the measured voltage U and the measured current I, such that the inductive power transmitted from the primary transmitter coil being a product of U and I integrated over a period equals zero by time compensating either of the measured U or measured I with the time compensation parameter, and

deriving a voltage amplification constant K U-MDT (f) using a an external voltmeter measuring a reference voltage over the primary transmitter coil, such that the voltage amplification constant K U-MDT (f) multiplied by the measured voltage U equals the reference voltage for each power frequency,

receiving, in a state where a secondary receiver coil of a calibrator unit is inductively connected to the primary transmitter coil, a signal containing data comprising information of actual inductive power transmitted from the primary transmitter coil P TX-ACTUAL (f) from the calibrator unit; and

deriving a current amplification constant K I-MDT (f) required for the calculated inductive power transmitted from the primary transmitter coil P TX (f) to be equal to that of the actual inductive power transmitted P TX-ACTUAL (f) for each power frequency, and store the amplification constant in a memory,

whereby the test unit can calculate inductive power transmitted from the primary coil using the time compensation parameter, the voltage amplification constant, and the current amplification constant.

15. The method ( 40 ) according to claim 14 , when the test unit ( 100 ) operates in a testing mode, further comprising the following steps:

for each relevant power frequency:

receiving ( 51 ) a signal containing data comprising information of a measure of received inductive power P RX (f) from a mobile device ( 30 ) comprising a secondary receiver coil ( 31 ) being inductively coupled to the primary transmitter coil ( 111 ) in use;

receiving ( 52 ) a measurement of a voltage U supplied over the primary transmitter coil ( 111 );

receiving ( 53 ) a measurement of a current I flowing through the primary transmitter coil ( 111 );

calculating ( 54 ) an inductive power transmitted from the primary transmitter coil P TX (f) using one of the two following formulae:

P TX ( f )=∫ UΔt ( f )* K U-MDT ( f )* I*K I-MDT ( f )− R ESR ( f )* I 2 *K I-MDT ( f ) 2 ,

wherein K U-MDT denotes the derived voltage amplification constant, UΔt(t) denotes the time compensated measured voltage U, I denotes the measured current, K I-MDT (f) denotes the derived current amplification, and R ESR (f) denotes a known transmitter coil equivalent series resistance for power frequency f, wherein the integration is made over a period, or

P TX ( f )=∫ U*K U-MDT ( f )* IΔt ( f )* K I-MDT ( f )− R ESR ( f )* I 2 *K I-MDT ( f ) 2 ,

wherein K U-MDT denotes the derived voltage amplification constant, IΔt(t) denotes the time compensated measured current I, U denotes the measured voltage, K I-MDT (f) denotes the derived current amplification, and R ESR (f) denotes a known transmitter coil equivalent series resistance for power frequency f, wherein the integration is made over a period, comparing ( 55 ) the received power P RX (f) with the calculated inductive power transmitted from the primary transmitter coil P TX (f) to determine a quality assessment of the inductive power charging capability of the mobile device for each power frequency, and

transmitting ( 56 ) a signal comprising information of the quality assessment of the inductive power charging capability of the mobile device.

16. The test unit according to claim 3 , wherein in the testing mode is configured to:

for each relevant power frequency:

receive a signal containing data comprising information of a measure of received inductive power P RX (f) from a mobile device ( 30 ) comprising a secondary receiver coil ( 31 ) being inductively coupled to the primary transmitter coil ( 111 ) in use;

receive a measurement of a voltage U supplied over the primary transmitter coil ( 111 );

receive a measurement of a current I flowing through the primary transmitter coil ( 111 );

calculate the inductive power transmitted from the primary transmitter coil P TX (f), and

compare the received power P RX (f) with the calculated inductive power transmitted from the primary transmitter coil P TX (f) to determine a quality assessment of the inductive power charging capability of the mobile device ( 30 ) for each power frequency, and

transmit a signal comprising information of the quality assessment of the inductive power charging capability of the mobile device.