DRV10983: I2C calculated motor current is zero or close to zero

Hi Marc,

Thanks for posting your question in MD forum. We recently updated DRV10983 Datasheet. Please check Table 9 (Page 44) in the datasheet for the updated equations to calculate Motor current. Please note that the values that are read back from motor current registers might have errors (especially at low currents) due to current sense amplifier and ADC offset, gain and noise.

Regards,

Vishnu.

Hi Vishnu,

1. I was aware of this equation on page 44 of SLVSCP6H

2. I implemented this particular equation already:

If ({ MotorCurrent[10:0]}) >=1023 Current (A) = 3 × ({MotorCurrent[10:0]}) - 1023) / 512

else Current (A) = 3 × ({MotorCurrent[10:0]}) / 512

in my software the following happened when calculating the motor current of the read register values:

- Reading back MotorCurrent[10:0] with a value of 0x3FE and put into equation => 5,988 A

- Reading back MotorCurrent[10:0] with a value of 0x3FF and put into equation => 0 A

- Reading back MotorCurrent[10:0] with a value of 0x400 and put into equation => 0,00586 A

3. As already told register values vary around 0x3FE and 0x401. Using the equation above makes things even worse than using equation shown in https://e2e.ti.com/support/motor-drivers/f/38/p/899148/3325060?tisearch=e2e-sitesearch&keymatch=DRV10983#3325060, because now the motor current value jumps around between 5,988 A, 0 and 0,00586 A which is definitely wrong. 

4. What is the solution?

Hi Marc,

FYI - We are having severe inclement weather in Dallas and multiple affected team members are without power. Please be patient with our response here.

Thanks,

Matt

Hi Marc,

Thanks for your patience. Motor current readback gets inaccurate either because current decays due to low inductance or if the BEMF waveform isn't very sinusoidal. Is it possible for you share the actual/measured phase current or the BEMF waveform of the motor? Did you ensure the Motorcurrent1 and Motorcurrent2 registers were refreshed before reading back their values? 

Regards,

Vishnu

Hi Vishnu,

- Phase to Phase inductance is 0,972 mH

- We do not have an actual measurement of the phase current, but I will measure it next week,  not sure if we can measure it at all yet.

- I did not find an information in the datasheet that one has to check if a value is refreshed before reading, beside the information that MotorCurrent1 needs to be read first which I do. Reading a value follows the process described in "Programming Guide for the DRV10983" (slvuaa5.pdf, version October 2014), Figure 9.

Which chapter and which register contains the information that tells whether the MotorCurrent register is ready to read or not?