motor current sensing

Hi,
I need some help from somebody who has experience in motor control.
It is about current sensing with a sense resistor. I know it is easier to sense the current in the low side that in high side, but high side is better because it allows to detect short to ground.
There is something else that I don t understand (need sbdy to explain me) : if the sense resistor is on the low side it disturbs the ground, which for instance is not allowed in automotive.??

There is something else I want to ask, for 3 phase motor whqt do you think qbout sensing the current in the phase (i don t speak about how to do it, but about the application of sensing the current in the phase of the motor (BLDC,AC).
Many thanks, if somebody can recommand me a book that will be nice, I found nothing up to now, so the only solution I have is to speak with people who have experience.

?

I haven't seen anyone try to current sense on the low-side. People tend to use the high-side so that the ground will not be distorted.

Why do you say its easier to sense on the low-side than high-side...is it because you are doing everything "manually" with an ADC right? And you can therefore the only voltages that you can measure across the resistor are referenced to ground, thereby making the low-side more convenient? Just use a current sense IC...you attach two of the leads to both ends of the resistor and it outputs a proportionally smaller current based on the voltage it detects in the resistor... connect this pin to your ADC and a resistor going to ground. The resistor will change the current to a voltage for your ADC. Voila!

I don't think current sensing is fast enough (or effective) to protect from short-circuits. The current just becomes too high too fast, so even if your MCU could shutdown the circuit in time, it wouldn't be able to detect the short. This is because the voltage across the resistor would jump to the higher motor supply voltage and this would destroy the ADC or the current-sense IC. If the motor supply was low enough not to destroy the ADC or current sense IC, it would still saturate the read-out so you could not tell if you were pulling maximum current or a excessive shorted current. In either case, your sense resistor would fry, and hopefully it would fry before everything else and disconnect the break the circuit from further damage.

For example:
**broken link removed**
or
https://www.electro-tech-online.com...sheetsactionAttachFiledogettargetzxct1009.pdf

It is easier to put the resistor in the low side because in the high side you have to sense a small difference voltage in presence of a high common mode voltage(you have to use special opamp).
But I don t really understand something I read: when the resistor is on the low side it perturbs the ground, explanation?
Any comments on the applications of sensing the current in the phase are welcomed.
many thanks

Bah, you put your responded before I finished editing my first post. Please reread it in case there is anything new or if anything is worded better.

Perturb the ground, distort the ground, it all means the same thing. In simple terms, when you current sense you want limit the effects of the sensing on the motor wiring as little as possible which means you don't want to shift the motor's voltage reference (ground) to be a something else. For low-side current sensing, this would create a "floating" reference for the motor. ie. The more current the motor draws, the higher the voltage drop across the current sense resistor. This causes the motor reference/ground to change with current = BAD.

Another obvious thing that comes to mind is that a current sense resistor on the low-side it will screw up the bypass capacitors for the motor and the motor controller which bypass to ground. The noise filtering will be very erratic and ineffective (if existent)...I think. All I know is a floating reference causes erratic behaviour, hard to predict what exactly...

most chopping stepper controllers use low side sensing. High side sensing requires differential inputs while low side only needs single ended. the trick is to keep the sense resistors traces short and have a seperate ground return for the sense resistor - "star ground pattern". A good example is the A3977 - **broken link removed**

In case of automotive applications
Mostly in Japan, we use low side current sensing, while in US and Europe high side current sensing is used.
In case of High side current sensing, the common mode voltage reference of the opamp input needs to be high. In case of low side current sensing the opamp input goes below the ground voltage.

dknguyen said:

For low-side current sensing, this would create a "floating" reference for the motor. ie. The more current the motor draws, the higher the voltage drop across the current sense resistor. This causes the motor reference/ground to change with current = BAD.

Click to expand...

The low side current sensing is use in case of current controlled solenoids/motors, which is basically done with PWM and the current sense resistor value is very small compare to motor resistance or other resistances in circuit (like driver ON resistance in case of MOSFETs) so vaoltage drop across sense resistor doesnot really matter
for high side current sensing very few special ICs are automotive grade and are comparatively expensive too, it is normally done using oapmps with High common mode input specification but these kinda opamp are not rail to rail so the output voltage range gets limited

philba said:

most chopping stepper controllers use low side sensing. High side sensing requires differential inputs while low side only needs single ended. the trick is to keep the sense resistors traces short and have a seperate ground return for the sense resistor

Click to expand...

Its always better to use the opamp in low side sense circuit in differential modes so that it will reduce/compensate the effect of the changes in ground voltage level due to other functions of the unit