you may start by considering what the measured motor current really is, how does one obtain single value when using a 3-phase motor and what is considered positive. pretty sure that motor current is not measured per phase but collectively before or after IGBT. but that would imply measuring magnitude.
if so, the next question would be - how does this ever become negative? perhaps the current is measured as always positive quantity and the sign is applied based on phase order used in particular moment.
axes use motors and gear ratios that are not the same. gear ratio is definitely a factor in this relationship. but not all gearboxes have input and output turning in same direction. check $RAT_MOT_AX and you will see some negative values there. on some robots, the exact same motors are used on few axes (arm or wrist) but the gear ratios are not all same sign. hence we can rule out something in the motor itself to be the cause for this.
but ultimately what difference does it make? if there is a constant k that is part of the relationship, why would it matter if it is positive or negative? it is just some fixed value...
Is it really constant on axis side? Nearly constant maybe. Afaik in newer robots its actually a polynom not a scaling factor. Since curr_act is motor sided to get to axis side you need as panic said gear ratio rat_mot_ax and also axis coupling comes to mind because the hand axes share motors to some extent. If I remember correctly the sign directly relates to the direction, so think of it maybe like the difference of push and pull. Probably also friction effects need to be calculated in when going from motor to axis torque.
Fubini
