What methods are there for continuously monitoring the current in a 3-phase servo motor for a CNC machine and giving an alarm when the current is less than some value?
Electronic – Continuously Measure 3-Phase Motor Current
cnccurrentcurrent measurementmotorservo
Related Solutions
There are two features of Motor control worth sampling. The first is the current you mention. There are two parts of the current path that are going to be interesting, and you probably need to measure them separately. The first half is the rise current, the part of the commutation when the motor is connected to power. The natural induction of the motor windings will cause the current to rise gradually, until the control loop shuts it off, either because it is using current limiting (chopper drive), or some other part of the control loop is turning the motor off. In the case of a normal H-Bridge, this is the only current path. In the off state, no current can flow in or out of the motor because it is completely disconnected. More elaborate circuits may also use regeneration, in which the motor is connected to another part of the circuit to assist in deceleration, either a dump resistor or a battery recharging circuit.
For small currents (about 1A or less), the usual way to do this is with a low value current sense resistor, connected in series with the motor windings. The current passing through the resistor causes a voltage difference on the opposite sides of the resistor, which can be measured quite easily.
Larger currents require fancier tricks, but a good option is a hall effect current sensor. The result is quite similar, an output voltage in proportion to the instantaneous current. This type of sensor has a slight propagation delay ( a few microseconds ) because of the multiple stages of amplification needed with the hall effect sensors.
The other feature of motor control worth looking at is Back EMF. This can be useful in some types of (really simple) servo control to determine motor velocity, since back EMF is proportional to motor speed. This feature can only be measured on windings that are 'off', since the voltage of the power supply will dominate the higher impedance windings. In the case of a PMDC motor, Back emf would be measured during the off periods of motor control. The back emf at the moment of switch-off will be equal to the supply voltage, but will decay, due to the natural inductance of the windings until it reaches the level determined by its speed.
Based on your (further) info, if there is no control wire, I think the reason it is not moving properly is that what you have now is basically a DC motor. To control a standard DC motor with PWM, you simply use a duty cycle between 0% (off) and 100% (full speed)
The current needed will be a lot higher than a servo control wire, as rather than controlling the feedback, you are controlling the actual power to the motor. This is okay as you have the L293D though, which will supply more current than a uC pin can, and exactly what they were designed for.
I suspect that the 2ms is too low to provide enough power to start the motor turning - if you have a 20ms period, 2/20 = 10% of full "power" (roughly - note actual power is I squared R, so at 10% will actually be 1/100th of power at 100% - RMS voltage would perhaps be a better term here) Try a higher value and see where the motor starts reliably. Let us know the results.
Here is a page that explains the differences quite well:
Best Answer
You need a current transformer (or three).
If you connect a resistor to the transformer's secondary winding a current through the sensed wire will cause a voltage across the resistor. The voltage is proportional to the current so will be a sine wave.
If you'd rather like a DC output proportional to the current you can use an i-Snail-V current transformer. It's self-powered and the i-Snail-V10 version for instance gives you 5 V out for 10 A. Other versions with other sensitivities exist.