From the forums – Current sensing for robot arm with PWM control of hacked servo motors

Adafriend Dustyn Roberts writes in the forums:

Im using an Adafruit motor shield to control two servos that act as the shoulder and elbow of a small robotic arm (photos and video here in this flickr set). The servos are hacked to remove the control board, so I control the robot directly through Arduino code and the Arduino PID library. Im sensing the current drawn by the motors by putting a tiny resistor in series with the motor power, sensing the voltage drop across it, then since V=IR and I know V and R I can get the current. Because the voltage drop is tiny, its amplified by op-amp before the Arduino reads it. Code and schematic for the current sense library and the Arduino code for the arm are on github. Its currently programmed to just draw a short vertical line over and over again.

Soooo… my issue is that Im trying to measure the power consumption each time through the loop by logging the current and voltage. But because the speed of the motor is controlled through PWM (at 1kHz through the motor shield M3 and M4 connections), the voltage input looks like a square wave. And since V=IR, the current looks like a square wave too. However, the point of using PWM to control the speed of the motor is to set an effective voltage between 0 and the maximum, by varying the duty cycle, so that if the input is say 0-5V, at 50% duty cycle the motor will only see 2.5V, and should feel a current pull in a similar way. So ideally both the current and voltage vs. time curves would be smooth because of PWM frequency being so high, but I get lower frequency spikes in the data. Any thoughts? I can only log data to serial at a rate of about 200 Hz, so my first guess is that is too slow and Im getting aliasing, but it seems that its not the whole story and Im not even sure if the whole Nyquist sampling theory applies here.

A picture of the current and voltage readings during the down stroke of drawing the vertical line is below.

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