Hello wise forum,
I'm 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 http://www.flickr.com/photos/dustynrobots/sets/72157627951190369/
). The servos are hacked to remove the control board, so I control the robot directly through Arduino code and the Arduino PID library. I'm 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, it's amplified by op-amp before the Arduino reads it. Code for the current sense library that was created and the schematic are here: https://github.com/caplingerc/IVSense/
and the Arduino code for the arm is here https://github.com/caplingerc/Roboarm
. It's currently programmed to just draw a short vertical line over and over again.
Soooo... my issue is that I'm 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 I'm getting aliasing, but it seems that it's not the whole story and I'm 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. I can send an excel file if anyone wants it.