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

Dustyn

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.

Current


Adafruit publishes a wide range of writing and video content, including interviews and reporting on the maker market and the wider technology world. Our standards page is intended as a guide to best practices that Adafruit uses, as well as an outline of the ethical standards Adafruit aspires to. While Adafruit is not an independent journalistic institution, Adafruit strives to be a fair, informative, and positive voice within the community – check it out here: adafruit.com/editorialstandards

Join Adafruit on Mastodon

Adafruit is on Mastodon, join in! adafruit.com/mastodon

Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.

Have an amazing project to share? The Electronics Show and Tell is every Wednesday at 7pm ET! To join, head over to YouTube and check out the show’s live chat – we’ll post the link there.

Join us every Wednesday night at 8pm ET for Ask an Engineer!

Join over 36,000+ makers on Adafruit’s Discord channels and be part of the community! http://adafru.it/discord

CircuitPython – The easiest way to program microcontrollers – CircuitPython.org


Maker Business — “Packaging” chips in the US

Wearables — Enclosures help fight body humidity in costumes

Electronics — Transformers: More than meets the eye!

Python for Microcontrollers — Python on Microcontrollers Newsletter: Silicon Labs introduces CircuitPython support, and more! #CircuitPython #Python #micropython @ThePSF @Raspberry_Pi

Adafruit IoT Monthly — Guardian Robot, Weather-wise Umbrella Stand, and more!

Microsoft MakeCode — MakeCode Thank You!

EYE on NPI — Maxim’s Himalaya uSLIC Step-Down Power Module #EyeOnNPI @maximintegrated @digikey

New Products – Adafruit Industries – Makers, hackers, artists, designers and engineers! — #NewProds 7/19/23 Feat. Adafruit Matrix Portal S3 CircuitPython Powered Internet Display!

Get the only spam-free daily newsletter about wearables, running a "maker business", electronic tips and more! Subscribe at AdafruitDaily.com !



1 Comment

  1. Thanks for the post! I’m following the forum thread, will post my next question shortly!

Sorry, the comment form is closed at this time.