×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Brushed DC motor encoder oscillations

Brushed DC motor encoder oscillations

Brushed DC motor encoder oscillations

(OP)
I purchased a Pittman brushed DC motor with a 500 count/rev encoder. I noticed quite a bit of oscillation in the encoder signal when the shaft is rotating at a constant rate. After running some tests I started to notice a trend. For instance, when I ran the motor at an average 600 RPM (averaged from the encoder oscillations), or 10 rev/s I measured a 10 Hz oscillation in the encoder signal. Running at an average 1200 RPM, or 20 rev/s I measured a 20 Hz oscillation in the encoder signal. I performed this test at various speeds and always found the oscillations to equal the motor's frequency. I tried running the motor at a very low rate (~1 Hz) and can actually see/hear the motor slow down at a certain point once every revolution.

I am speculating that perhaps there is a commutation issue with one of the brushes. I am curious if anyone else has some experience with this and if they have any suggestions. I ultimately want to use the encoder for feedback control and would like to solve the issue itself rather than just adding filters. Any recommendations would be greatly appreciated.

:LabGuy330

RE: Brushed DC motor encoder oscillations

Monitor the armature current with an oscope. If there is any variation in speed due to loading or commutator issues it will show up there. If not then you may have a bad encoder.

RE: Brushed DC motor encoder oscillations

Optical encoders have a once per revolution position error due to eccentricity of the encoder disk. Disks in small motors often have higher eccentricities.

RE: Brushed DC motor encoder oscillations

(OP)
Thank you both for your responses. I in fact did measure the current to the armature and saw oscillations. This morning I contacted Pittman motors and spoke with one of there engineers. He informed me that these oscillations are due to cogging (unknown to me before) due to an iron armature. Apparently all Pittman motors experience this cogging, though they try to minimize it as much as possible. He did recommend other companies that sell ironless armature (zero-cogg) motors: Maxon and Micro Mo. According to him these motors will not exhibit the oscillations I am seeing. If anyone has some experience with such coggless motors I am all ears. Would really like some input before I make another purchase. Many thanks.

:LabGuy330

RE: Brushed DC motor encoder oscillations

The MicroMo motors have ironless cores, and no physical structures that could cause cogging. They don't cog. They may cost a bit more than the Pittman motors.

You may be able to improve the performance of your system even with the Pittman motors by adjusting the phasing between the encoder and the motor's commutator.

Mike Halloran
Pembroke Pines, FL, USA

RE: Brushed DC motor encoder oscillations

How are you controlling the motor, using the encoder tach output? If so, then the eccentricity of the encoder will affect the motor control. You possibly need to try and run the motor open loop or with a different tach to see if it is the motor by itself.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

RE: Brushed DC motor encoder oscillations

(OP)
Valid question. I am running the motor in open loop and just monitoring the encoder readings. My plan was to step test the motor to empirically find the mechanical time constant, and then I noticed the trend in encoder oscillations. Encoder eccentricity is also something new to me and I will look much more into this as well. Thank you for everyone's insights.

:LabGuy330

RE: Brushed DC motor encoder oscillations

"running open loop"

what exactly does that mean?

your motor does NOT - note - NOT - have a once per rev cogging - it is impossible.

SO. AGAIN, how are you running this motor where is the voltage and current coming from?

please do not jump to some conclusion about cogging when your data makes that not a possible cause.

RE: Brushed DC motor encoder oscillations

i suppose I got too excited in hitting submit button wo an edit button. My factual statement that you cannot have a 1/rev cogging was in answer to your and other suggestions that it was caused by the motor design. Know that a motor has an even no of poles so you MUST have at least 2 'cogs' per mechanical rev if it is caused by the motor design. buying an ironless motor may fix your problem but not because this one has a 1/rev cog due to iron in its rotor.

RE: Brushed DC motor encoder oscillations

(OP)
The motor is hooked up to a power supply. When I said 'running open loop' I simply meant the encoder is not being used in feedback, just for visual monitoring.

No conclusions have been made. This is a discussion. I only relayed the information I received after calling Pittman motors. I am still very open to the advice from others.

:LabGuy330

RE: Brushed DC motor encoder oscillations

Maybe the armature is out of round? Off center?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources

White Paper - The State of Product Development and Hardware Design 2019
The most successful companies are usually the ones that never rest. They are always striving to get better, to deliver more innovative products and get them to market faster. Onshape surveyed 850 product design and manufacturing pros to learn what techniques and strategies they’ve employed to keep ahead of their competition and in this white paper their answers are distilled. Download Now
Research Report - Mass Customization Placing Design in the Customers Hands
Products come in all shapes and sizes, and customers demand solutions tailored to their needs. With mass customization software, these two facts can be reconciled in an automated, customer-facing experience called a product configurator. In this engineering.com research report, we examine the different levels of mass customization with the pyramid of product configurators. Taking a look at real-world case studies, we discover how the highest level of the pyramid can enable true engineering automation, saving time and increasing customer satisfaction. Download Now
Research Report - Test and Simulation Survey
We at engineering.com continually hear how engineers and designers will be doing simulation, such as FEA and CFD, and that they will be doing it early in the design cycle. Is this wishful thinking? We found out what it is really like in industry. Read more now. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close