# Increase/Decrease rpm over time

Thread Starter

#### M clif

Joined Mar 16, 2015
2
I am building a Kinetic art piece that has a rotating disc that ideally could increase from about 10rpm to 20rpm over a span of about 30 seconds then decrease at the same rate and continue that cycling until switched off. Only a few ounces of torque will be required to drive the disc. RPM and time accuracy is not required. As I know very little to nothing about circuits and motor controls should I be looking at a stepper motor and controller or something to increase/decrease voltage over time to a dc motor/reduction geared set up? Not trying to be vague about anything I just do not know the proper questions to ask. Any suggestions would be appreciated.
Thanks
Mark

#### crutschow

Joined Mar 14, 2008
33,367
I believe a stepper motor would be the easiest since it operates synchronously to give a known RPM for a given input frequency and thus requires no feedback to control the speed.
You simply have to ramp the stepper frequency up and down to give the desired speed variation which can be done with a voltage to frequency converter chip.

Thread Starter

#### M clif

Joined Mar 16, 2015
2
Thanks for the reply will get to researching.

#### wayneh

Joined Sep 9, 2010
17,475
You could use a 556 dual timer IC to do this. One timer, B, would be set to give a frequency to drive the stepper motor and disc at ~15rpm. The other timer, A, could be set to a cycle of about 1 minute. Link the two timers so that the output of A drives the "control" pin of timer B with an RC delay (I can explain). When timer A output is high for 30 seconds, the voltage at the control pin of B will rise and so will the output of B that is driving the motor speed. Then A will go low for 30 seconds and drive down the control voltage and speed of B.

If this sounds remotely interesting, I can offer a schematic. It's not terribly elegant, but it does work and is something a newbie can handle. A microcontroller would be the fancier approach but that requires a learning curve.

#### Bernard

Joined Aug 7, 2008
5,784
Stepper might be a good choice but I would go for a DC gear reduced motor controlled by PWM, pulse width modulation. Mostly use 556 for voltage controlled PWM.

#### Sensacell

Joined Jun 19, 2012
3,331
Stepper is going to be most precise, but most expensive and noisy!
Also the large inertia of the disk may cause problems with the motor loosing sync with the drive signal if it changes abruptly, causing it to stall.

I would do -> linear ramp generator - > PWM motor drive -> DC gear motor.

#### Reloadron

Joined Jan 15, 2015
7,256
I am building a Kinetic art piece that has a rotating disc that ideally could increase from about 10rpm to 20rpm over a span of about 30 seconds then decrease at the same rate and continue that cycling until switched off. Only a few ounces of torque will be required to drive the disc. RPM and time accuracy is not required. As I know very little to nothing about circuits and motor controls should I be looking at a stepper motor and controller or something to increase/decrease voltage over time to a dc motor/reduction geared set up? Not trying to be vague about anything I just do not know the proper questions to ask. Any suggestions would be appreciated.
Thanks
Mark
All things considered here is what I would think about. Not knowing your location makes suggesting parts houses and vendors a little difficult but I would suggest, as mentioned, a stepper motor. I also agree that other DC motors as suggested could be PWM driven. I suggest a stepper motor because of kit availability from suppliers like SparkFun Electronics. A good stepper motor kit will include the stepper motor as well as a stepper motor driver. These are nice because getting up and running is relatively easy. Finally to make it "Go" I would suggest a uC (micro-controller) like an Arduino kit. While the uC will need to be programmed that can be a matter of copy and paste the code from dozens of published code out there on the web. Likewise, even a PWM (Pulse Width Modulation) control method can also be done using a uC. My thinking in your case, based on your post is to keep things modular and as simple as possible to connect the blocks.

Just My Thinking
Ron

#### wayneh

Joined Sep 9, 2010
17,475
The circuit I mentioned in #4 is shown here. It's part of a larger circuit but if you look at just the 556, you'll see what I described - one timer controlling the second. You can get a max of ~4X control using the control pin of the 555 or 556, for example 5-20 Hz or 10Hz±2X. Since you want speeds in a 2X range of 10-20rpm, this should be doable.

Just be forewarned, this analog circuit will be far less versatile than a µC. But you can build it on a breadboard quickly.