PWM and H-Bridge (R/C project)

[AMSA84]

Hello there. I'm new here.

I'm gonna make an R/C project. Right now I'm testing the H-Bridge, so I have made an PWM based on 555.

There is here the first problem. When I start making a frequency variation through the potentiometer, just right DC motor start rolling there is a strange sound coming from the DC motor. I think it might be something related with the frequency and the DC motor (coils?).

I'd like to know how can I eliminate that sound.

 

[jpanhalt]

The sound you hear may be the coils in the motor acting like miniature speakers. If you change the frequency, the noise should change. It is very common. Some people go high frequency (e.g., 20KHz); some just live with it.

John

 

[AMSA84]

Ok. I just thought about that. But imagine..

There is a contorl signal that will allow the PWM signal drive the transistors to make the car go fwd and backwd. If I want the car to ride kinda slow, wich frequency I'll need? Something like what, 2khz? But maybe you have are right, because I'm not experimenting the car at the ground, he's on air (supported).

But do you understand what I'm trying to say?

Best regards

 

[BeeBop]

Ok. I just thought about that. But imagine..

There is a contorl signal that will allow the PWM signal drive the transistors to make the car go fwd and backwd. If I want the car to ride kinda slow, wich frequency I'll need?

Best regards

It is the duty cycle, which controls the speed, not the frequency.

 

[jpanhalt]

Something like what, 2khz? But maybe you have are right, because I'm not experimenting the car at the ground, he's on air (supported).

I find 2KHz quite annoying. At 20KHz, hearing response is fairly low (gone if you are old like me), but there are harmonics. Just make the frequency something you can live with and control speed with duty cycle, as already mentioned. John

 

[audioguru]

The inductance of a motor reduces the current when the PWM frequency is high.
If you want to have full power then the frequency should be from 200Hz to about 4kHz.

 

[Jim Aman]

Hello there. I'm new here.

I'm gonna make an R/C project. Right now I'm testing the H-Bridge, so I have made an PWM based on 555.

There is here the first problem. When I start making a frequency variation through the potentiometer, just right DC motor start rolling there is a strange sound coming from the DC motor. I think it might be something related with the frequency and the DC motor (coils?).

I'd like to know how can I eliminate that sound.

The sound could be generated in the motor because its being driven by a square wave instead of a sine wave. The square wave has multiple frequencies see FFT which will shake the coils and make noises. You may be able to dampen the effect if you put a capacitor across the motor.

 

[jpanhalt]

The inductance of a motor reduces the current when the PWM frequency is high.
If you want to have full power then the frequency should be from 200Hz to about 4kHz.

That point has been argued many times. I think 4QD has a fairly balanced discussion of it and makes both high- and low-frequency drives.

http://www.4qd.co.uk/faq/bmnc4.html#switfreq

The lower switching frequencies are better with smaller, higher voltage motors where the motor has a much larger inductance. However high current, low voltage motors have a very low inductance so a high switching frequency is necessary for best performance.

Ultimately, it becomes a balance of linearity, efficiency, and noise. The OP did not say much about his motor.

If I were just building a toy to run around the house, I would opt for higher frequency to avoid the annoyance.

John

 

[AMSA84]

Ok. Thank you guys. There is another doubt:

I've noted that the frequency on the output of 555 (PWM mode) is diferent from the frequency at the motor input.

Can anyone tell me why? That is not supposed to be equal? The transistors are switching at the same frequency than the 555? Or what determines the frequency at the motor input is the ON time of the duty-cycle?

Best regards

 

[Sceadwian]

You could smooth the current out a bit with a large capacitor right at the motor, similiar to the noise blocking caps that are on most DC motors. Small motor run capacitors may work as they're not usually polarized. I've heard some people suggest using back to back electrolytics as a non-polarized cap but I don't know how reliable this is.

 

[AMSA84]

What you mean by "smooth the current out a bit with a large capacitor right at the motor"?

Please explain yourself.

Best regards!

 

[jpanhalt]

As for the frequency shift, my first guess is that you are seeing an artifact of they way you are measuring it. Are you using an o'scope or something like a digital multimeter? Does the error change with duty cycle?

John

 

[AMSA84]

Well, I'm using a digital multimeter (but tomorrow I'll watch on a scope). The frequency at the motor input changes a little bit. Variantions ~400Hz.

So my doubt is that the frequency at the motor input is defined by the ON time of the duty-cycle?

Another question, how can you guys explain me why the output of the PWM 555 signal changes when we vary the potentiometer? I know that the "speed" of the motor is based on the ON time of the duty-cycle, but what I cannot understand is why the voltage changes. For example, 15% of the duty-cycle corresponds to 2.5V and 50% carresponds to 5.0V. Can you understand what I mean?

Best regards

 

[jpanhalt]

Your voltmeter is showing a time averaged voltage, not peak to peak. That is, on DC, the sampling time includes several cycles. When you use the o'scope, you will see the peaks are fairly constant in amplitude. John

 

[AMSA84]

Ok then. Tomorrow I'll study the wave forms in every single part of the cricuitry and if doubt persist I'm gonna let you know.

Best regards

 

[Sceadwian]

If you're measuring things on a multimeter all you're getting is the RMS voltage, and that's in a very narrow frequency range, if you're measuring above the frequency your meter is good for you're going to get a distorted reading. Even the best RMS multimeters are usually only good into the low KHZ range, mine is only good up to a couple hundred hertz. If your meter isn't a true RMS meter then the readings you get can be pretty much discarded as useless as the internal circuitry will affect it's reading more than the signal itself.

Depending on how the 555 circuit is built changing the duty cycle will affect the frequency a bit, not much you can do about it on a 555, it's an analog device, if you want a more precise drive frequency and duty cycle you can get micro controllers that have glitch free PWM outputs that will give you very high precision frequency and duty cycle.

If you use a capacitor across the motor leads in series with a diode to prevent it from discharging to ground during the off cycle it will provide hold up power and smooth out the abrupt changes in voltages from a PWM source. When a discharged capacitor is connected it will suck a LOT of power as it's a low impedance load to a square wave so it will slow the sharpness of the power rise to the motor as it's own impedance will draw less power, then when the PWM is turned off the capacitor will be isolated by the diode and discharge it's available power to the motor smooth it's shutdown voltage by drawing power from the capacitor. Making the curves less sharp will decrease the noise the motor can generator, as a hard square wave will generator many odd harmonics well above it's natural frequency into the audible range. This would probably most benefit from a low PWM frequency, in the hundreds of hertz range. But the capacitor and diode will introduce some loss which need to be taken into account.

 

[AMSA84]

Well i've seen the wave forms already. Now I've an idea.

We'll, I'm gonna take advantage from this post to ask you guys if it is possible you to give me ideas to my R/C car transmitter. I'd like to use a PIC or simple analog or digital methods.

I'm open to sugestions.

Best regards

 

[jpanhalt]

Which part of the transmitter do you want to build? The RF module inputs (i.e., the modulator) are pretty well defined...see various PC-based trainer systems. Some transmitters have outputs for trainer cords. You can use that output to see what you will need to do for your own modulator.

If you want to build the RF section, that is quite another matter and raises licensing issues. John

 

[AMSA84]

The modulator I intend to buy it. Those PCB very small TX and RX working at 433Mhz..

My question is related with the way to put the information on the modulator. Wich could be the best way to say go fwd and the information is created and then go to the modulator. Can you understand? I can't explain it in english, it's quite wierd to explain x) but I hope made myself explained!

btw, can you tell where are selling on the internet those kinds of potentiometer used in R/C car trasmitters, looking like a pistol..

Best regards

 

[jpanhalt]

They are called joysticks.

The cheapest source is probably from an old, discarded transmitter. We don't know where you are. In the US most modeling clubs either have some to give away or sell at rummage sales, or the members will give them to interested new members.

John