I can't seem to make a little 6v DC motor start gradually. I want the turntable to start very very slowly, then ramp up, but no matter what I have tried (NPN transistor circuit, capacitive charging) it just sits there until a certain minimum threshold voltage is reached, then it jumps to a start.
How can this be done ?

 

Simple PWM circuit http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm
Or $3.00 to $5.00 on ebay.
Or with a 12F picmicro.
You may be seeing a case of break-away torque when a load present, the current has to be sufficient in order to see any motion.
Max.

 

Simple PWM circuit http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm
Or $3.00 to $5.00 on ebay.
Or with a 12F picmicro.
You may be seeing a case of break-away torque when a load present, the current has to be sufficient in order to see any motion.
Max.

thanks for the reply. yes, I thought about a PWM arduino setup, but I couldn't see any difference between a pot slowly increasing the voltage (which doesn't work) and widening the PW's until an equivalent voltage is reached (to roughly the same RMS voltage as starts it using the pot). It doesn't move until a certain voltage is reached, then it jerks to life.

As far as inertia and friction go, it slows to a gradual stop once it has been going. Buy I'm not sure what that proves. When I see DC motors start in youtube electronics videos, I don't think I have seen one where it doesn't jerk to life. Once it is going, it can be gradually ramped up, but the very beginning is always a jerk.

 

You explaine dit well. You need a much better closed loop control system with some sort of feedback, namely acceleration. See http://www.ni.com/product-documentation/4824/en/

PWM essentially means you have the same torque at all speeds since torque is proportional to Voltage.

An "S-curve" and linear are too different things. See also:

 

A method I have used with a micro is to apply full voltage for a fraction of a second followed by the pwm ramp up.

 

You explaine dit well. You need a much better closed loop control system with some sort of feedback, namely acceleration. See http://www.ni.com/product-documentation/4824/en/

PWM essentially means you have the same torque at all speeds since torque is proportional to Voltage. /QUOTE]

But if the RMS voltage changes with the pulse width, how can the voltage and torque always be the same ?

 

A method I have used with a micro is to apply full voltage for a fraction of a second followed by the pwm ramp up.

That sounds interesting. Do you get a much more gradual start ?

 

But if the RMS voltage changes with the pulse width, how can the voltage and torque always be the same ?

The RMS voltage is somewhat proportional to speed. Vrms = Vm-I*Rm; A DC motor can be used as a tachometer because I is nearly zero. Rm is the resistance of the motor. Some speed controls in 8 track players used this speed control method.

Torque is proportional to the peak voltage applied. Applying 12 V for 50% of the time is not equivalent to 6 V 100% of the time. Speed wise it might be close, but torque wise the 12 V version will be higher.

 

Can you post a pic of said motor? Some modern DC motors are actually of the brushless type, which means that they already have their own PWM and back-emf or encoder or resolver or whatever already included to regulate their speed.

 

Can you post a pic of said motor? Some modern DC motors are actually of the brushless type, which means that they already have their own PWM and back-emf or encoder or resolver or whatever already included to regulate their speed.

here they are. it is a TRW motor, and I think it is for windshield wipers. in any case, it is for a car, so i should have said 12v.
the one picture shows my latest configuration : press the button and it starts rotating. you can see the 12v power jack.

 

View attachment 142672 View attachment 142673 View attachment 142674
here they are. it is a TRW motor, and I think it is for windshield wipers. in any case, it is for a car, so i should have said 12v.
the one picture shows my latest configuration : press the button and it starts rotating. you can see the 12v power jack.

I still can't see the motor itself... but if it's for windshield wipers, then it should be of the common universal type, which can easily be controlled using PWM. And if it really is a for windshield wipers, then there's a 99% chance that it's actually a gearmotor, which should make it easier for you to lower its speed.

 

I still can't see the motor itself... but if it's for windshield wipers, then it should be of the common universal type, which can easily be controlled using PWM. And if it really is a for windshield wipers, then there's a 99% chance that it's actually a gearmotor, which should make it easier for you to lower its speed.

ok. Being that you and another guy suggest PWM, I will go ahead and set it up and write the sketch and see. I don't think it will make any difference, but I will try it.

 

ok. Being that you and another guy suggest PWM, I will go ahead and set it up and write the sketch and see. I don't think it will make any difference, but I will try it.

It might make a difference, but so far all we've seen is a plastic enclosure, and not the real motor with its wiring and all....

 

PWM essentially means you have the same torque at all speeds since torque is proportional to Voltage.

[/MEDIA]

Torque is dependent on current, rpm on voltage.
The Mean current level using PWM is dependent on the width of the pulse.
Max.

 

I recently delivered a project to activate all dials, gauges & lights of a 1972 jaguar inst. panel to be used as a desk display. The speedometer uses a direct drive from a Johnson 12 V, 200 mA motor. The load is jerky & PWM was not satisfactory, so I added a 20 slot code disc to motor shaft. Motor now runs at about 20 RPM to 3,000 RPM, 0 to 150 MPH.

 

I recently delivered a project to activate all dials, gauges & lights of a 1972 jaguar inst. panel to be used as a desk display. The speedometer uses a direct drive from a Johnson 12 V, 200 mA motor. The load is jerky & PWM was not satisfactory, so I added a 20 slot code disc to motor shaft. Motor now runs at about 20 RPM to 3,000 RPM, 0 to 150 MPH. View attachment 142690

This is very interesting. Can you elaborate on how the circuit works?

 

ok. Being that you and another guy suggest PWM, I will go ahead and set it up and write the sketch and see. I don't think it will make any difference, but I will try it.

What is the final RPM you are aiming for, IOW do you reach or need the full rpm of the existing motor?
Max.

 

I can try. Power is 14V SMPS @ 2A. dropped to 13.5 with a series diode. Motor goes from + supply ( P+ ) thru 10 ohm R to drain of U4. Advancing speed control puts a small V on + input of comparator turning motor full on.
As motor starts turning, pulses from the slot detector trigger OS U3 giving strong 10 us + pulses, integrated by R8, D1, C3 & R9 giving a noisy + V to - input of comparator. Output of U2-B is PWM with duty cycle controlled by speed control & slotted disc. C4 is hash filter.

 

Torque is dependent on current, rpm on voltage.
The Mean current level using PWM is dependent on the width of the pulse.
Max.

When it comes to creating that initial movement, breaking the static friction, is there a difference between a constant 20% current and a 20% duty cycle alternating max and zero current?

I've forgotten the terminology, but I know when you're using pwm control to set the position of direct acting proportional solenoid valves, you can program in a small amount of constant oscillation in order to facilitate smaller step changes in position (overcoming stiction?)

Is there a similar effect when starting a motor, where the brief moment of full current in pwm pulses provides enough instantaneous torque to break friction and achieve movement, but the average current is lower, so it's a smoother start?

 

I can try. Power is 14V SMPS @ 2A. dropped to 13.5 with a series diode. Motor goes from + supply ( P+ ) thru 10 ohm R to drain of U4. Advancing speed control puts a small V on + input of comparator turning motor full on.
As motor starts turning, pulses from the slot detector trigger OS U3 giving strong 10 us + pulses, integrated by R8, D1, C3 & R9 giving a noisy + V to - input of comparator. Output of U2-B is PWM with duty cycle controlled by speed control & slotted disc. C4 is hash filter.

Let me try to understand it better... is the 555 configured as a monostable "one shot" ? Are pins 6 and 7 tied together? because I'm not sure I'm seeing the schematic correctly.