Help with DC motor driver circuit

Thread Starter

herher

Joined Oct 18, 2011
53
Hi,


Thanks for the reply.

I am sorry, I don't understand what do you mean. The +-15V is the supply voltage for the OPA544 but the allowable operating voltage is the supply voltage minus 4-5V, so the voltage range passing through the dc motor will only between +-10V.

Please advice. Please let me know if I have any misconception. Thank you very much.


Best Regards,
herher
 

Bernard

Joined Aug 7, 2008
5,778
Hi,


Thanks for the reply.

I am sorry, I don't understand what do you mean. The +-15V is the supply voltage for the OPA544 but the allowable operating voltage is the supply voltage minus 4-5V, so the voltage range passing through the dc motor will only between +-10V.

Please advice. Please let me know if I have any misconception. Thank you very much.


Best Regards,
herher
Sounds like you understand the motor drive just fine.
 

Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

I have tested my circuit, and it is working. Sgtwookie, I have tried to put a 1k or 10k resistors in series with the input signal (the signal to the OPA544 input pin), my dc motors are still able to rotate. Does it mean that the power amplification function of my circuit is working finely? But why? I thought the resistor is just used to drop down the voltage supplied to the input pin? Please clear my doubt. Haha.

Thank you very much.

Best Regards,
herher
 

Bernard

Joined Aug 7, 2008
5,778
The input impedance of the OPA544[ as a V follower or buffer ] is verry high so adding some resistance in series with the input, R1, will have little effect on the output. R2, as suggested by The Sgt., never allows for a floating input. R3, hash filter or Boucherot cell[ new word for me]. R3 commonly 2X motor DC resistance C non critical, .1 to .3. Rx convenient test point to measure motor current, helps with start-uo & quick reversals. As large as possible, but not to limit desired torque at max output. Sorry Ifixit for messing up your drawing.
 

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Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

Thank you very much for the reply.

Thanks for the circuit. Ya, in my circuit, I did parallel a 1 ohms resistor in series with a 0.01 microfarad capacitor (what is the difference between 0.1 and 0.01?) to ground. Besides, can I know more about the usage of the resistor Rx?

You mentioned in your post that the OPA544 input has very high input impedance. Does it mean that the input impedance will remain high regardless of what types of signal is being supplied to it (can be the low current output signal from a microcontroller or a voltage signal generate from a high wattage power supply)? As a matter of fact, the microbox output I am using is like the output from a micocontroller which is initially unable to drive DC motor. But now the microbox is not with me, the input signal I am using is generated from a 9V alkaline battery (which is able to drive my dc motor even without the OPA544 in between).

So, in order to acquire the signal similar to the microbox output Sgtwookie have suggested me to series a 1k or 10k resistor to the input, in the hope of determining the functionability of the power amplification as well. This is the reason why I series a resistor in the input pin.

Please advice, thank you very much.


Best Regards,
herher
 

SgtWookie

Joined Jul 17, 2007
22,221
what is the difference between 0.1 and 0.01?
0.1 = 0.1uF (microfarad) or 100nF (nanofarad)
0.01 = 0.01uF or 10nF.
0.1uF is 10 times as large as a 0.01uF.

You mentioned in your post that the OPA544 input has very high input impedance. Does it mean that the input impedance will remain high regardless of what types of signal is being supplied to it (can be the low current output signal from a microcontroller or a voltage signal generate from a high wattage power supply)?
Yes, as long as you do not exceed the power rails. Then the ESD protection diodes will start drawing current. They are designed to protect the device against static discharge, not to be used as routine.

As a matter of fact, the microbox output I am using is like the output from a micocontroller which is initially unable to drive DC motor.
That is because it is a high-impedance output.

But now the microbox is not with me, the input signal I am using is generated from a 9V alkaline battery (which is able to drive my dc motor even without the OPA544 in between).
This we knew.

So, in order to acquire the signal similar to the microbox output Sgtwookie have suggested me to series a 1k or 10k resistor to the input, in the hope of determining the functionability of the power amplification as well. This is the reason why I series a resistor in the input pin.
Wire one "end" of the pot to +15v.
Wire the other end of the pot to -15v.
Wire the wiper to the 10k resistor that goes to the input of the opamp.

This thread is going on way, way too long for such a simple project.
 

Bernard

Joined Aug 7, 2008
5,778
Value of C. Assume hash has a frequency of one mega Hz. thn .01 μF, X of C is about 15Ω, which is large compared to 3X motor resistance. .1μF is about 1.5Ω, would be better-or better .2 to .5μF.
 

Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

Thanks for the replies...you guys make the project looks easy, haha anyway thanks a lot.

I have a few problems here with the circuit...

How can I dissipate the heat of the amplifier more efficiently? I have tried to use heat sink and heat sink compound, but the heat sink will easily get hot once I run the DC motor.

What is the operating voltage range of a usual 12V motor? I have tried to run the dc motor with the circuit, I found out that my dc motor is not rotating at all if the supply voltage is small (e.g: -1.8V to 1.8V). My project require dc motor to rotate even it is supplied with little amount of voltage (for PID algorithmic, position control). Or dc motor is generally unable to rotate if the supplied voltage is much more lesser than the rated voltage?

Here are some info of the dc motor I am using:
DC12V
Output Power: 1.1 Watt
Rated Speed: 58RPM
Rated Current: 410mA
Rated Torque: 254.8mN.m
Sample Application: lightweight mechanism such as: bank note machine, handling machine, educational robot, etc

Please advice. Thank you very much.

Best Regards,
herher
 

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ifixit

Joined Nov 20, 2008
650
Hi Herher,

Sounds like you need a bigger heatsink, or you could use a fan and blow air over the heatsink to help take the heat away.

You are using a non-precision motor with a gearbox on it that reduces the RPM while increasing the torque. The side-effect of this is increased mechcanical fiction. To get the motor to move from a stop you need to overpower the friction to get it moving then back off once it is moving. The "D" part of the PID control algorythm should do this if set up correctly.

You should have a tachometer on the motor as well for better control.

Regards,
Ifixit
Don't forget to have fun.
 

Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

Thanks for the reply.

If geared dc motor is not suitable, then what type of dc motor is better? I have checked it before, the price for brushless dc motor is very expensive.

To get the motor to move from a stop you need to overpower the friction to get it moving then back off once it is moving. The "D" part of the PID control algorythm should do this if set up correctly.
Can you explain more on this? I am using matlab to design the PID controller...

Please advice. Thank you very much.


Best Regards,
herher
 

ifixit

Joined Nov 20, 2008
650
If geared dc motor is not suitable, then what type of dc motor is better? I have checked it before, the price for brushless dc motor is very expensive.
I didn't say it was not suitable, just difficult to get working the way you want due to; slack in the gear train, and friction. Whither it is a brushed motor, or not, doesn't make much difference.

Just try to make the motor you have work by learning more about electronics and mechanics. Google "PID tuning". Look at some of the links for tips on how to tune a PID. It will be a "trial and error" process if you don't understand the exact characteristics of the devices you are trying to control.

Regards,
Ifixit
 

Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

Thanks again for the reply.

I explain my concept here, can you please verify if it is correct or wrong? Thanks.

From what you have mentioned, I can amplify the controller magnitude (Adjust the P, I and D) so that the signal to the dc motor will be big enough to overcome the friction when it is approaching the set position right? As an illustration, when the system is approaching to the set position that I have assigned in the set point, the signal to the motor will become smaller and smaller as the error is decreasing...So all I need is to ensure that when the signal is decrease to a specific point (below 2V), my PID control is able to produce the signal that is big enough to rotate so that my system can still move towards to the set position. Is it correct?

Please advice, thanks

Regards,
herher
 
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Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

Ifizit and Sgtwookie, how are you?

I have met some problems with my circuit. Could you please give me some comments about my problems. Thanks a lot.

First of all, I found out that my motor driver circuit (with operational amplifier OPA544) is very easy to get hot. It was ok and no heat was generated from the OPA544 when there was no input voltage supplied to its input pin, the circuit was just simply powered up by the dual power supply. But when there was an input signal being supplied to the relevant pin, the OPA544 will start getting hot very quickly, and the worst thing was that the voltage follower was not functioning anymore as it will output -1.5V at 0V input and around 7V at 8.5V input. It was like the output voltage was moving 1.5V backward comparing to the input. Can I know what is the reason causing this malfunction? Actually, I got accidentally touched the different leads together before ( I am sorry, it was because I forgot the tab as well as the heat sink is carrying the negative supply), and there was a spark. Do you think this is the main reason? Besides, I am temporarily using voltage signal from a 9V alkaline battery passing through a potentiometer as my input, I share the common of the battery with the ground of my circuit.

The other problem is everytime when I finish tesitng my circuit and was about to remove the wires (line, neutral and ground) from the +15V switching power supplies ( there are two), the ELCB circuit breaker switch in my house will suddenly cut off by itself. Is it because I have used too much supply and has reach the cut off limit of the switch? But why the electricity was not cut off when I am doing my experiment? It was cut off only when I switch off everything and was about to remove the wire cables from the switching power supplies. Besides, I get the AC supply for the two switching power supplies from a household electrical socket.

Please give me some suggestions. Thanks a lot.

Best Regards,
herher
 
Last edited:

ifixit

Joined Nov 20, 2008
650
Hi,

Show a schematic of what you have exactly. Take pictures so we can see where the problem might be.

Regards,
Ifixit
 

Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

Thanks a lot for the reply.

I have attached some pictures over here. Please have a look. Please advice. Hope to hear from you soon. Thanks.

Best Regards,
herher
 

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ifixit

Joined Nov 20, 2008
650
Hi Herher,

It is possible that your circuit is oscillating because you don't have any power decoupling on the power pins of the OPA544s. Refer to the specification on page 6, figure 1. A 0.1uF and a 10uF capacitor are requied as close to the opamp pins as you can get them. You can check for oscillations with an oscilloscope, or if you don't have one you can hold a small AM radio near your circuit and see if you hear anything... it should be quiet. The switching power supplies may have a noise of there own so stay away from them.

I don't know why your house breaker would trip because your power supplies are a light load. Perhapes that circuit in your house is already loaded to near its limit.

Always power up both power supplies at the same time so that the opamps see both plus and minus 15 volt rails come on or off at the same time.

Those heatsinks look small. However, the opamps are designed to run hot if required, 85°C maximum case (Tc) temperature would be my recomendation as an upper limit. Refer to figure 2 on page 6. The opamp with heat up more when the motor load is high, or RPMs are low with high current. You may need bigger heatsinks, or forced air cooling.

Let me know how your testing goes after you have installed the bypass capacitors.

Good Luck,
Ifixit
 

Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

Thanks again for the reply.

But currently I have only electrolytic type capacitor for the 10 microfarad one, can I temporarily used it to substitute the recommended tantalum type? Besides, can I know why the two leads of my 1 ohms resistor are connected? I thought usually the two leads of the resistor should be disconnected...please advice. Thanks again.

Best Regards,
herher
 

ifixit

Joined Nov 20, 2008
650
Hi Herher,

  • Use the 10u electrolytic for now
  • I don't understand your question about the 1Ω resistor
Regards,
Ifixit
 

Thread Starter

herher

Joined Oct 18, 2011
53
Hi,

Ifixit and Sgtwookie.

Can I know why I can't simulate this circuit? When I try to run it, a spice error stating that "Unknown subcircuit called in: xu1 in1 n002 v+ v- n002 opa544" will jump out. Please see the attached file. I am using LTSpice software.

Ifizit, I will test my circuit with the capacitors you have suggested as soon as possible, as these few days I am busying with other stuffs. Thank you very much.

Best regards,
herher
 

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