i lose torque with my Pwm motor speed control

[oscyes]

hello everybody thank you for reading this post. I have been working on a project wich i need to reduce my dc motor speed to a very slow one, but i cant allow the lost of torque because it will make my robot not to move because of its weight.

i tried to use a pwm control with arduino board but when reducing the speed my robot doesn move, it seems not to have enough torque for moving my robot`s arm

does anybody know how can i reduce my dc motor speed to a very low one without losing torque , i also need my motor to be stopped but doing force and torque

thank you so much and please excuse me for my english level i tried my best 

[Azraels]

Your English is fine.
 Did you build your own H-bridge or are you using a chip? If you built your own did you use transistors or fets? Fets work better because there is very little current loss. If you use transistor try to use ones designed for driving inductive loads.

A gear reduction would work better for torque. Go from small gears on your motor shaft to the final larger gear running what you want to turn.

[oscyes]

thank you for your reply, i am using a l293d chip wich has 4 half h bridge inside, the thing is i need to create a kind of pid aproaching code and when the arm aproaches the ground i need very low speed and high torque thats why i cant use the mechanical transmision that would make the movement slow since the beginning where i need high speed.

it is supposed that the pwm speed controller keeps a constant torque but it seems not to work for me, maybe it is not a constant torque speed variation or maybe i am doing it wrong

i read something about using two 555 chips so i can drive my motor at a very slow speed withou losing torque but i didnt understand

this is the link if you want to check it out =)
http://homemadecircuitsandschematics.blogspot.com/2011/12/constant-torque-dc-motor-speed.html
thank you so much

[Azraels]

Do you mean to say you have your motor's shaft connected directly to the arm you are trying to move? This will never work with a dc motor. You must have some type of transmission. You can have a gear reduction which will still enable for a decent speed with your arm and then use the PWM to slow it down as it reaches the ground.
I would say that is why the PWM is not working. You will never generate enough torque at low speed to turn an arm hooked directly to the shaft.

[oscyes]

yes i connected my motor directly to my robot's arm but it is a micromotor with a reduction of 298:1 in rpm ill add the link of my motor so you can see. the motor have enough torque for moving my arm but when i want to move it at a very slow speed the arm just stay stopped

thank you so much for your replies =)
http://www.dynamoelectronics.com/dynamo-tienda-virtual.html?page=shop.product_details&flypage=dynamo.tpl&category_id=99&product_id=216

[Billy]

maybe it is not a constant torque speed variation or maybe i am doing it wrong

The torque of a motor is determined by the current through the motor. A general purpose controller that claims to be constant torque would have to monitor current, and the circuit you link to does not monitor current....so it is not constant torque. The author of the page is incorrect in his/her claim.

As stated in other thread, the driver requires a controller to modulate the PWM to give the desired motor behavior.

[oscyes]

Thanks for your replies, Now I understand a lot and this url helped me a lot too:

http://arduino.cc/forum/index.php?topic=87473.0

The conclusion I have done is:
When you vary pwm duty cycle is going to behave similar to vary the source voltage, when u reduce ur duty cycle then the average voltage in the motor decreases as well, and decreasing the current passing trought it too  and afecting the torque the motor can raise.

I have decided to control my pwm duty cycle as much as the motor speed decreases with load for maintain velocity trough a pid control as Billy says.

[voodoo]

Best to use transmission,  like a small worm drive. The problem is here, without transmission, the motor will be more strained, thus shorten the life significantly of you motor 

[oscyes]

thats true but i need also the high speed reaction thank you so much

[Pogertt]

Have you experimented with both Duty Cycle and Frequency of the PWM?
Try 10, 100, 500, or 1000 for the frequency and see what difference that makes in low speed torque.