Greetings to everyone…
I’m working with DC motors to build servosystems and many other robotic devices, mainly a polypod, an hexapod and a 4-legged robot called QUADDRA.
The purpose of this post, is to get some information on how to build a circuit that allow me to control the speed of a dc motor but mantaining torque, I’m a novice in electronics and need some help, so if anyone has a good schematics it’s help will be very useful.
Thanks in advance…
CIGV
MX ROBOTICS
I am going to guess that you are talking about maintaining a constant torque at a specific rpm. torque is proportional to current. back-emf from the motor is proportional to rpm. so the concept is to measure the back-emf in an off part of a pwm cycle and then setting the current level of the pwm based on that reading. it is not simple. the equation relating the current setpoint to the rpm is not especially linear. usually you wind up with an approximation of the torque that is relatively constant over a range of loads and rpm. unless you have significant engineering support behind you I would suggest you just purchase a control that does this for you.
I’m not sure how this would relate to a hex (etc) since those are usual built from servos.
I’m interested though in the control aspect.
Would an encoder help? Back-emf is employed in one of the controllers I use, while another uses encoder ticks. It may be a bit more accurate and ‘do-able’ with the encoder scheme, where you just measure how many ticks of the encoder are going at a time with respect to how much power is applied to the motor. The more load on the motor, the more power needed to reach the tick-rate. You can then set up a fairly easy to understand PID control scheme. I have some experience with this (senior year design course focused pretty heavily on PID control) and would be open to discussion.
Mike
you would want/need a fairly high resolution encoder to make it run smoothly without a lot of dsp power. back-emf is ‘almost’ free. 
Seems to me DC motor speed can be controllec by the voltage (and/or Current) being fed ( usually through a variable resistor) to it. More modern DC motor controllers keep the voltage constant and vary the “duty cyle” of a square wave being sent to the motor. 10% on duty cycle is slow, 90% is fast and 100% is top speed.
You may want to look at how the variable speed control works for the cheap battery powered drills.
man this is an older thread.
most cheap battery powered drills appear to keep constant torque and RPM by using a huge gear ratio.
in a DC motor the back-EMF is proportional to RPM, and the current is proportional to torque. To create a constant torque servo you need to measure both properties and drive your PWM based on them. If you are also using this as a positional servo with position feedback the equations can get complex and use either a fast micro or a small dsp. most common cheap servos are not controlling either velocity or torque as those parameters are defined by the supply voltage and gear ratios. this is observed by such and such a servo has X amount of torque at 4.8V and Y amount at 6V. and industrial servo will deliver specific levels of torque at specified angular velocity over a specified operating voltage range, but that kind of control isn’t inexpensive and usually doesn’t work off of a r/c servo type input.
