Need Help!- Best and cheap bipolar controller

[Margean]

Hello.
Please help me with choosing a better bipolar controller , and that can handle currents greater than 5A.
Also please help me with electrical diagram of the controller.

Thank you very much

[Margean]

Nobody knows any drivers?

[eaglezsoar]

See:
http://www.cnczone.com/forums/showthread.php?t=20905

There is apparently no open source drivers that handle that much current.
You may have to use a commercial driver such as those made by Gecko.

Regards!

[wildwestpat]

Hi Margean

The problem is that to drive a motor you need a driver that can handle voltage and current. A further complication is that the motor uses coils to generate the required electro motive force and this means they are inductive. I am assuming that you have a stepper motor drive in mind and if this is so then:-

In electrical terms the motor may need just a few volts when stationary but as soon as the step speed increases more voltage is needed to provide the required current in the time available before the current is switched for the next step in the sequence. The increased voltage is related to the inductance of the winding and a low value is better. Pulse width modulation of the coil applied voltage is used to limit the peak current to the safe value and this demands more applied voltage to sustain sufficient current to generate the electro motive force needed to turn the motor. This is also why the torque falls with increasing rpm and why the higher voltages are needed to get nippy movements. (There is a balance to be struck for each machine / motor / driver and value for the performance required. It is a luck person who can have it all!)

The maximum current the motor can take is determined by the gauge of wire used in those coils and the maximum voltage by the insulation on the coil wire and the frame to coils insulation. The frame insulation could be ignored in a perfect construction but life is not like that and the sharp bending of the coil wires can introduce frame to coil insulation weakneses.

To design a driver the easy bit is the PWM rrives as there are several integrated circuits available. Some of these include the power output stages but as you have discovered they are limited in terms of both voltage and current. Whilst it would be practical to design and build power output stages to work with the available integrated circuit bipolar drivers you would have to source suitable semiconductor switches for the voltage, current and frequency needed. These are not cheap and are difficult to obtain in small quantities although a suitable open source design might improve the availablity / cost.

Thus the advice you have from Eaglzsoar to go with a commercial drive is a good one unless you wish to spend a lot of time and cash developing your own. Think about the cost of the available open source drivers in terms of component cost and deduct this from the cost of the commercial products and this will give you a feel for the price sensitivity. Also there is a possibility you will kill several sets of output devices (MOSFET) further adding to the cost. This is why there is no 'open source' design for bipolar drivers capable of 5amps and more with a voltage rating to match.

Go with the commercial offerings and these are many or start an electronics project of your own.

Hope this helps clarify why - Regards Pat

[Margean]

Hello. Thank you for answers. The problem lies in the following way, currently I have a small CNC, which has this controller http://www.epsicom.com/docdown/ep0300.pdf . This is a unipolar controller up to 6A and I would like something similar but to be bipolar. I thought the driver with TB6560AHQ, but do not know if it can be combined with MOSFETs. Or another solution would be A3986, but again I do not know if it can be combined with MOSFETs.
Please help me, I want to make a larger CNC, and I need higher precision and more power so I want to be a bipolar driver.

Thanks

[wildwestpat]

Hi Margean

I have to ask but why the desire for bipolar over the unipolar you have. Have you sorted out the torque you require for the speed you want to move the total mass of your machine as seen on that axis? I am asking as the unipolar has as good or better higher speed potential with some motors. For a lot of people and I am not one - top speed is the goal! If this is also your desire then find motors which are at the top of the capacity of your drives and do some sums to see if unipolar operation will give you the required umph. If not redo the calculation for bipolar operation. This link gives a simple graph of torque v speed for both types of connection but please use the data for the motors you intend to use as there are differences between them.

I can not help you with the addition of power out put stages to integrated circuits as this requires a lot of bench testing as well as a sound theoretical approach. High speed switching of highly inductive coils that also generate a large back emf due to the motion are not a walk in the park but serious design stuff.

Sorry to be so gloomy - regards Pat

http://www.probotix.com/stepper_moto...polar_bipolar/

[Margean]

Hi Thanks for the reply. I know you are right, I want speed to, but I do not like to lose,
that the unipolar , one third of motor power.
I wish to use the motors at maximum torque.
If I need 3Nm power, and using unipolar drivers, I will need to buy at least 3.8 Nm motors,
and price increases with each 0.1Nm.
Here's my dilemma
Thanks again, and wait for your advice !

[wildwestpat]

Hi Margean

Yes but! You need to think about torque and motor pulses per minute as the effects on the type of motor are not linear due to the inductance of the winding and the dynamics of the back emf from the rotating magnetic rotor. What I am trying to explain is that you need – firstly a value of torque to get your machine axis to budge from stand still – secondly then you need a value of torque to get it to accelerate to the cruising speed for the traverse and thirdly a value of torque sufficient to maintain that traversing speed.

When you have an idea of these three values of torque you can then select a motor and part of selection involves selecting the motor driver. Obviously to determine the three values of torque is difficult and requires a physical machine on which to measure the applied torque as well as some test instrumentation. The static torque to get the axis to move is easy to measure with a spring balance and a lever. Unfortunately many motors only specify the stalled torque and the actual just turning torque is less - this is why most people use what appears to be over size motors or copy an existing design.

The running torque of the bipolar and unipolar drive for many motors is not as straight forward as saying bipolar is best. At the higher speeds unipolar can provide a bit more torque and hence the axis can traverse quicker provided the motor has not stalled whilst accelerating. It is for this reason that I was suggesting that you might be OK with unipolar drivers. The general advice that unipolar gives about half the power of a bipolar drive is valid for slow and medium speeds but depending on the motor windings unipolar can give better performance at the higher speeds - just depends on your machine and those torque requirements.

Can you find a similar machine in both size and the axis mechanics (dovetail – linear ball etc.) to yours where the motor size is stated along with the traverse speed. There is a further complication in that the cutting forces have also to be over come wich is s further reason for increasing motor size.

Irving2008 has produced a spread sheet that helps with the calculation of torque and this may help you explore the impact of acceleration and traverse speed on motor torque as well as posting a lot of valuable information on steppers amongst other things. Just do a search of the CNC Zone for Irving2008.

Regards - Pat

[Margean]

Well, you convinced me that unipolar driver is better, but now have to buy at least 4.2 Nm motors to have a power of 3 Nm, unipolar, and the budget has increased significantly from 240euro to 400euro. But there is another problem, not really seen unipolar driver with microstep , I saw only with full /step and half / step. So my request is if you somehow know about the unipolar scheme with microstep driver ...?

[wildwestpat]

Hi Margen

I am not trying to convince you one way or the other what I am trying to get you to see is that both types of drive have their own advantages and disadvantages.

You need to consider the torque curves for the motor you have selected. The microstepping technique will loose torque very rapidly and it is better to gear the motor to the axis rather than microstep. Assume that with 16 microsteps the torque has reduced to one tenth and you won't go far wrong.

Microstepping has a real use in giving smoother movement but for this a low number of steps is all that is required.

At high PPS there is a problem driving the current into the winding inductance. Hence the lower the inductance of the coils in the motor the better. Connecting windings in parallel will make current driving easier as the inductance is reduced by a quarter. Think of inductance as resistance to the high speed current pulses but do not use Ohms law as it is the 'resistnace' to alternating current not direct current - this is a gross simplification so don't get hooked up on it!

Here is the link to Irving2008's calculator which will answer a lot of questions about stepper motors,

http://www.mycncuk.com/forums/showth......-A-tutorial

Having got the torque sorted then you can pick actual motors and use their spread sheets to determine the best driver vv motor size vv driver cost.

Sorry but this is a itterative design process and there is no one size fits all solution. What I intended was to make sure you understood why you were selecting bipolar or unipolar drive as in either case you are about to spend some serious money.

Try various values in Irvings spread sheet and then look at actual torque performance graphs for any motors you intend to purchase. Some motors do not appear to have torque curves and you would need to have some assurance that these are OK or avoid them. With torque being such an inportant design parameter I have to ask what is being hidden! Try and get the most modern design of motor as the magnets used have improved a lot. Second hand motors are best avoided unless you have some assurance they have not been opened. Opening the motor without the correct shunts can result in some of the magnetising force being lost.

Hope this help - Regards - Pat

[Margean]

Thanks wildwestpat.
You gave me very useful information.
I decided to use unipolar drivers that i have , namely those with L297 and MOSFETs.

Thank you so much!!!

[wildwestpat]

Hi Margen

That sounds like a good decision but please do the sums before spending any cash.

Have fun and a good build. Regards Pat