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Project Help and Ideas » control of large steppers

January 15, 2012
by sask55
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I am looking at different options I could use to gain control of stepper motors for a CNC project. I could purchase a motor controller board and stepper drivers when I order the motors. That option would greatly simplify the motor control for me but is not very interesting or rewarding to me. On the other hand I am not at all certain that I would be successful in dealing with the design considerations such as back EMF, voltage spikes, current monitoring and chopping circuits, heat dissipation and so forth. There are undoubtedly a number of issues that I am not even aware of that could make the likely hood of success remote.

I was thinking that by using a motor control chip I may be able to take advantage of the chips features and still have a project to design and build the supporting circuits and software (that is likely to be challenging enough for me).

http://www.marcmart.com/ebay/o/OT400/TB6560AHQ_AFG_datasheet.pdf

I have read over the date sheet for a Toshiba stepper motor control chip. I believe that I have a general grasp of the functions available with this chip. I think that it may work well for my application and should be able to use a AtMega mirco to control the external inputs. The MCU could control various parameters and modes on the chip using the input pins. I do have one question I was hoping someone could help me with. My questions are related to controlling the motors rotational speed and starts /stops thru the external input pins.

It is not directly addressed in the data sheet. The only way that I can think of to control the motor speed and stop/start, so that it will restart at the appropriate spot in the cycle, is to control the CLK input pin. Does anyone know is this correct? Do I simply control the frequency of the CLK to control motor speed and delay the CLK pulses to stop the motor?

Thanks

Darryl

January 15, 2012
by missle3944
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Darryl,

I use a ULN 2803 NPN transistor array for controlling a 12v stepper. It takes care of the diodes and you dont have to mess around with tons of messy wiring with diffrent descrete components. I've used a shift register with it. I suppose you could just use that ic.

As far as your ic that you suggested I have no idea how to control it.

January 15, 2012
by sask55
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In the past I have used ULN 2804 to control smaller stepper motors, that does work well. The motors I am using in this project are rated at 3A max. The current draw and Back EMF produced would both be considerably too large in this project for that chip. The power supply I am using is a 24V.

my understanding is that, typically with stepper motor control systems the motor coils are overdriven with high voltage levels to accomplish fast phase changes. As the voltage is initially applied to the coil the inductance of the motor coil slows the rate of change in the current flow. The high initial voltage compensates for this increased impedance and speeds up the phase change, giving quicker torque rise for the motor. Then as the current level within the coils reaches a predetermined level circuitry in the control reduces or chops the voltage applied to the motor coil and holds it at that level to protect the motor. The motor control chip provides a number of other interesting capabilities that could not be achieved with the ULN 2803 type circuit.

thanks for the thought

PS I found a good reference on line. I believe I have now have a good understanding of what it is I am attempting to do just waiting on some parts.

January 16, 2012
by Rick_S
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Sounds like a job for an L298 4A stepper motor driver IC. You can either drive that from your mcu directly or from an L297 driven from the mcu. In either case you would be providing an external source for the higher stepper voltage. These work real well and are easily found.

Rick

January 16, 2012
by sask55
sask55's Avatar

Rick

The data sheets for the L298 4A and the L297 are interesting; they give some good details on some of the supporting components, and board layouts. That is what seems to be somewhat lacking on the Toshiba data sheet(TB6560AHQ). I have ordered a couple of the Toshiba chips. After I get a chance to play around with them a little on a prototype setup I should have a better idea if I can get along with them. From the data sheets the Toshiba chip seams to do much the same things as the L298 and the L297 combined. The Toshiba chip also has the capability for vary fine built in micro stepping and precise control of current decay timing. I don’t believe I will require those features so it likely does not matter for my application.

With the Toshiba chip I am a little uncertain about anti back EMF diodes on the outputs. The Toshiba data sheet does not show any in place on the application circuit example diagrams. The data sheet goes into some considerable detail with regards to current discharge path thru the chip, and internal parasitic diodes. However the reference that I have found stresses the need for fast, high current anti back EMF diodes at the outputs. I don’t have any such diodes on hand so I guess I should order some and but them in place just as a precaution even if I read the data sheet as not requiring them.

Since I have started down the Toshiba chip road I will continue and see how that works out. I will certainly keep the L298 L297 option in mind should I decided to change paths.

Thanks for the info.

Darryl

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