Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: John40131 on September 23, 2017, 07:52:42 AM
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Hi, Thanks for allowing me to join forum, I am new to CNC but experienced in Electronics and Computers, I am building a CNC Machine based on a NYC CNC Project but a bit higher spec Using Mach3 driving a BOB and 4 TB6600 stepper Drivers to 2 x "X" axis, "Y" axis and "Z" axis and a spindle motor working on the PWM port of driver board.
My Stepper motor power source is 24 volts.
The first quiry I have is the Stepper drivers, I am using Nema 17 for X and Y motion but there are 2 settings on TB6600 2/A 400 steps or 2/B 400 steps, which one do I select, also the motor Rating is 1.68amps/Phase so I selected 1.5 amp switch, for the Z axis I am using Nema 23 which has 1.8deg so 200 steps and set to 3 amp.
My other concern is reading different forums its a little worrying to find Stepper drivers are burning out so is there something I can do to protect this.
Also the BOB has the USB supply from the Computer which is running quite a few things i.e Limit switches, and the PUL+, DIR+ and ENA+ is there enough power being supplied by PC to power all these things as standard USB 2 will only supply 400ma.
Most grateful for any advise
Regards John
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Hi,
you have the opportunity to set full or half step. In full step mode 200 pulses are required to drive a motor one revolution, in
half step mode 400 pulses are required.
Those drivers blow up a lot. If you don't want that to happen DON'T power them up! When you get sick of replacing them have a look
at Gecko drivers, expensive but you only ever have to buy them once.
Craig
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Hi John,
Don’t be too disheartened – I have used quite a few of those TB6600 drivers in different projects and found them to be pretty reliable. OK, Gecko is better but use what you have until such time as you wish to move up.
For smooth CNC operation I would recommend that you use 1/8th microsteps (1600 steps per rev.) and or course, never exceed the motors specified current rating. Also don’t disconnect or connect a motor whilst the drive is powered-up.
I can’t answer the question about your BoB and driver’s current requirements with any certainty but I am not aware of anyone else finding the 400mA limit of USB2 ever causing a problem.
Which BoB are you using ?
Tweakie.
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Thanks for replies, I was having nightmares over 1st reply, but I have read quite a lot about does and does and don't, and like you say don't disconnect or unplug power source from motor while turned on and also I think I will ground all 0 volt lines to my case, I am using an old PC Desktop case to mount all drivers and Power supplies.
One thing I did find is from www.twovolt.com is a stepper driver schematic with Schottky Barrier Diodes to reduce spiking from motors I may experiment with this but not sure what the motor pulses look like so wont try with the TB6600 drivers.
The thought of buying Gecko Drivers is a no no at over £100 each would break the bank as I am retired and just using as a hobby.
Thanks for reply
Regards John
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Hi John,
I suggest you do not ground 0Volts to case unless you have established that you will not create any GND loops. Your case should be connected to protective Earth and (depending on your configuration) 0Volts is often connected to GND by the electronics of the BoB or drivers.
GND and Earth should really be treated as two different circuits.
Just a thought.
Tweakie.
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Hi John,
the extra diodes only double up whats already inside the IC. The diodes shown are schottcky and so have a lower forward voltage drop and ultra fast
reverse recovery which may help. As tweakie says they do work and are cheap, even if you do ping one is it the end of the world?
What puts these drivers under real stress and in fact ALL divers is when a motor overruns on deceleration. So you have the motor singing away
at 1000 rpm and you want to stop....and for CNC purposes with as high deceleration as you can, the motor now becomes a generator and power
flows back to the driver IC and into the DC link capacitors of your power supply. If the voltage exceeds the breakdown voltage the switching devices
in the IC go into second breakdown with consequences for reliability.
As I say ALL motor drivers have to accommodate this problem, it just plain physics. You may have seen 'braking resistors' which are sold as an add-on
for VFDs, all they are is a place to dump the extra energy under intense deceleration and try to stop the DC link voltage going thru the roof.
Craig
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Hi John,
if you wish to try to protect your driver from over voltage excursions as a result of deceleration try something like this...
It goes across your 24V supply, when the MOSFET turns on it loads the supply with the 10ohm resistor to absorb the
energy of the excursion. Obviously the 10ohm resistor has to be grunty, I would recommend 100W
The op amp is just a comparator, when the 24V zener starts to conduct a few mA then it will turn the gate of the MOSFET on.
This is broadly the same idea used in VDFs and servo drives, of course the DC link voltages are likely to be 320V for single phase
or 580V three phase rather than 24V as here.
Craig
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Thanks for input, I know what you mean about Earth loops I did go round all case and remove any earth connections, the PSU GRN I will take directly so nearest point of socket, I did have an earth connection inside a TB6600 but after I took apart it disappeared,
I take it I need to connect both 0 volt lines of the 2 power supplies larger one is connected TB6600 and the small is the spindle supply which at the moment don't have a connected 0 volts the only other issue I have is the PC power supply I am using for BOB 12 volt supply and a few misc items but this supply is earthed inside the case.
Many thanks for help
Regards John
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I take it I need to connect both 0 volt lines of the 2 power supplies
In my opinion - Yes.
Tweakie.
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Thanks for all comments this has been very useful, I got to finish the Bed yet bit more wiring and tidy up, one problem I may have to fix is, I didn't realize the weight of "Z" axis assembly so may have to change to supported bar, think I have a deflection of about 0.5mm on the "X" axis in the middle, more expense LOL
I will post when we are up and running, fingers crossed.
Regards John
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It’s all looking good John ;)
(As you progress I think you will find that there is too much flex in the unsupported rails – at some later date, changing to the fully supported type will make a considerable difference in performance).
Tweakie.
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Thanks for good wishes.
Just another point I think I have picked up looking at the BOB config, I am using BOB pwm connection but have Spindle motor supply connected directly to the power supply, and looking at Mach3 Ports & Pins config to enable spindle motor and spindle setup do I need to connect power to spindle controller via the relay on the BOB so the turns motor on when relay is energized.
The pink wire in pic is the PWM connection
Also Motor output config I think as I have 5 volts connected to the 3 TB6600 5v +Dir, +Pulse and +Enable input I am using common-Anode wiring
Thanks John
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Best to connect the spindle via the relay on the BoB then set-up Mach control (M3/M5) on Mach Spindle and Output Signals pages (PWM is irrelevant).
Within the terms used for CNC you are correct that you are using common-anode wiring for your controllers.
Tweakie.
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Connecting system bit by bit, I have just connected parallel cable and USB lead to breakout board, I have a "?" in Mach3 in Device manager also on breakout board the long terminal block has 5v going to TB6600 stepper drivers but cant find any reference to the connection next to this 5v the "GND" going anywhere, have you any idea if this should connect to the general 0volt of the power supplies or is it a PC only ground, and is opto coupled on the breakout board.
Regards John
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Think I found answer on Yu-Tube, NO it is not connected its only used in common cathode mode to the stepper drivers BUT can this be confirmed also will the "?" mark go away once I power everything and configure all.
Regards John