Picstep and heat
I have recently built three picsteps which I am now testing. The picsteps are being used with the pmino 4 port BOB.
My steppers are 1.8 volt, 3 amp. As such I have used a 6k8 resistor for the current set which should drive the motors at 2.94 amps (6k8 being the closest I could find to 6k6). This should be within the limit of the lmd18245's.
The steppers are wired serial bipolar (6 wire steppers).
The power supply provides is 27 volts 16 amps.
Within a few minutes of running the LMD's are very hot - too hot to touch. I have fitted some heat sinks to help with cooling but find they are still fairly hot. I'm not sure a better heatsink/fan combo would keep these things cool.
Does anybody have any experience in dealing with the heat coming off the LMD's at 3 amps, or should I be looking for a problem elsewhere ?
They indeed run very hot at 3 amps.
You MUST have a heatsink on the LMD's even if only used at 1 amp.
At 3 amp you will need a fan or the overtemp protection will kick in and give some funny symptoms.
I have 4 Picsteps and 2 80mm fans blowing directly on the heatsinks and now it works without problems.
That may be the problems I am having. The steppers turn but stall and miss steps continually with no load. I have attached heat sinks but they probably aren't big enough for the task, and fans will be necessary with the heat being generated.
I'll add a large set of heat sinks and fans and see what happens. I'm guessing the metal tabs on the lmd's need to be kept electrically isolated from each other.
The metal tabs are connected to PGND, there's no need to insulate them.
Did you use heat conductive paste between the chips and the heatsink?
The heatsinks I use are only 5 x 5 cm and this seems sufficient with the active cooling.
Without fan's I have the same problem as you describe.
unipolar or bipolar with Picstep
If you have 6 wire motors, the motors are probably wired internally for unipolar drivers.
Each set of 3 wires that connect to each phase has a center tap that attaches to the High voltage.
The other two wires drain two oppositely wound coils that drive one phase of the stepper motor.
If you want to drive it with a bipolar driver like Picstep, you must only drive one of the coils on each phase. The second oppositely wound coil cannot be connected as it will buck the force of the first coil.
You can find the center tap by measuring the resistance of the coils.
The center tap will have half the resistance as the measurement across both coils.
Connect the Picstep driver across only one coil for each phase.
You will find it runs smoother and has much more power.
Otherwise, you need a Unipolar driver such as the SLA7062m chips.
You cannot drive a 6 wire stepper motor "serial bipolar"
If you want to drive "serial bipolar", you must open the motor and separate the center tap feeds into two lines and bring them out of the motor.
This will give you an 8 wire motor which you can drive serial bipolar.
That will give you the maximum power out of the motor.
A discussion of this is at the following thread
http://www.cnczone.com/forums/showthread.php?t=14287 (Bipolar from unipolar motors)
That makes sense... I assume if I am driving one on one side of the coils I will need to drop the amperage to 1.5 amps as well. I will try it and see how it goes.
You should be able to drive at 3 amps if your motor is rated 3 amps.
If the motor is driven unipolar, you will drive each phase to 3 amps.
If you rewire the motor to 8 wire and run the phases in series, you would then have to reduce the current.
The stepping action of the motor and the fact that the phases are at 90 degrees to each other, also reduces the effective current through the motor.
If the motor runs cool, you know it is not being overdriven.
>The second oppositely wound coil cannot be connected as it >will buck the force of the first coil.
Is there any way to identify the matched coils (i.e. same winding). I am guessing that when running 1/2 phase bipolar it is important to match the correct coils (same winding).
I had assumed that an incorrect matching would result in a 'vibrating' stepper. No go - all combinations I have tried seem to yield the same results.
Finding matched coils
On a 6 wire motor, use your ohmmeter to find the two sets of three wires that are connected.
Unipolar 6 wire steppers have two sets of coils.
The common wire to each set of poles connects to the High voltage.
The other two wires connect to the unipolar driver.
The unipolar driver grounds the coil when the drive signal for that coil is energised and the current flows through the coil, then through the drive transistor to ground.
If you use the ohmeter, the center connection will have half the resistance when measured to the outside wires.
The two outside wires will have 2 times the resistance across them as the resistance to the center tap.
If you have no connection between two wires, you do not have the right ends to the coil you are looking for.
If the coils are connected to the wrong outputs on the stepper driver, the motor will not turn evenly.
Reversing connections to coils may cause the motor to reverse direction.
There is a good diagram at http://www.damencnc.nl/
Click on the Tutorial link