Submitted by threeaxis on Sat, 01/16/2010 - 02:40
Posted in
Stepper motor / easydriver question.
Apologies in advance for the longish post, I'm really just hopping for a short answer.
So up until this point I've been running everything off of a 9V battery just for testing purposes, but a 9v battery only lasts about 5-10 minutes, depending on the motor. So I'm looking for something that I can just plug into the breadboard and practice playing with writing different scripts to dig a little deeper.
I've been reading up a little on the Easy Driver and power sources but it's still over my head.
Brian, the guy that designed it has a great explanation here:
http://forum.sparkfun.com/viewtopic.php?t=17945
Which does ease my fears about plugging something in and frying a board(like the last time I tried something like that) ;)
So I know that a 9v can run both of these just fine, but still not sure how many amps is coming out of a 9v or if it matters?
http://www1.duracell.com/oem/Primary/Alkaline/mn1604.asp
Since the easydriver is a chopper, I know it'll cut voltage down from 30v... but is it dealing with amps as well?
Here are the specs on the 2 steppers:
---------------------------------------------------------------
Stepper from Sparkfun http://www.sparkfun.com/commerce/product_info.php?products_id=9238
STEP ANGLE 1.8° 5%
PHASES 2
INSULATION RESISTANCE 100Mohm(500V DC)
CLASS OF INSULATION B
WEIGHT 0.20Kg
VOLTAGE 12V
CURRENT 0.33A
INDUCTANCE 46 20% Mh
RESISTANCE 34 10%
HOLDING TORQUE 0.23N.M
---------------------------------------------------------------
2 phase stepper (Chinese Brand)
Voltage V 2.5
Current A 1.7
Resistance Om 1.5
Inductance MH 3.2
Holding Torque N.cm 42
Detent Torge g.cm <220
Rotor Inertia g.cm2 32
Step Angle Accuracy ±5%（full step, no load）
Resistance Accuracy ±10%
Inductance Accuracy ±20%
Temperature Rise 80℃Max.(reted current,2 phase on)
Ambient Temperature -10℃- +50℃
Insulation Resistance 100M ΩMin.500 VDC
Dielectric Strength 500VAC for one minute
Radial Play 0.06Max.(450 g-load)
End Play 0.08Max.(450 g-load)
---------------------------------------------------------------
I have 1.5-12v 300ma wall wart from radio shack and based on Brian's description I should just set the pot to it's lowest point and move it on up until i can feel the motor heat up? But I'm still curious about the "ma"? The lower amps the better microstepping?
I've also read that stepper hooked up to worms don't need as much power since they have so much torque with the worm? Which probably makes that chinese stepper overkill but again no idea what half those specs mean.
So on that note will the wall wart work a.o.k? and thinking ahead I'm assuming a 12V not higher would be fine as well?
thanks in advance!
threeaxis
With the easy driver, you can
With the easy driver, you can think more if it as limiting power, not just reducing voltage. It keeps an eye on the current drawn, and reduces the voltage when it needs to adjust the amount of power being consumed. What that means is that you will want to keep the power input within the reasonable range of the motor, depending on the voltage you're trying to drive the motor at.
But first, the wall wart from Radioshack won't work. To tune the 12V motor from sparkfun to its rated torque at 12V, you'd use 330 mA (milli-Amps) for that motor alone, which is higher than the 300 mA power supply you have.
It's possible to run the 12V motor at a lower current (though not a lot lower), and give up some holding torque on the motor. You reduce the holding torque of the motor approximately in half by setting the easydriver to it's lowest current setting (150mA). If the motor is rated at or around the minimum torque ratio for your setup, you'll want to keep the motors closest to their measured power rating to get the listed torque -- thus you would need a power supply capable of supplying 330 mA or greater.
What he's saying about microstepping, is that when you exceed the power input rating for your motor at the selected voltage you then run the risk of losing the ability to microstep and over-heating the motor. This is caused by the driver not being able to limit the current at or below the maximum the motor can handle given the current input voltage. At the same time, if you're forced to run the driver configured to its minimum current setting, it will often have to reduce the input voltage going to the motor. This will result in quite a bit of heat in the easydriver.
Since the motor will be able to handle a certain amount of power (Watts) at a given voltage, you'll need to reduce the current when increasing the voltage to keep the amount of power input the same. The basic formula for determining power is P = IV. So, if we look at your second motor, that's rated at 1.7A at 2.5V, or 4.25 watts. So to make the power stay the same at 12V, just divide the current until you end up at 4.25 watts: 4.25 = 0.3542A * 12V (I = W/V or V = W/I). In this case, this is well above the minimum power rating for the easydriver (150 mA < 354 mA), meaning you will not run the risk of losing microstepping by running too much current through the motor.
At 9V, you will again need to calculate the correct settings to run the motors properly. Following the same formula we used above, we can calculate the proper current limit to set on the easydrivers for each motor. The sparkfun motor runs at 3.96W of power, so at 9V, that would be 3.96 = 0.44A * 9V (440mA), and the other motor rated at 4.25W would be 4.25 = 0.472A * 9V (472mA). Fortunately, both numbers are well below the maximum current rating (750mA) for the easydriver, meaning you will be able to operate them at spec at 9V. However, If we add the total current draw on each motor, we get 912mA. To speak to the point at to why the 9V battery only lasts a few minutes, you'll see from the last two charts shown on the page for the battery you listed, the battery only has a capacity of a few minutes at this current draw. So, at 9V, you need an actual power supply, or larger capacity battery that is able to provide more than 912mA, preferably a couple times this.
Obviously, just like we find 9V can then calculate the maximum voltage we can support as well, by just plugging in the voltage and watts for each motor in the formula we used above. As long as the current is within the easydriver's spec, you can use that voltage.
Does that help? I know it can seem confusing at first, but that simple formula will let you understand the general requirements for your motors given a certain voltage, and what it do-able. You can then always manually tweak the current flow to see how much more you can get out of a motor, or how much less you can consume, based on your needs. In some cases, you can push the motor quite a bit further than the specs say.
!c