My HUGE oversight in a new OEM development and lessons to be learned

idbruce

Hello Everyone

Okay here's the story!

In my endeavors to get another product ready for market, which includes putting tamper resistant labels on my packaging, I became discouraged with the label dispenser that the labels arrived in, so I decided to create my own label dispenser from surplus materials. Being a naturally lazy guy and having a love for stepper motors, I decided to motorize the label dispenser with a surplus stepper motor. All in all, the dispenser turned out quite nice, being constructed entirely out of metal, with the exception of a steel studded black plastic knob. From this point, I had two options, either power it with my major power supply for my CNC equipment or provide it with power from an AC adapter. Well the dispenser turned out so very nice that I decided to keep going forward and make it one of my future products. As a result of this decision, I decided to make a small stepper driver and power it with a decent size AC adapter.



After going back and forth with the stepper driver design, I finally decided to build an inexpensive four transistor (TIP120) unipolar stepper driver, driven by a Basic STAMP OEM, and utilizing Chris Savage's BS2 OEM "Lite" schematic (USB2SER modified for PropPlug). Having very little space to work with, I had to design the driver to fit in a 3"X3" space and still allow enough room to fasten a cover for the electronics. In addition to space limitations, I also had to consider current control for the motor. Instead of using power resistors to limit the current, I decided to run the motor at it's rated voltage, thus eliminating current control all together. Here are the specs for the unipolar wiring:

• 0.95 A/phase
• 4.2 Ohms/phase

Using Ohm's law it was determined that the rated voltage of each phase was as follows:

0.95 amps * 4.2 ohms = 3.99 volts per phase

So now I had a board that needed two different votages, 5.0V and 3.9. My solution was to use the LM2940CT-5.0 that came with the BASIC Stamp 2 OEM Module for the 5.0V, and use a seperate LM317T adjustable regulator to obtain the 3.9V.

Anyhow to make a long story short, after 2 1/2 days of doing a nice neat job on a 2 7/8 X 1 7/8 tiny prototype board, the LM317T goes into thermal shut down. During my design and thought process, I completely forgot that in half step mode there will still be two coils being energized, which results in 1.9 amps, and there is no room for a heatsink. DUH It is definitely getting very hot and then the voltage drops out.

For those of you that might have experience with voltage regulators, would a change to a LM138 rectify this problem without requiring a heatsink? I have looked at the datasheets, but I am having trouble deciphering them.

Additionally, both regulators are being supplied with a well regulated 12V/1.2A AC adapter, which will also need to be changed due to current requirements.

Bruce

RobotWorkshop

Hello Bruce,

I think that sort of thing can happen to anyone of us at some point. I'd really consider swapping it out for one of the small 3-pin switching regulator modules that are meant as a drop in replacement for the standard TO-220 package regulators. They are usually more efficient and may fix your issue. Here is a list of some available from Mouser.

http://www.mouser.com/Power/DC-DC-Converters-Regulators/_/N-5gc7?P=1z0wdj3&Keyword=switching+regulator&FS=True

Robert

davejames

Hi Bruce - I don't think changing to an LM138 w/no heatsink is going to remedy the situation.

If the requirements of the LM138 is to drop 12V to 3.9 (8.1V across the regulator) and pass 1.9A, that's >15W! YEOW!

I wish the datasheets for linear voltage regulators would somehow tie in the expression of "free air power dissapation", but the only thing I've ever seen listed was junction temperature. Although at 15W I can almost guarantee (is that an oxymoron?) that the LM138 maximum junction temperature is exceeded.

Is there anyway the regulators can be mounted to a casing? If so, watch out for differences between the two regulator's tabs; one is ground, the other is not. So insulating material is required for mounting the LM138.

Regards and good success,

DJ

idbruce

@davidjames - I also thought about mounting the regulator to the main frame of the dispenser, which is a 2X3X6 piece of aluminum angle, additionally the circuit board has a provision for a fan to plug in.

@RobotWorkshop - Thanks for the link.

Kevin McCullough

Just as DJ described, if you are using a linear regulator the device will have to dissipate lots of heat at that voltage and current. You may really want to consider changing to a switching regulator (much more efficient for larger currents), or use an AC adapter with a lower output voltage closer to what is required by your circuit (so the voltage drop across the regulator won't need to be as large). Power (heat) is also dissipated in the motor too. So really, the total power dissipated inside your project enclosure could be up to 22 watts. 12V x 1.9A (possible when half-stepping) = 22.8W.

Because you are planning to make this as a product, the power dissipation is also an aesthetic concern as well. In other words, customers might get nervous if their label dispenser becomes hot to the touch during normal operation. And at 22W, the product will definitely get pretty warm.

kwinn

Since you are dispensing labels I am assuming that the motor is not running continuously. If that is the case you can reduce power dissipation by always stopping on a full step and turn off power to the coils. Even better would be to run from the unregulated input voltage and PWM the drivers.

idbruce

@Kevin McCullough

In other words, customers might get nervous if their label dispenser becomes hot to the touch during normal operation. And at 22W, the product will definitely get pretty warm.

LOL You forgot to mention the melting adhesive from the labels, the labels slipping off the backing, and the gooey mess at the bottom of the label dispenser

As kwinn mentions, the labels will not be coming off continuously, only one approximately every minute and ten seconds, and it was my intention to shut off all current going to the motor during down time.

or use an AC adapter with a lower output voltage closer to what is required by your circuit (so the voltage drop across the regulator won't need to be as large).

I also thought about this, and my thought was something like a 7.5V/2A power supply, but that would be hard to find and expensive, and I am sure it would still produce a lot of heat.

@kwinn

Even better would be to run from the unregulated input voltage and PWM the drivers.

I realize this, but it is also much more complicated and expensive. My first intention was to take the BS2 OEM Module and make my own little module the would have 6 pins that would plug into a G251 stepper drive from Gecko Drive, with 2 of the 6 pins passing VIN, but that is like a $100 solution (I don't think so).

@everyone - There is another possible solution that nobody mentioned. I could always swap out the motor for one that excepts a much higher voltage.

Bruce

idbruce

Hello Everyone

Just for giggles, I decided to adjust the voltage up to 7.5V, and to my amazing surprise, everything works PERFECTLY! The regulator and motor both remain very cool

Before you say it, I know this is not good for my beautiful little stepper motor, but until I get a chance to swap out the motor, I believe that will do the trick. Considering the duty usage, the motor might stand up for several years as is.

Additionally, like kwinn said (and which I already intended to do, but had not implemented it yet), I also shut off the coils to the motor after advancing the label. Between raising the voltage and shutting off the current when not in use, I now have a very nice addition to my product line. Very, very, sweet! All I have to do now is make a cover for the electronics.

After I make the cover, I will post a photo of it, so you can add it to your wish list. Retail value will be in the ballpark of $150 - $200.

Bruce

jmg

idbruce wrote: »

@everyone - There is another possible solution that nobody mentioned. I could always swap out the motor for one that excepts a much higher voltage.

Perhaps, but it's best to choose the cheapest motor and use the Electronics you already have to line everything else up.

So, you start from the Motor spec, and work in the power domain :
0.95A*0.95A*4.2Ohms = 3.7905Watts, if two of those are on, that's 7.581W, (peak) whiich at 12V, is appx 0.75A at 85% efficient.

To get the desired 3.99V, from 12V, simply use that as a preset PWM (33-38%, depending on the PWM topology) , and you can likely do some Step/ hold shaping too, so that 0.95A is time-shaped, and that lowers the power further.


Use linear regulation only where the load demands it - and a stepper motor is not such a load.

idbruce

@jmg

Use linear regulation only where the load demands it - and a stepper motor is not such a load.

I agree with that because steppers should be run from an unregulated power supply. However you always pay for something one way or another. As previously mentioned, current control can get quite expensive if you are not an electronic genius. To effectively use PWM to control the current within stepper motor coils, you must have have some type of current sensor, as is included with all major stepper drive ICs. I could have used a stepper driver IC that permits PWM, but those chips are not cheap either, and the boards are much more difficult to design, not to mention all the necessary additional components.

Bruce

jmg

idbruce wrote: »

@jmg

I agree with that because steppers should be run from an unregulated power supply. However you always pay for something one way or another. As previously mentioned, current control can get quite expensive if you are not an electronic genius. To effectively use PWM to control the current within stepper motor coils, you must have have some type of current sensor, as is included with all major stepper drive ICs.
Bruce

I'd say that was true for Microstepping, but for simpler drives, you can measure the Raw Vcc, and use ohms law, and a simple time-constant to get a good enough handle on current to do non micro-stepping easily.
The AllegroMicro drive chips, with power devices and microstepping, are quite good value, compared with a discrete alternative.