Servo's vs Stepper

AIman

I am working on a robot that requires a driver motor. Originally I was considering using a stepper and then started thinking about continous rotation servos.

So here's the question:

The total weight will be 5 lbs on a liberal estimate. The drive wheels will need to move the weight. The steering and drive power can be done with either a servo or stepper.

Is it better to use a servo for the drive wheels or to use a stepper?

daidalos62

Either motor can provide that kind of torque if your willing to pay for it...

There may also be a size to power ratio that would work better with one or the other, depending on your design.

Do you mean a hacked servo?
The thing with hacked servos is the loss of precision upon taking out the little stopper that keeps it to a certain degree of movement. Movement in this case would be based on more trial and error by giving the servo commands to move for a certain amount of pulses for each specific movement I suppose.

Steppers are great for precise movement, but a bit more tricky to work with. Bipolar motors are typically stronger, but a little harder to wire up, while unipolar steppers tend to be a bit weaker, but easier to wire. In the case of a stepper, you'll need a motor controller IC, such as the ULN2803, which I believe they sell at Sparkfun for about a buck.

I hope this helps somewhat, let me know if you need any help with the schematics for unipolar steppers and code to drive them.

AIman

No hacked servos.

I am refering to the stepper and continous rotation servo that Parallax sells.

allanlane5

Well, some would call the "modified" continuous rotation servo "hacked" -- but it's not common usage, true.

Though the "modified" servo DOES lose the 'absolute positioning' of an unmodified servo -- it can only go forward, backward, and stop -- picking the exact point it stops at is no longer possible.

Unless, of course, you add an "encoder" to track the exact position of the wheel. But that's another story.

AIman

I am not worried about exact locations of the servo or stepper (and I do have a couple of different encoders). The robot only goes about 5 feet per minute. Given that speed a stop can be done very easily and precise control of the drive wheels is not needed.

Lets look at this another way -·The robot weighs about 5 lbs and my biggest concern right now is getting something that will be able to move the weight without problems. A stepper will require some sort of gearing and a servo - unless I am mistaken - would require one on each side.

For that matter - has anyone tried to drag anything to see what these things can pull?

Which is better to move a 5lb load - two servo's or a stepper?

slamer

It really depends on what size servo motor(s) you plan on using and how much voltage you plan on feeding them. EVA weighs over 5 lbs normally·when carrying a can of·soda it's closer to 6 lbs. She use (4) standard 44 oz. in. servos over volted to 9.6 VDC. These servos have had all of their electronics removed and·I power the motors directly using speed controllers. With the same size servos in another robot that weighed 1 lb and only using 2·servos over volted to 12 VDC it was able to carry a 12 lb battery on top of the robot. I only did this briefly in an very exstreame test.·All of this testing was done with 3 inch diameter·tires. If you want to run stock 4.8-6 VDC voltage in a 5 lb robot with a similar diameter tire· I would look for 2 servos in at least the 100 oz. in.·torque range. ·Also metal gears are a good idea as well for reliability.

Fun with servos...

·http://www.teamkiss.com/antweight.html



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Steven Kirk Nelson (slamer)

Team K.I.S.S
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T Chap

I just designed a controller and driver for a brushless DC motor closed loop system. After looking at tons of motor designs this past week, the steppers always win out at Nema 23 size and below for torque without gearing. There are tradeoff's for each choice. I can provide you with a very cool bipolar design that uses the Allegro 3986 controller, it features whole, half, 1/2 and 1/16 stepping, current limiting, and is very easy to use if you know how to solder smt. If you wanted to go brushless I have a design for that as well under way, a similar chip called the Allegro 3938 3 phase BLDC driver. Either driver will run up to 50v, your choice of mosfets can get you over 30 amps if needed, low cost stuff to build. For power without having to add gears go with a stepper. Even a Nema17 will produce a lot of torque, allelectronics has them for 12/50 They also have some bipolars for 10.00. The steppers do not require encoders at all, you can use them to check positioning but if you overarate the stepper for the application, it will not miss steps. The DC or brushless motor does not require encoders for continuous motion either. With DC or brushless DC, you can just set the speed via current adjust or PWM and let it run. For two motors, one per side, you could feed each driver board the same PWM(Speed) and same current thresholdm and have near mathces sides, maybe some slight tuning. With steppers both will run identically unless the step resolution is varied.

Personally I would use a Nema 23 stepper, maybe 3 or 4 amp and it will be like tank pushing 5 pounds, let me know you need help, I have either board(bipolar stepper or BLDC 3 phase DC) in Eagle.

Nema 23's wont budge under 18 volts, a 17 will run somewhat maybe arund 12v-15v. I have used two 12v 7aH batteries in series to produce 24 volts, if you have the space this would be ideal.

Here is a comparison:

Nema 23 4 amp stepper from USdigital.com no gearing = running torque 110 oz inch
Nema 23 brushless DC motor from Kelinginc.net with no gearing = 60oz inch

But, the stepper takes a nose dive on torque once you start hitting some rpms, thats why you have to consider all the variables. The DC will give the same torque with gearing.

Post Edited (originator) : 3/7/2007 7:11:36 AM GMT

AIman

Because of the slow speed of the robot a stepper is becoming more and more the way I lean. I never thought about the metal gears but that makes sense on servos.

I·am very interested in learning more about what you have, there are two parts of this robot that need real working power.

Could you post something?



·Oh ya - I can adapt the body to fit.

T Chap

If you have the space, you could drop in two 12v 7Ah batteries in series. Allelectronics has various sizes, there are 12 and 17 amp hour versions as well, here is the 7 aH for reference to the size, see their site for other batteries.


http://www.allelectronics.com/cgi-bin/item/GC-1270/160/12V_7AH_LEAD_ACID_BATTERY_.html


If you could fit 3, that would five you 36 volts which would be the ultimate for a Nema23 stepper. I haven't tried these cheap surplus steppers, but they are bipolar, new, and cheap The most recent steppers I bought were around 50-69 bucks for 4 amp models. To get the serious torque, you need high voltage drivers. Nevermind what the voltage rating is on the stepper, the 3986 driver has current limiting so you can set the current to the motors specs and not have a problem.

Here is the IC mentioned. It is fine pitch Tssop(.5mm pin to pin) and requires some experience or learning curve in SMT. I have a few basic tuturials on the process if you need help with it.

www.allegromicro.com/sf/3986/index.asp

The parts for one board are around $50 or less at digikey.

Here is the schematic for the driver. I assume you don't need control signals since you have a controller alreadt, but if you do then I have a 4 motor Propeller controller board as well. I have Eagle files for these, there are some obvious wires missing on the motor driver section, those parts were hand routed on the board. The copper pour(fill) is not shown in the pic so you could see the layout better.

You drive it with Step and Dir signals. A typical stepper has a 1/4 shaft, you can couple it to your drive train with a coupler or timing belt and pulley. When looking at steppers, you don't need encoders, look at 2 - 4 amps, higher is better, 200 steps per rev.

One side note, once you get this down, you are ready to build cnc machines rather easily.

Post Edited (originator) : 3/7/2007 11:48:49 PM GMT

slamer

So much for the 5 lb weight limit

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Steven Kirk Nelson (slamer)

Team K.I.S.S
Build Safe, Build Mean, Build Strong!