Roomba Discovery Wheel Motor Specs?

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Roomba Discovery Wheel Motor Specs?

Postby brodam » February 20th, 2010, 11:52 pm

Sorry for the long post, and thanks for taking time to read... but if you want Cliff notes: what are the motor wheel specs for the Roomba Discovery?

For a class project, my group is refitting a Roomba Discovery (I think it's the 4210 series) with an Arduino and Adafruit motor shield. We stripped everything out of a used Roomba, except the wheel DC motors. I was warned by my professor that the motors might draw more current than the motor shield would allow, but of course I wanted to try, since we already put in time and money.

So after uploading a DC motor test program, we were able to see the motors moving... but one of the L293D chips caught on fire on the motor shield! Obviously the Roomba motors were demanding more current than the 1.2 A max allowed with the motor shield chips.

Before we completely give up on using the Roomba for our project, can anyone tell me the exact specs of the DC motors used for this Roomba model? DC motor part name, current and voltage requirement, etc specs? I have searched online and could not find any exact specs for the wheel motors, only some posts that it might be a RS-360SH-10500 ... D=rs_360sh
It obviously wasn't the 10500, since the max current for that one is below the max the motor shield will allow. I hope it's not the RS-360SH-2885, because the max for that is very high!

Thanks for any help,
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Re: Roomba Discovery Wheel Motor Specs?

Postby quadfighter » February 21st, 2010, 12:28 am

I'm not very knowledgeable with with this, but here goes:
If the wheel motor actually drew more than 1.2 amps, then Roomba's battery, which has a capacity of about 3000mAh, would not be able to power the robot (with four motors all running simultaneously) for more than several minutes! So I don't think the motors even exceed 1 amp.

Also, see this table, it may help:
Current Limits.jpg

These are the values iRobot uses.
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Re: Roomba Discovery Wheel Motor Specs?

Postby mfortuna » February 21st, 2010, 9:30 am

When I drive my create around the extra load amounts to about 150 to 200ma per drive wheel. The micro controller uses PWM to run the motor and it is not on continuously. Maybe in your case full on running drew a lot more current.
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The wheel-motor is not at fault!

Postby Gordon » February 22nd, 2010, 9:39 pm

I doubt that Karl will be back. He answered his question by having found the best we have to offer (i.e., the suspected Mabuchi P/N). It was only last night that I spotted his thread. My interest got piqued upon reading this (abbreviated) introduction to his problem:
brodam wrote:...For a class project, my group is refitting a Roomba Discovery ...4210 ...with an Arduino and Adafruit motor shield. We stripped everything..., except the wheel DC motors. I was warned by my professor that the motors might draw more current than the motor shield would allow, but of course I wanted to try, since we already put in time and money.
Karl might want to pay more attention to what the Prof. says, in the future!
... but one of the L293D chips caught on fire on the motor shield! Obviously the Roomba motors were demanding more current than the 1.2 A max allowed with the motor shield chips.
Then, immediately, my thoughts were:

A) The motor is not at fault in this situation!
B) What the Hell is a "motor shield" (coupled with "Adafruit", or NOT!)?

I was forced (by me) to google for item (B). Here is what I found:

There is this sort of "shyster", "vulture-like" outfit on-line that must spend a lot of time cooking up what they believe are cute names for products it hopes to trap (as customers) characters who think those product names are cute too. The bandits do that rather than spend the same amount of time preparing decent, useful informational dox about those products, and which may help customers get some use from them without burning up anything! Natch! When things burn up, ya get to sell more -- sweet!

The product that gave Karl a bad experience is not a "shield" of any kind, it is a Motor-Driver / Controller PWB Assembly, which has this partial, ugh, "specification" posted at its home:
specs wrote:4 H-Bridges: L293D chipset provides 0.6A per bridge (1.2A peak) with thermal shutdown protection, internal kickback protection diodes. Can run motors on 4.5VDC to 36VDC.
That is all the tech-info that Karl was given prior to purchasing the so-called "motor shield".

I claim the above statements to be very incomplete hardware specifications, to say the least. Without further delay, I will jump on the most critical one, the one that relates to a flaming IC!

To get a leg-up on what may of happened in Karl's trial, I was then forced to find & d/l a data-sheet for the touted "L293D" IC (which, BTW, I can't see that IC as part of a "chipset" in its "shield" application). At page top, the L293D data sheet claims these features:
Well !! Does any of that look familiar? Hmmm? The vultures have spiked those lines, and added some from further down in the data-sheet to apply to their own product! AND, that could be all right, IF, they had also provided additional details, critical details, which CAN be ferreted from the data-sheet.

Here is THE germane detail: See that "* 1.2A PEAK OUTPUT CURRENT (non repetitive)", just above? Now compare it to the "specification" line item: "L293D chipset provides 0.6A per bridge (1.2A peak) with...". Wouldn't you say that something got truncated in the copy & paste? Like the "non-repetitive" detail! Stand by, it gets worse...

Looking further into the data sheet, in an effort to nail down that high-current operating limit, I found this line in the MAX-table: "Io {cap-i, sub-oh} | Peak Output Current (100 ms non repetitive) | 1.2 | A". Now, we have a time period throughout which the (so-called) peak current may exist before ruination occurs. Any longer than 100ms, and poof!

If Karl had been given that detailed information, he might have (should have) asked himself: When I apply my system's supply voltage in a step-fashion, a zero_V to V_sys step, to that stopped wheel-assembly, what peak current will be drawn? And, will current fall below 1.2A within 100ms? Answers should have been empirically obtained before spending money on hardware.

Even Mabuchi's data-sheet provided no help by showing a locked-rotor current of 0.8A for the (alluded) wheel motor. The manner in which that parameter is stated makes it difficult for me to make sense out of it. The uncertain part is applied voltage. But, fortunately I happen to have the stuff on hand to make an empirical measurement, not of locked-rotor data, but for the more applicable start from dead-still condition -- at least doing so on a wheel-assembly that is free of bearing weight and free of the forces of 'road' travel.

I test-clipped a 16Vdc PSU to one motor wire, wired the other motor-lead to a 0.5(+/-1%) ohm resistor (to serve as current shunt for developing a voltage to the O-scope's input), then to an ammeter (for magnitude verification), and back to the low side of the PSU. The scope's probe was connected across the 0.5 ohm shunt. I closed the circuit and ran with 16Vdc applied to the motor for several minutes, to warm it up and verify steady-state current -- it was on the order of 0.25A.

I then opened the circuit, set up the O-scope, and did a few single-shot captures showing current-surge required to get the stopped motor running up to steady state rate. Here is one of the oscillograms:

The initial surge current ran up to a peak in excess of two amperes, then settled to steady-state current of about 0.22A.

Keep in mind, this wheel-assembly was lying on its side -- tread touching nothing, therefore there was no mechanical load on the wheel-assembly for this check. IOW, don't be surprised to see these data saying there would be NO L293D flames. The V_shunt value required to ruin an L293D driving this motor-test is at the 0.6 voltage level on the graph. In this example, shunt-volts in excess of 0.6-volt represents the L293D's exposure to 1.2A and larger. For this test, we see that time interval, where 0.6V < V_shunt, is only dt ~ 20ms wide. Five times that amount of time is needed to kill the IC.

Observe that actual peak current went off-scale, exceeding I = 1V / 0.5 ohm = 2 amps.

I claim it is reasonable that a much higher peak, and therefore a great increase in dt at the 1.2A level, can occur when normal starting loads are experienced by a robot-mounted Roomba-4210 wheel assembly. I'm not surprised the IC burned out -- its bridge-components are quite wimpy as compared to the quad-bridge of TO-126-cased BJTs that were built into Roomba's wheel-driver circuit!

L293D's failure is not the motor's fault, it is the result of inattention to technical detail, and to improper check out of driver-hardware prior to powering driven-H/W. Also, there is the aspect of incomplete project planning, viz., this driver / motor sub-system should include motor-current sensors that are capable of protecting driver & driven.
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Re: Roomba Discovery Wheel Motor Specs?

Postby brodam » February 23rd, 2010, 11:19 pm

Thank you all for the responses, and thank you Gordon for the detailed analysis.

I did end up measuring the current during "steady" operation, and it was 0.23A, and 0.9A for "stalled" operation (manually holding the wheel). I also got a response from robotshop that the wheel motors for the Roomba Discovery are RP360-ST, with 0.21A to 0.81A (stall). ... =60&page=1

Yes you're right Gordon, I was suspecting that the motors demand more current to initially start up, since the L293D's *should* be able to handle the current used by the motor.

Adafruit does explain that SN754410 motor driver chips can be used in place of the L293D's, for current up to 2A. I would need to add my own protection diodes.

While this could provide for our current needs, this is going to delay our time-sensitive project.

It does come down to me and my group members' lack of planning too. Our professor encouraged us to try something different for our project (refitting an existing robot with an Arduino microcontroller/Adafruit motor shield combo, adding IR sensors, programming it to track stationary targets). I thought the Roomba would be good for it. But I had a hard time finding the motor specs before purchasing, and our first lab (controlling the motors) was due 1 week after we had a chance to hack the hardware apart. So I foolishly pushed my luck with the L293D's. Again this could have been avoided, but as an EE undergrad, I don't know any better! :?

Sadly our group is pressed for time, and we are leaning towards using a different bot, like the Wall-E toy robot (which was used successfully by previous students). Hopefully in a later project I will make use my Roomba skeleton.

Thanks again for the information, and I'll probably end up posting my other project problems soon!

- Karl
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Re: Roomba Discovery Wheel Motor Specs?

Postby DancesWithRobots » March 12th, 2011, 2:56 pm

I hate to necropost, but I'd feel bad if I didn't defend the virtue of Lady Ada and Adafruit Industries.

While I have absolutely NO issue with Gordon's excellent analysis of and tutorial of LM293D motor control specifications, I feel he may be somewhat undereducated on the subject of Arduino interfacing "culture" (So to speak.)

A "shield" in Arduino terms, is a circuit board meant to physically integrate with a typical Arduino board. To put it simply--they plug right in. This particular shield, along with its library makes it a little easier to connect and control an LM293D motor control IC with an Arduino.

If the terminology seems cute or simplistic, well, that's Arduino for you. I know there's something of a bias towards Arduino's by more advanced hobbyists and engineers, but I'm not going to open up that can of napalm.

Arduino's are THE product that got my daughter into electronics, and Lady Ada is something of a hero(ine) to her.

As a resource for electronic hobbyists, Adafruit Industries has an active forum, tutorials, and support.
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Re: Roomba Discovery Wheel Motor Specs?

Postby brodam » March 14th, 2011, 1:42 am

Wow, I didn't have time to post any updates... but my team did end up sticking with the Roomba and Motor Shield combo. We had to cut and desolder the LM293D chips, then install 16 pin sockets, and two 754410 chips piggybacked. We attached a heatsink with thermal grease to the top chip.

Anyway, we did end up completing the objective, which was to find 6 target plaques and stop in front of them. Our sensors were attached to a stepper motor.

Thanks again to everyone for the advice and information, and of course Lady Ada for designing the shield!

Here is the Roomba at work:
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