raspirobot - magician chassis - motors do not turn

[windy54]

Hi,

I have just built a magician chassis robot with the raspirobot board designed by Simon Monk.

I can control the LEDs using Simon python example code, but the motors do not turn.

When I issue the command rr.forward() there is a high pitch sound coming from the board, when I issue the command rr.stop() the sound stops.

The motors do not turn. Motors are okay when I connect them direct to a battery.

I am using a power traveller USB battery to power the raspi and robot board so I do not believe it is a battery issue.

Could I have damaged one of the chips when soldering it?

What to check next, bearing in mind all I have is a cheap multi meter!

[windy54]

Of course it was the battery, 5v is not enough for the motors.

Got confused by the instructions where it said do not connect The PI power connection and the robot board at the same time.

I took this to mean either or !

Now to find a better power source, using 8 AA cells at the moment

[lucyg]

Windy54, can you explain what you meant about getting confused by the instructions?

I also assumed they meant to do either or, what did they actually mean? More importantly, what should I not do, so that my board isn't destroyed. Also, why would they include a 6 AA pack if it doesn't supply enough energy? I thought the motors were broken at first because a single AA had no effect (I had to lift the wheel off the ground... hardly useful for moving anything).

[Ravenous]

I haven't built one of these, but here's what I think they mean.

They may have meant connect the raspi to the power, then connect the motors. Or the other way around. In other words don't make both connections at the same time. (The sudden load on the batteries might be the problem.)

As for how much power is needed, 6AA cells might give up to 9V (for normal non-rechargeable cells) or 7.2V (for rechargeables like Ni-MH or similar). When the motors are running the voltage will probably be a bit less, expecially for the non-rechargeables as these have a higher "internal resistance" so don't work as well at high power. Also, the motor driver used might waste a volt or two depending on what design it is. All this means the motors will never see the full battery voltage, and might struggle if the batteries are a bit low. This is normal...

If the motors are having trouble turning, running it on a flat table top instead of carpet might work better too. Just something to try to get it moving.

There are plenty of other robot buillders here so if you read a few more of the existing posts you might find a bit more. There's lots of info around but it's pretty hard to find enough of it to get started...

[Ravenous]

OR - if the robot board has a 5V regulator already built in, perhaps it means connect the batteries to the board only, and it will send 5V to the raspi... ? (I've never been able to find a schematic for this so don't know what it does. Other people have got them working though, perhaps soemone else will chip in...)

[Ravenous]

I finally found some instructions here:
http://www.doctormonk.com/2014/06/build ... robot.html
Turns out he's saying what we guessed above; if the raspberry pi is on the robot and connected up, it takes its power from the robot's batteries. So don't connect a power supply of any sort to the raspberry pi's normal micro-USB power connector. (I guess it may harm the regulator on the robot's own board.)

[lucyg]

Should I just get a bigger battery box then? It seems unlikely that they would supply the kit with parts that don't work together. Maybe something is screwed too tightly, and if I loosen it, the motors will turn better?

[BMS Doug]

Should I just get a bigger battery box then? It seems unlikely that they would supply the kit with parts that don't work together. Maybe something is screwed too tightly, and if I loosen it, the motors will turn better?

did you try it with the 6 cells Lucyg?
Windy54's issue was that he had tried to replace the 6 cell battery box with a 5v power supply and found that it wasn't enough.

The motors of the magician kit shouldn't have any issues with friction regardless of where or how they are fastened, unless you can see somewhere that they are rubbing?*

*unless you have tightened the screws hard enough to deform the plastic.

[lucyg]

Yes, my kit came with both the 4 and 6 cell battery packs (weirdly), and I've been using only the 6. I can start the raspi and even the led_test script works. No obvious sources of friction, so I'll assume that's not the problem.

[Ravenous]

If it runs OK with the wheels off the ground, then maybe the batteries and motors don't have enough "kick" (technical term) to run properly. (Maybe build yourself a little cardboard stand so the chassis can be held off the table for testing.)

First try running it on a smooth desk, not on a carpet (and definitely not uphill)

Try running it while lifting the battery box and holding it in your hand, to take the weight off the wheels

Expensive option: try fresh rechargeable batteries (Rechargeables apparently can provide a bit more current than alkalines, though of course they're expensive)

An 8-cell battery box might help provide more power too, if there's room to fit on that chassis... (Another 4-cell put in series with your existing 4-cell might work)

Hopefully one of the above will help. I haven't tried one of these kits so I don't know if the motors and drivers are any good...

[mikerr]

I finally found some instructions here:
http://www.doctormonk.com/2014/06/build ... robot.html

Note the last line from that link:

Although the program is running, the robot will not start to move until SW2 on the RRB is closed. So using a screwdriver, momentarily short together the connections for SW2 as shown below.

[Ravenous]

How embarrassing that would be - if the example program is just waiting for a button press to start it

[lucyg]

Ok, I've tried running the script while holding it off the ground. The wheels still don't move unless I sort of push them. Then they do what the script says (forward, off, backward, off, etc.) for a few seconds, then they quit. Also, if I move one, the other stops; there's not enough power for both of them. I also tried shorting SW2 and it had no discernible effect. I've been keeping it plugged into the wall through the microUSB adapter, which says it supplies 5V. Maybe that's the problem. I tried to boot from the batteries before, but couldn't even get past the loading screen (online searches told me power was the problem - even though the pack of batteries comes up as 7.05V on the multimeter). Any help is very appreciated.

[Ravenous]

That intermittent stop/start behaviour sounds like the batteries are having trouble - the raspi might even be switching off at that point! (The motors will draw a lot of current and possibly shut down the raspi hortly afterwards.)

Can you do this with a monitor plugged into the raspi, just to test it isn't suddenly going off? (I appreciate this might be impossible.)

Another thing I would do is to run the robot off a long wire to a good 9V power supply (rated at a few amps). I would always try a mobile robot "tethered" like that to start with as it saves fiddling with batteries. But that suggestion's not much help if you don't have wire, connectors, power supply etc. handy. If you're doing this at a school you might be able to borrow something...

Going to 8 AA cells might work too... yes this hobby is getting expensive isn't it

[lucyg]

I finally resolved the problem. I don't quite believe it, but it was actually just that the batteries needed replacing...

To clarify (and for anyone else who might have this problem), the raspi will boot up and run just fine off of the 5V microUSB, but that's not nearly enough power to run the motors. I was having some connectivity issues when I used the batteries, even though they had 7.05V running through them (not sure why on that one...). I replaced them all and got a reading of 9.60V, which was plenty to get the motors running full power, even on carpet.

In short, only use the batteries when you are totally ready to test the motors. Not when you just want to write code, no need to waste the batteries on something that stands still.

[Ravenous]

I don't quite believe it, but...

I do. Guess how I learned that one...

Batteries might measure a decent voltage when they're just being measured, but if you connect an old-style moving-needle style of voltmeter and run the robot, you will see that needle start to dip every time the motors do something. Especially turning corners or carrying extra weight...

[BMS Doug]

I finally resolved the problem. I don't quite believe it, but it was actually just that the batteries needed replacing...

To clarify (and for anyone else who might have this problem), the raspi will boot up and run just fine off of the 5V microUSB, but that's not nearly enough power to run the motors. I was having some connectivity issues when I used the batteries, even though they had 7.05V running through them (not sure why on that one...). I replaced them all and got a reading of 9.60V, which was plenty to get the motors running full power, even on carpet.

In short, only use the batteries when you are totally ready to test the motors. Not when you just want to write code, no need to waste the batteries on something that stands still.

I have a multi voltage dc power adapter, which is how I tested my pi bot, in fact I still haven't sorted out the battery completely (the battery is ready but the charger isn't). I think that a long lead power supply is the best way of testing a pi bot, only add batteries once everything else* is working.

*for a fairly flexible value if everything else, it's entirely reasonable to get the pi bot rolling under its own steam and then add a variety of sensors.