Arduino Forum

Using Arduino => Motors, Mechanics, Power and CNC => Topic started by: brokenAvocado on Nov 09, 2015, 01:42 am

Title: Need a small motor that can be used for moving my robots
Post by: brokenAvocado on Nov 09, 2015, 01:42 am
Hi everyone,

I bought two motors from RobotShop: http://www.robotshop.com/en/solarbotics-regular-pager-motor-rpm2.html and they are both pager motors for my very small robots (5 x 7 centimeters) and I was soldering some solid core wire for easy breadboard use, and the wire snapped off from the motor, and now I can't power it in any way. Also, I found that no wheel can fit on the shaft of the motor, and I've tried making my own adapter and trying to connect them. Now I'm reluctant to use these small motors. I want to know if a small robot like mine can use servos, or if there any other small motors out there that can be used for small robots like mine.
Title: Re: Need a small motor that can be used for moving my robots
Post by: OldSteve on Nov 09, 2015, 04:08 pm
Hi everyone,

I bought two motors from RobotShop: http://www.robotshop.com/en/solarbotics-regular-pager-motor-rpm2.html and they are both pager motors for my very small robots (5 x 7 centimeters) and I was soldering some solid core wire for easy breadboard use, and the wire snapped off from the motor, and now I can't power it in any way. Also, I found that no wheel can fit on the shaft of the motor, and I've tried making my own adapter and trying to connect them. Now I'm reluctant to use these small motors. I want to know if a small robot like mine can use servos, or if there any other small motors out there that can be used for small robots like mine.
I don't know about DC motors, there are so many to choose from, just do a search.
And yes, you could use continuous-rotation servos. That's how I power my robot car. I found a seller here in Australia who sells continuous-rotation servos with wheels to fit the output shaft. (Pic attached.)
If you search on the internet, I'm sure you'll find similar in your part of the world.
(Servos might be a bit big for your application though. My car is about 250mm long.)
Title: Re: Need a small motor that can be used for moving my robots
Post by: DuaneDegn on Nov 09, 2015, 04:49 pm
Adafruit sells wheels for micro servos (https://www.adafruit.com/products/2744).

Unfortunately the CR micro servos they sell have very poor speed control. My friend erco wrote about his experience (http://www.robotrebels.org/index.php?topic=186.0) using the Fitec servos.

He thinks modified HXT900 servos (http://www.hobbyking.com/hobbyking/store/__662__HXT900_Micro_Servo_1_6kg_0_12sec_9g.html) are much better then the ready made CR servos.
Title: Re: Need a small motor that can be used for moving my robots
Post by: brokenAvocado on Nov 10, 2015, 02:48 am
Cool about the servos, but would they work with a dual motor driver? I have spark fun's dual motor driver: https://www.sparkfun.com/products/9457 and I'm just wondering if the servo can be controlled that way.
Title: Re: Need a small motor that can be used for moving my robots
Post by: OldSteve on Nov 10, 2015, 03:32 am
Cool about the servos, but would they work with a dual motor driver? I have spark fun's dual motor driver: https://www.sparkfun.com/products/9457 and I'm just wondering if the servo can be controlled that way.
You don't need a motor driver for a servo. You just send a stream of pulses out on one pin per servo using the Servo library. You send a value between 0 and 180 to each servo. With continuous-rotation servos, 90 is 'stop', 0 is full speed in one direction and 180 is full speed in the other direction.
The servos have 3 wires, one for signal from the Arduino, and two for connection to a 5V power supply. Then you connect the power supply's ground to the Arduino ground and Bob's your uncle.

Still, there's no special reason to change to servos. I only mentioned it because you did.
Title: Re: Need a small motor that can be used for moving my robots
Post by: brokenAvocado on Nov 10, 2015, 03:39 am
You don't need a motor driver for a servo. You just send a stream of pulses out on one pin per servo using the Servo library. You send a value between 0 and 180 to each servo. With continuous-rotation servos, 90 is 'stop', 0 is full speed in one direction and 180 is full speed in the other direction.
The servos have 3 wires, one for signal from the Arduino, and two for connection to a 5V power supply. Then you connect the power supply's ground to the Arduino ground and Bob's your uncle.

Still, there's no special reason to change to servos. I only mentioned it because you did.
Ok, but I wanted to change to a servo because the small dc motors don't have a wheel to fit on, and that's really the main reason why I wanted to change.
Title: Re: Need a small motor that can be used for moving my robots
Post by: OldSteve on Nov 10, 2015, 03:51 am
Ok, but I wanted to change to a servo because the small dc motors don't have a wheel to fit on, and that's really the main reason why I wanted to change.
I have no experience with the micro servos, but keep what Duane said about speed control in mind. My Dagu standard CR servos aren't the best for speed control either, but I only run them at two speeds anyway - stopped or flat-out. :D
Also it would pay to ensure that the Adafruit micro servo wheels will fit the servos you decide to buy. If micro servos are anything like standard ones, the shafts will vary from manufacturer to manufacturer, with different numbers of splines. That's why I bought servos that were supplied with wheels.
Title: Re: Need a small motor that can be used for moving my robots
Post by: DuaneDegn on Nov 10, 2015, 04:21 am
The micro servos don't seem to have as much variation in the shape of the output shaft as normal servos. Though I may just think this is the case since I don't have as much experience with micro servos.

The metal gear micro servo I've used has different splines than the plastic gear micro servos.

It's not too hard to attach wheels to servo horns. This thread shows several examples of attaching wheels to servos. The first page of the thread shows a Lego wheel attached to a servo horn and the second page shows a Vex wheel attached to a servo (without using the horn).

I think it's easier to attach wheels or gears to metal gear servos than it is to attach things to plastic gear servos because metal gear servos have a nice tapped hole for the a machine screw on its output shaft.

The original motors you purchased didn't have a gearbox. It's very unusual for a DC motor to have enough torque to propel a robot without a gearbox.

Pololu sells all sorts of nice little motors. They sell some without a gearbox but these are intended to be as replacements to be used with a gearbox. IMO, you shouldn't purchase motors to power a robot unless the motor includes a gearbox.

Oldsteve's strategy of purchasing a combination of wheel with gearmotor is a good one unless you know how to adapt wheels to various output shafts. It's harder than one would first expect to attach a wheel to a motor not specifically designed to fit on the motor. (http://forums.parallax.com/discussion/145826/cheap-bot-a-possible-bsa-robotics-merit-badge-robot/p1)
Title: Re: Need a small motor that can be used for moving my robots
Post by: MarkT on Nov 10, 2015, 11:36 am
Those coreless motors are designed for small quadcopters and RC helicopter tail props - you'll find
props that fit the shaft easily - gear trains even, if you look for RC helicopter parts.  Not my
first choice for robotics - micrometal motors are available in several voltages and many reduction
ratios and have metal gears, and are good value on eBay, for instance:
http://www.ebay.co.uk/itm/1Pcs-6V-100RPM-Micro-Torque-Gear-Box-Motor-Hot-Sale-/231466738662?hash=item35e47e23e6:g:r8wAAOSw1vlUyzNE (http://www.ebay.co.uk/itm/1Pcs-6V-100RPM-Micro-Torque-Gear-Box-Motor-Hot-Sale-/231466738662?hash=item35e47e23e6:g:r8wAAOSw1vlUyzNE)

But first work out what speed and torque you actually need before choosing a motor, there's little need
for guesswork.
Title: Re: Need a small motor that can be used for moving my robots
Post by: zoomkat on Nov 10, 2015, 07:40 pm
two points of interest:

1) Continuous rotation servos have better control using microsecond commands than using degree commands. The microsecond control band is typically 1400us to 1600us when the servo is tweaked for 1500us being the neutral/stopped value.

2) Attach a servo horn to the servo, then attach the wheel to the servo horn using a small drop or two of hot glue.
Title: Re: Need a small motor that can be used for moving my robots
Post by: OldSteve on Nov 10, 2015, 10:52 pm
2) Attach a servo horn to the servo, then attach the wheel to the servo horn using a small drop or two of hot glue.
Still a matter of finding a servo horn that's sold separately and fits the shaft. It does give a wider range of options though.
Title: Re: Need a small motor that can be used for moving my robots
Post by: DuaneDegn on Nov 10, 2015, 11:07 pm
Still a matter of finding a servo horn that's sold separately and fits the shaft. It does give a wider range of options though.
Most servos come with several different horns. I imagine zoomkat's suggestion was to use one of these horns.

Some servos come with half a dozen horns. The micro servos I've purchased usually some with three servo horns.

Some horns lend themselves to be attached to wheels more than others. If the servo comes with a nice "X" shaped horn, they're usually the easiest to attach to wheels.

Make sure the horn can be removed with the wheel attached. You don't want to have to undo the wheel/horn connection to remove a wheel. I've almost made this mistake several times.
Title: Re: Need a small motor that can be used for moving my robots
Post by: OldSteve on Nov 10, 2015, 11:12 pm
Most servos come with several different horns. I imagine zoomkat's suggestion was to use one of these horns.

Some servos come with half a dozen horns. The micro servos I've purchased usually some with three servo horns.

Some horns lend themselves to be attached to wheels more than others. If the servo comes with a nice "X" shaped horn, they're usually the easiest to attach to wheels.

Make sure the horn can be removed with the wheel attached. You don't want to have to undo the wheel/horn connection to remove a wheel. I've almost made this mistake several times.
Of course. My bad. I was thinking of my continuous-rotation servos, which came without horns. But that's because they came with wheels. Duh.
I just got up a short time ago and am still half asleeep.
Title: Re: Need a small motor that can be used for moving my robots
Post by: brokenAvocado on Nov 11, 2015, 01:25 am
The original motors you purchased didn't have a gearbox. It's very unusual for a DC motor to have enough torque to propel a robot without a gearbox.

Pololu sells all sorts of nice little motors. They sell some without a gearbox but these are intended to be as replacements to be used with a gearbox. IMO, you shouldn't purchase motors to power a robot unless the motor includes a gearbox.

Oldsteve's strategy of purchasing a combination of wheel with gearmotor is a good one unless you know how to adapt wheels to various output shafts. It's harder than one would first expect to attach a wheel to a motor not specifically designed to fit on the motor.
The problem is, for my robots I really didn't think I needed a gear box because they are so small and they're only traveling on a 4 foot x 2 foot table with no load.

Those coreless motors are designed for small quadcopters and RC helicopter tail props - you'll find
props that fit the shaft easily - gear trains even, if you look for RC helicopter parts.  Not my
first choice for robotics - micrometal motors are available in several voltages and many reduction
ratios and have metal gears, and are good value on eBay, for instance:
http://www.ebay.co.uk/itm/1Pcs-6V-100RPM-Micro-Torque-Gear-Box-Motor-Hot-Sale-/231466738662?hash=item35e47e23e6:g:r8wAAOSw1vlUyzNE (http://www.ebay.co.uk/itm/1Pcs-6V-100RPM-Micro-Torque-Gear-Box-Motor-Hot-Sale-/231466738662?hash=item35e47e23e6:g:r8wAAOSw1vlUyzNE)

But first work out what speed and torque you actually need before choosing a motor, there's little need
for guesswork.
Really I thought that the pager motors I bought were going to have a well sized motor shaft to fit a wheel with a little work, but turns out they are really small. I wanted to find other motors in that size or a little bit bigger, and I really don't need a lot of torque, but enough speed so it doesn't crawl on my 4 x 2 foot table like a turtle, since my robots are only 5 x 7 centimeters...
Title: Re: Need a small motor that can be used for moving my robots
Post by: MarkT on Nov 11, 2015, 01:52 am
The need for reduction gearing _increases_ as the motors get smaller because small motors have
less torque and higher rotation speeds.