EV3 vs Arduino using RC servos

[oracid]

In this video, I show that you can use RC servos with the EV3 as easily as with an Arduino, thanks to the NXTServo-v3 from Mindsensors.

http://www.mindsensors.com/ev3-and-nxt/25-8-channel-servo-controller-for-nxt-or-ev3

 

 

[Jonas]

Nice. Which servo do you use? And what is the load?

[oracid]

10 hours ago, Jonas said:

Nice. Which servo do you use? And what is the load?

Thank you.

In this video I use the JX BLS-HV7146MG, I have notice it at 0'5", which is 46kg.cm torque. You can find it here : https://www.banggood.com/fr/JX-BLS-HV7146MG-46KG-180-Degrees-HV-High-Precision-Steel-Gear-Digital-Brushless-Servo-For-RC-Robot-p-1180610.html?rmmds=myorder&cur_warehouse=CN

But for my quadruped project, https://www.eurobricks.com/forum/index.php?/forums/topic/168446-gato-my-quadruped-project/  I have choosed the JX B7640HV RD-360 which is a 40kg.cm torque, here :https://fr.aliexpress.com/item/JX-RD-B7640MI-360-40-KG-Double-axe-Brushless-servo-induction-Magn-tique-complet-CNC-en/32866647426.html?spm=a2g0s.9042311.0.0.40696c37ZEg6mX

 

To compare, remember that Lego says is big servo is 4.5kg.cm with 9V and 175RPM, but it is only 5V output in EV3. 

 

Edited November 13, 2018 by oracid

[MajorAlvega]

Nice.
Since I use ev3dev I prefer to use a USB servo controller so I don't need to use a EV3 OUT port and if/when I'm crazy enough I can use more than 8 RC servos. But I keep looking for this Mindsensors device, one of these days I might get one.

Just a note: the EV3 servos use more than 5V volt. The output ports can give almost the full battery voltage so around 7V with the EV3 LiPo battery and around 9V with a good pack of 6 Alkaline AA batteries.

 

[oracid]

4 hours ago, MajorAlvega said:

Nice.
Since I use ev3dev I prefer to use a USB servo controller so I don't need to use a EV3 OUT port and if/when I'm crazy enough I can use more than 8 RC servos. But I keep looking for this Mindsensors device, one of these days I might get one.

Just a note: the EV3 servos use more than 5V volt. The output ports can give almost the full battery voltage so around 7V with the EV3 LiPo battery and around 9V with a good pack of 6 Alkaline AA batteries.

 

Hi Jorge

Can you give us a link about your USB servo controller ?

About EV3 voltage, did you measure it ?

Myself, I have an EV3 with a 7.4v LiPo battery and don't use Alkaline AA.

[MajorAlvega]

Sure!

I have two different Pololu Maestro: https://www.pololu.com/category/102/maestro-usb-servo-controllers

They have several models, I have 24-channel and a 6-channel. Really only used the 24-ch and never used all the channels, don't have enough RC servos for that (but I'm collecting some chinese small servos with LEGO form factor, Kittenbot Geek Servo).

You can see both (Maestro and Kittenbot) in action here:

The Maestro is USB based but it works like a COM port and accepts simple messages. There is at least one program for Windows and other for Linux but I use simple my own (linux) bash script or python script, they work seamless on my laptop, my Raspberry Pi and my EV3 (ev3dev).

Now for the EV3 output port... no I didn't measure. I know that it uses an h-bridge circuit but didn't find the reference, just found this post from Dexter where they state they used a SN754410NE on their first version of the BrickPi (https://www.dexterindustries.com/howto/lego-mindstorms-motors-with-raspberry-pi-brickpi-0-1/)

but the SN754410NE isn't an efficient circuit, if I read the datasheet correctly it typically drops 1 volt on each transistor so with a 7.4 Volt LiPo you would get just 5 Volt, that's terrible. I have the idea (probably wrong) that both EV3 and Power Functions IR receiver share the same output circuit, the LB1936 that has a lower saturation voltage.

Edited November 13, 2018 by MajorAlvega

[oracid]

Thank you Jorge, you should make a topic about this Pololu Maestro USB Servo Controllers. It seems very good. Nice video.

Here is the official EV3 Lego documentation, https://education.lego.com/en-us/support/mindstorms-ev3/developer-kits at the bottom of this page you can see a link to download "the EV3 Hardware Developer Kit". In this zip file, there is the file "LEGO MINDSTORMS EV3 Hardware Developer Kit".

In this document, at the first page you can see this :

POWER

9V battery
5 V buck boost 2 A>
3V3 1 A
1V8 1 A
1V2 1 A
Power up sequence
5 V Current limiter
5V on/off
Current measurement on overall current
Voltage check on Batteries
5V BURN OUT

and this

OUTPUT

4 Output
Pin 1: Motor Out PWM
Pin 2: Motor Out PWM PU to pin 6
Pin 3: Ground
Pin 4: 5 Volt +20 mA
Pin 5: I/O (OC), Tacho, PD,PU, ADC
Pin 6: I Tacho, PU from Mx1
Auto Detect Motor and Sensor

 

This means that the output is definitely 5V. The same with the LiPo 7.4V ou with 6 AA batteries. 

 

[MajorAlvega]

I am not sure... the last page of the main hardware schematics has "Battery Positive" directly connected to "VCC9VBAT" and after that, through R245 (1.1A/16V proteccion component, resetable fuse?) to VCC9V.

And the motor drivers (LB1836M and not 1936 as posted above) use this voltages (VS1 and VS2).

That part of 5V Buck Boost 2A is the TPS40210DRCR circuit, it converts VCC9V in 5V.

But now I got curious and decided to measure. With the EV3 LiPo, I get 6.9V (cheap chinese multimeter) with the old 9V micro motor connected to OUT A through a NXT/EV3 to RCX cable adapter.

Don't have fresh AA batteries to test, will have to wait a day or two.

[oracid]

Ok, me too, I will try to measure. So we can compare.

[MajorAlvega]

Fresh new Energizer AA alkaline batteries.

ev3dev reports:

cat /sys/class/power_supply/lego-ev3-battery/voltage_now
9177466

when the micro-motor is running my voltmeter reads 9.02 V

The voltage drop is really small but the micro-motor only uses a few mA so I would expect just a little more than 8 Volt with a medium EV3 servo motor and a moderate load.

[oracid]

Things are not simple.

I agree  with you. I have tested this morning. The motor is in 8V for me, but the output voltage for servo electronic is in 5V.

I would like to make a new topic and a video to show all this. I hope tomorrow.