Servolulu: Polulu-compatible servo motor driver I've made some progress on the Polulu-compatible servo motor driver that I proposed in an earlier post. I'm calling it the Servolulu. It consists of a PIC18F2331 microcontroller, which has a quadrature encoder interface, and an A4953 (2 amp) motor driver chip. The processor is loaded with PIC-SERVO controller code which provides PID servo control with Step & Direction command signals. It all fits in the same footprint as a standard Polulu driver, and the relevant signals (Vmot, +5v, Gnd, M+, M-, Step, Direction, Enable) all pass though to the Servolulu board from the printer controller board below. (The first photo shows a Servolulu plugged into a Teensylu board.) The unused pins (MS1, MS2, MS3, RST, SLP and 2 unused motor pins) are clipped from the bottom side that plugs into the printer board (see last photo) but are extended on the top to give you a place to plug in your encoder, a TTL-level serial port (for programming gain parameters into EEPROM) and also a couple of pins (PGD, PGC) for flashing the micro. Unfortunately, the PIC-SERVO assembly code I'm using to test this is proprietary so I can't publish it. However, I am planning on writing a stripped down version written in C that will be open-source. Also, once I get the design documented and posted somewhere, anyone else will also be able to write their own servo control code. With servo control, you can generally run cooler, quieter, and at higher speeds without fear of losing steps. And with the serial port, it is also possible to monitor the servo performance in real-time while moving - a feature that can be used to detect end-stop collisions, jammed filament and even running out of filament. Please let me know what you think and if you'd like to see this go into some sort of production. https://reprap.org/forum/read.php?1,654278,654278#msg-654278 Fri, 11 Sep 2026 09:45:31 -0400 Phorum 5.2.23 https://reprap.org/forum/read.php?1,654278,739239#msg-739239 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,739239#msg-739239 rklauco General Mon, 16 Jan 2017 04:37:06 -0500 https://reprap.org/forum/read.php?1,654278,738799#msg-738799 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,738799#msg-738799
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Tangent1001


Can anyone give me a link where I can buy a pair of these?

Unfortunately, there was not a huge amount of interest in this so I've pretty much dropped the project.]]> LoboCNC General Sat, 14 Jan 2017 16:22:24 -0500 https://reprap.org/forum/read.php?1,654278,738676#msg-738676 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,738676#msg-738676
Can anyone give me a link where I can buy a pair of these?]]> Tangent1001 General Sat, 14 Jan 2017 07:33:07 -0500 https://reprap.org/forum/read.php?1,654278,674136#msg-674136 Re: Servolulu: Pololu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,674136#msg-674136 LoboCNC General Mon, 13 Jun 2016 21:32:07 -0400 https://reprap.org/forum/read.php?1,654278,674110#msg-674110 Re: Servolulu: Pololu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,674110#msg-674110 I understand the driving frequency - and based on my ATtiny85 timer set-up, I should operate somewhere within 32kHz. The only other electronics I am using is 2 caps - 470uF and 100nF.
Wiring:
Pin1 - GND
Pin2 - input1
Pin3 - input2
Pin4 - VCC (5V)
Pin5 - VBB (12V)
Pin6 - output to motor 1
Pin7 - GND
Pin8 - output to motor 2
I am not using anything special here - one of the inputs is always low, the other one has PWM with the mentioned frequency and desired pulse width. When I change direction, the other pin gets PWM while the previously active sets to LOW.
Apparently, according the datasheet, I am using fast decay mode (chopping). But I cannot find the reason why - as my current sensing input is connected to ground directly, the mode should be only simple forward/reverse, not fast decay one.
Any chance you can share your schematics? I know the code for the controller is closed source, but the schematics should be fine...]]> rklauco General Mon, 13 Jun 2016 17:33:01 -0400 https://reprap.org/forum/read.php?1,654278,674079#msg-674079 Re: Servolulu: Pololu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,674079#msg-674079
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rklauco
However, unlike TB6612 I am still getting some unwanted "audible" feedback - especially when breaking and stationary. With the same PWM the TB6612 is totally silent. It seems like some sub-harmonic frequency is causing it.
It looks like there is not much documentation/experience discussion on the web about the A4953.
@LoboCNC: Did you already test it with your chip? Did you experience any sound feedback?

With chopper-type drives, it is not uncommon to get audible subharmonics. A lot depends on your exact motor and the components you select for the chopping frequency. In the Servolulu, I'm not actually using the A4953's in chopping mode to control the current. I'm just turning them on or off completely at the servo controller's frequency of 20KHz, and I get no whining.]]> LoboCNC General Mon, 13 Jun 2016 15:25:34 -0400 https://reprap.org/forum/read.php?1,654278,673900#msg-673900 Re: Servolulu: Pololu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,673900#msg-673900 B4Me General Mon, 13 Jun 2016 05:37:45 -0400 https://reprap.org/forum/read.php?1,654278,673894#msg-673894 Re: Servolulu: Pololu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,673894#msg-673894
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LoboCNC
How much current are you running through the motor? The total on resistance of the A4953 is about 1 ohm, so I'd guess that with no connection to the thermal pad, it'll get pretty hot even if you are drawing less than 1amp.
Thanks for the hint.
Normally the motor takes less then 150mA, but with problems during movement (e.g. start or hitting a minimum/maximum position) it can take almost 700mA.
1 ohm is quite a lot, so I put on a heat sink (stolen from my stepper driver :D) and it's now quite fine - not more then 39 degrees.
However, unlike TB6612 I am still getting some unwanted "audible" feedback - especially when breaking and stationary. With the same PWM the TB6612 is totally silent. It seems like some sub-harmonic frequency is causing it.
It looks like there is not much documentation/experience discussion on the web about the A4953.
@LoboCNC: Did you already test it with your chip? Did you experience any sound feedback?]]> rklauco General Mon, 13 Jun 2016 05:24:57 -0400 https://reprap.org/forum/read.php?1,654278,672153#msg-672153 Re: Servolulu: Pololu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,672153#msg-672153
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rklauco
LoboCNC: Do you have experience with A4953? Mine is quite disappointing - I got it yesterday and tested it today and it is quite hot during position hold. Is it due to my board design? (No heat pads or anything similar, but driving only 3.6W 12V motor...)

How much current are you running through the motor? The total on resistance of the A4953 is about 1 ohm, so I'd guess that with no connection to the thermal pad, it'll get pretty hot even if you are drawing less than 1amp.]]> LoboCNC General Tue, 07 Jun 2016 18:47:40 -0400 https://reprap.org/forum/read.php?1,654278,672087#msg-672087 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,672087#msg-672087 rklauco General Tue, 07 Jun 2016 15:46:20 -0400 https://reprap.org/forum/read.php?1,654278,661986#msg-661986 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661986#msg-661986 pamalofeev General Mon, 09 May 2016 13:22:52 -0400 https://reprap.org/forum/read.php?1,654278,661931#msg-661931 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661931#msg-661931
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pamalofeev
If you overwrite project in C, you did not think to use other mcu? For example stm32f030f4p6.

Yes, it looks like this processor would be a much better choice for writing new code. (I used the PIC18F2331, a much older processor, because I have access to the PIC-SERVO SC code.) The control code itself, whether a simple PID loop or a fancier multi-loop architecture is all fairly easy to do. Doing a good job of the user interface and documenting it is much more work. And for me, the amount of work required to learn how to use the STM32 processor and development tools would totally eclipse the amount of work in writing the code.

Ultimately, the choice of processor is not really that important. The goal is not optimality but to come up with a servo controller solution that is easy for people to use.]]> LoboCNC General Mon, 09 May 2016 11:09:18 -0400 https://reprap.org/forum/read.php?1,654278,661861#msg-661861 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661861#msg-661861 this project. This is a real DC motor servo. He even has the current loop, but not really working on the motor model. I used to be studied for this specialty, a little understand the topic. A good drive will not work with only loop position and PID. Only simple, non-demanding systems, and this is not our case. And I do not propose to abandon the concept of a simple replacement of the stepper motor driver, only propose to review the use of pic18. He did not give the project the possibility to grow.]]> pamalofeev General Mon, 09 May 2016 07:22:54 -0400 https://reprap.org/forum/read.php?1,654278,661770#msg-661770 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661770#msg-661770 rklauco General Mon, 09 May 2016 02:43:24 -0400 https://reprap.org/forum/read.php?1,654278,661760#msg-661760 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661760#msg-661760
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LoboCNC
The current version of the Servolulu uses PIC-SERVO SC code, and you can view all of the gory details in the PIC-SERVO SC data sheet. .

Hmmm... In STEP/DIR mode only position loop.. Thus, this system is not much different from mysan servo, except that used a more expensive controller. The only plus - the hardware encoder interface.
If you overwrite project in C, you did not think to use other mcu? For example stm32f030f4p6. On aliexpress cost ten mcu such as a PIC18F2331. Also it has a hardware encoder interface, and it is a 32-bit processor.]]> pamalofeev General Mon, 09 May 2016 01:49:07 -0400 https://reprap.org/forum/read.php?1,654278,661705#msg-661705 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661705#msg-661705 rklauco General Sun, 08 May 2016 16:56:06 -0400 https://reprap.org/forum/read.php?1,654278,661665#msg-661665 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661665#msg-661665
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pamalofeev
LoboCNC,
I like that there are more attempts to do good dc servo reprap. You can show your servo Operation diagram, which will be open source? Just like here. Or tell which loops will be used?

The current version of the Servolulu uses PIC-SERVO SC code, and you can view all of the gory details in the PIC-SERVO SC data sheet. The PID servo controller is described in Section 4.3, and the various operating modes are described in Section 4.4.

What I'm planning for an open-source version of Servolulu code is to duplicate just the PID controller section with Step & Direction inputs and then give it a simpler ASCII communication interface for setup with any simple terminal program. I'll also write it in C rather than assembly so that other people might have a hope of hacking on it.]]> LoboCNC General Sun, 08 May 2016 15:12:58 -0400 https://reprap.org/forum/read.php?1,654278,661637#msg-661637 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661637#msg-661637 LoboCNC,
I like that there are more attempts to do good dc servo reprap. You can show your servo Operation diagram, which will be open source? Just like here. Or tell which loops will be used?]]> pamalofeev General Sun, 08 May 2016 13:04:52 -0400 https://reprap.org/forum/read.php?1,654278,661618#msg-661618 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661618#msg-661618
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rklauco
But the gearing is quite serious with the little motor (so is the torque), so 12 impulses per revolution should be plenty...

Indeed. Some of these micro-motors have up to 1000:1 gearing, so 12000 pulses is close to 1/64 microstepping with a typical 1.8 deg stepper, which is way overkill for our application. The torque you can get from these is rather nice (see the above ultralight extruder) and you can temporarily boost the speed with higher voltage for retractions.]]> ipcalit General Sun, 08 May 2016 12:15:17 -0400 https://reprap.org/forum/read.php?1,654278,661606#msg-661606 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661606#msg-661606 rklauco General Sun, 08 May 2016 11:45:47 -0400 https://reprap.org/forum/read.php?1,654278,661603#msg-661603 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661603#msg-661603
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ipcalit
Would Servolulu work with one of these magnetic encoders from Pololu?

Magnetic Encoder Pair Kit for Micro Metal Gearmotors, 12 CPR, 2.7-18V (HPCB compatible) [www.pololu.com]

It would be great for this ultralight extruder [forums.reprap.org]

Cant see any reasons they shouldnt work, but your resolution are only 12 pulses per revolution, and then your gearing]]> B4Me General Sun, 08 May 2016 11:41:30 -0400 https://reprap.org/forum/read.php?1,654278,661591#msg-661591 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,661591#msg-661591
Magnetic Encoder Pair Kit for Micro Metal Gearmotors, 12 CPR, 2.7-18V (HPCB compatible) [www.pololu.com]

It would be great for this ultralight extruder [forums.reprap.org]]]> ipcalit General Sun, 08 May 2016 10:56:53 -0400 https://reprap.org/forum/read.php?1,654278,659994#msg-659994 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,659994#msg-659994
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LoboCNC
rklauko: Great to see someone else working on servo control, but interleaving progress on your controller with the Servolulu development in the same thread is getting a little confusing. Perhaps you could start your own topic.
Sorry for spamming your thread. I started a new one and I'll refrain from further posts here. Good luck with your project.]]> rklauco General Wed, 04 May 2016 02:23:13 -0400 https://reprap.org/forum/read.php?1,654278,659923#msg-659923 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,659923#msg-659923 LoboCNC General Tue, 03 May 2016 19:03:08 -0400 https://reprap.org/forum/read.php?1,654278,659919#msg-659919 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,659919#msg-659919 And I have to say it's fantastic.
I created a simple code that does "homing" in the beginning.
I verify if the encoder direction is consistent with motor direction - if it's not, I'll adjust the motor direction. This was a frequent problem for me as I undid and put together the printer and by accident connected the motor cable the other way.
Next, I slowly move to minimal position I can find - and I am checking here the encoder input to see if the carriage is still moving.
As soon as it stops, I kill the motor and consider current position as minimum (I also reset the encoder value to 0).
Next I will verify the maximum using the minimal speed in the opposite direction.
And as soon as I have it, I'll return back to 0 position.
Funny thing here is - I no longer need minimum and maximum switches - I can get the info from the feedback loop :)
I'm starting to love the system. Have to talk to Repetier firmware guys if it's possible to implement some feedback communication to their code directly.
But even if not, this will be very nice this already works OK.
And, did I mention it's extremely quiet? The only think I can hear is the linear bearings. And I'll try to print them as a next step to see, if that can help further.]]> rklauco General Tue, 03 May 2016 18:44:51 -0400 https://reprap.org/forum/read.php?1,654278,659816#msg-659816 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,659816#msg-659816 pcbng_jh General Tue, 03 May 2016 12:24:28 -0400 https://reprap.org/forum/read.php?1,654278,659815#msg-659815 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,659815#msg-659815 I started with the project but am having difficulty finding any compatible chips on cut tape... (required per the PCB NG guidelines).
I'll keep looking otherwise I think we may have to find another IC.. (maybe ATiny85?) but always difficult to find cut-tape and not whole rolls (2000+ units).

Jon]]> jonnycowboy General Tue, 03 May 2016 12:21:07 -0400 https://reprap.org/forum/read.php?1,654278,659613#msg-659613 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,659613#msg-659613
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o_lampe
Dumb question:
What makes a DC motor a DC Servo motor? Is it just the encoder or number of poles or???
A servo is a closed loop system. This means that there is a feedback on the actual position that is then used to correct positioning to reach the intended position. This is usually done via a PID control. This can be used to stabilize RPMs as well.
Sometimes you don't even need a specific encoder, with brushless motors it is possible to use the inactive coils for measurements. Many gimbals use this method.]]> Srek General Tue, 03 May 2016 03:17:01 -0400 https://reprap.org/forum/read.php?1,654278,659612#msg-659612 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,659612#msg-659612 And the plan was to make a new controller board with dc in the heart, and a STM32xxxxxx as the mcu..
But it seems to be good enough just to use this approach instead ! :)]]> B4Me General Tue, 03 May 2016 03:14:58 -0400 https://reprap.org/forum/read.php?1,654278,659610#msg-659610 Re: Servolulu: Polulu-compatible servo motor driver https://reprap.org/forum/read.php?1,654278,659610#msg-659610
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o_lampe
Dumb question:
What makes a DC motor a DC Servo motor? Is it just the encoder or number of poles or???
Basicaly normal motor with some sort of feedback control (even potenciometer and comparator works OK in RC servos...).]]> rklauco General Tue, 03 May 2016 03:05:50 -0400