Replacement servo amplifier

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Tobe
Posts: 503
Joined: 16 Feb 2018, 06:19
Location: Varberg or Stockholm, Sweden

Replacement servo amplifier

Post by Tobe »

From Tobe's workbench.
I have been playing with the idea to retrofit old servos with a modern amplifier so they will just look old but the soul would be up to the standards of today.
So I have updated on old faithful Kraft kPS-14 servo
20220513_171845.jpg
20220513_170713.jpg
What you see in the picture is an old Kraft KP14 amp compared to the new test unit which eventually will use a DRV8837 directly soldered to the board ad I that configuration I hope to be able to shrink slightly more.
20220513_165333.jpg
As processor I use an ATMega168P
JohnH
Posts: 24
Joined: 18 Jun 2019, 03:48
Location: Eden, MD USA

Re: Replacement servo amplifier

Post by JohnH »

Tobe,

A super idea! I have boxes full of old servos, I would love to use again with new amps. Especially some of the original proportional 4 and 5 wire metal box servos and KPS-9's.

John
John H.
"Here's another fine mess you've gotten me into."
Tobe
Posts: 503
Joined: 16 Feb 2018, 06:19
Location: Varberg or Stockholm, Sweden

Re: Replacement servo amplifier

Post by Tobe »

This shows what have come up with this little exercise:
Maxi/MiniPelle own flight pack with fully proportional servos.
20220515_110716.jpg
20220515_110437.jpg
...and a peek inside
20220515_110427.jpg
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Al Clark
Posts: 39
Joined: 17 Aug 2018, 13:40

Re: Replacement servo amplifier

Post by Al Clark »

Excellent design as usual Tobe! Now old servos can be converted just like old TXs. Look forward to seeing the details.
Al Clark
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Wayne_H
Posts: 702
Joined: 17 Feb 2018, 05:26
Location: Wellington, NZ
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Re: Replacement servo amplifier

Post by Wayne_H »

Great stuff again Tobe 👏

Potential users re-birthing old servos will just have to be extra careful checking them for signs of the plastic having gone brittle. I think this risk may have been mentioned in another thread some time back. Anybody have any advice and/or experience treating the brittleness issue (arthritis treatment for servos :shock: ?)
Cheers,

Wayne
Once a Retrobate, always a Retrobate............ ;)
bluejets
Posts: 255
Joined: 19 Jun 2019, 04:09

Re: Replacement servo amplifier

Post by bluejets »

Maybe soak with thin "ZAP" ...stripped down case naturally.
Attachments
PT07.jpg
Pchristy
Posts: 323
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: Replacement servo amplifier

Post by Pchristy »

Nice project, Tobe, but I have a question: Many older servos used a centre-tapped (4-wire) supply. The motors in these servos were a different impedance to the ones used in later (3-wire) "bridge" amplified servos.

How do you get around the issue of running motors designed for 2.4 volt operation on 4.8 volt (nominal)?

I can think of at least one way of doing it, but it would mean different firmware (or switchable?) for the different motor types. Have you found a way to detect and select automatically?

Cheers,

--
Pete
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Mike_K
Posts: 568
Joined: 16 Feb 2018, 06:35
Location: Hertfordshire

Re: Replacement servo amplifier

Post by Mike_K »

Hi Pete

I did the software for Tobe and included pwm on the outputs, so if you have a 4-wire servo, then just set the pwm to 50% duty cycle and you can run it as a 3-wire servo. In fact many of the motors used were rated above 2.4V and the drive voltage to the motors would have been less than 2V allowing for the 0.7V drop of the final drive transistors, so you could actually improve the servo performance from original by running at a slightly higher voltage. As long as the servo has a motor and feedback pot, it should work.

I based the "amp" on an ATMega328P and DRV8838 as they were what I had to hand, but it would easily fit on an ATmega48P or many of the newer ATtiny processors or an alternative motor driver chips like an DRV8835, TB6612FNG, etc.

Cheers Mike
Pchristy
Posts: 323
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: Replacement servo amplifier

Post by Pchristy »

Thanks for the prompt reply, Mike!

Yes, when I said I could think of one way, I was thinking of the mark-space ratio on the drive output!

One last question: Do you know what the input impedance of the amplifier is? I'm asking because I might renovate an old 27 MHz system that used an SCS decoder. These were notoriously fussy about the load placed on them by the amps. I'm guessing that you are going straight into one of the inputs of the processor, in which case that would be very high impedance and suitable for SCS decoders. But its always wise to check...!
;)

--
Pete
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Mike_K
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Joined: 16 Feb 2018, 06:35
Location: Hertfordshire

Re: Replacement servo amplifier

Post by Mike_K »

Hi Pete,

Yes, the ppm signal goes straight into the Atmega input. I'd have to look at a datasheet to confirm, but with the pull-up resistor, I think the input impedance is between 20K to 50K, I'm sure an SCS decoder would cope OK with that. What is the Rx, an RCS, Sprengbrook or similar era?

Cheers

Mike
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