About this Blog

This blog is an engineering log book; for me to record the progress on my many projects and hopefully to inspire you.

Some projects do not get off the ground, they remain as interesting thoughts, a select few get some work done on them, even fewer get close to completion, and none get completed because unfortunately I subscribe to the theory: "If something ain't broke then it doesn't have enough features". If you'd like to collaborate on some work to get something to a useable state then send me some communications.

Being a blog, posts are listed in chronological order. However I usually have multiple projects on the go and will try to post some of my earlier work. With this in mind I'll try to add labels to each post so all posts relevant to one project can be easily extracted.

Enjoy and happy hacking.

Wednesday, August 20, 2008

What's Inside the MG995 Servo?

The servo of choice (MG995) is the cheapest high torque (~15 kg/cm) servo that seems to be available world wide (at least it used to be). There is also a big brother servo (~20 kg/cm) that comes in Jumbo size and with nylon gears). I believe they are an OEM servo made in china like all cheap hardware. They are traded under a number of names, commonly Tower Pro.

Now lets break it open and work out what the nitty gritty bits inside are. The servos are labeled as "digi", which some take to mean digital. There are discussions on RC forums about these servos and if they are actually digital and how good they are. Apparently digital servos are better - better holding torque, less dead band and more responsive. The forum concensus was that they wern't digital (because digital servo's should be expensive). I'm not into my RC vehicles so I don't know much about servos, but I am a trained electronic engineer. As we shall soon see these servos are in fact digital (and i'd suggest that analogue servos would be more expensive).

Inspecting the circuit board inside we find:
  • three SOIC-8 ICs (5 in the big brother) which have had their marking removed (thanks Mr Manufacturer guy),
  • a couple of electrolytic capacitors,
  • some surface mount capacitors and resistors,
  • an 8MHz ceramic resonator,
  • a couple of transistors,
  • a 3.3V linear voltage regulator.
All but one of the SOIC-8 packages, (those next to the motor connections) are MOSFETs two N types and two P types in a full H Bridge configuration. On the MG995 board the two chips appear to be something like an IRF7309, housing a pair on N an P type MOSFETs, making a half bridge each.

The transistors provide the inverted full supply voltage drive signals to the P-type MOSFETs.

The final component of question is the 'contol' IC. In an attempt to protect the design the top of the IC has been scraped to remove the markings. I suspect the IC is a microcontroller.

The micro's pins are connected to:
Pin 1. VCC
Pin 2. Oscillator
Pin 3. Oscillator
Pin 4. H-bridge control
Pin 5. Servo's PWM control input wire
Pin 6. H-bridge control
Pin 7. Potentiometer input (ADC?)
Pin 8. GND

The micro is probably a PIC, since they are cheap and well known. After a bit of internet searching the pin-out matches the 8-pin microcontrollers of the 12-series PICs. The PIC12F510 being the most likely.

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