Serious About Servos

Serious About Servos

A bunch of servos, from the excellent Sparkfun servo tutorial
A bunch of servos, from the excellent Sparkfun servo tutorial

Something strange started happening to my Particle Photons when I began working with the Clunky #1 gumball machine -- the Photons started dying, violently. Sometimes in a plume of electric smoke, a little whiff of scary.  

I went on the Particle community boards, to try to figure it out. 

Slowly a theory emerged: there were power issues (volts 'n' amps) that I was blissfully unaware of, and had been probably just avoiding, without being aware of it.  

But now I had to address them. $19 (the cost of a Photon) is too high a price to pay for my mistakes. 

This discussion in particular -- Killed my Photon... How? -- focused my attention. It made me feel like I had been living on borrowed time. 

Turns out that servos operate at the limits of the Photon, in terms of volts, and amps. A little too much torque, a small, dirty spike in electricity, and it's curtains for the Photon.  

So I decided to get serious about servos. 

Servo Resources

Fortunately, I discovered ServoCity

And HobbyKing, which also has a good Servo section. 

Adafruit put servos in context, in their Motor Selection Guide

Sparkfun also has an excellent Servo Tutorial

For example it has this: a table that summarizes common color schemes. A useful mnemonic is that the most drab color (black or brown) is usually ground, and red is usually the power supply.

Pin Number Signal Name Color Scheme 1
(Futaba)
Color Scheme 2
(JR)
Color Scheme 3
(Hitec)
1 Ground Black Brown Black
2 Power Supply Red Red Red or Brown
3 Control Signal White Orange Yellow or White

Why Servos are Worth It

Because: 180 degrees of motion.

And stops anywhere along the way. And you can put on the brakes at any point, and wait for a few seconds.  And then start back up again, in a different direction if you want to. 

A continuous servo, an almost trivial variation, can spin 360 degrees, in both directions, and stop suddenly, and wait, and start spinning again. 

Think of the possibilities for powering automata:

Or Light Play:

Light Play at the Exploratorium
Light Play at the Exploratorium

Or some variation on Paper Signals, which, significantly, relies on servos to make the vision work. 

paper signals
Paper Signals

I ordered two:

The Futaba S3003 Standard Servo

https://www.futabarc.com/servos/specs-lineart/specs-futm0031.html

futaba

and the 

Hitec HS-422 Deluxe Standard Servo (which sounds like a contradiction of terms). 

HiTec servo

Both were about $10. 

When they arrived, I eagerly consulted the datasheets for both servos. And I was disappointed to learn that the power-consuming information was sketchy. 

For the Futaba:

Technical Specifications:

  • Operating Voltage

– 4.8V – 6.0V

  • No-Load Speed (4.8V)

– 0.23sec/60°

  • No-Load Speed (6.0V)

– 0.19sec/60°

  • Stall Torque (4.8V)

– 3.2kg.cm

  • Stall Torque (6.0V)

– 4.1kg.cm

  • Pulse Amplitude

– 3 - 5V

  • Operating Temperature

– -20 to +60°C

  • Continuous Rotation Modifiable

– Yes

  • Motor Type

– 3 Pole Ferrite

  • Gear Type

– Straight Cut Spear

  • Cable Length

– 150mm

  • Dimensions

– 41 x 20 x 36mm

  • Weight

– 37.2g

 

For the Hitec

HS-422 Servo Specifications

Performance Specifications
Operating Voltage Range (Volts DC) 4.8V ~ 6.0V
Speed (Second @ 60°) 0.21 ~ 0.16
Maximum Torque Range oz. / in. 46 ~ 57
Maximum Torque Range kg. / cm. 3.3 ~ 4.1
Current Draw at Idle 8 mA
No Load Operating Current Draw 150 mA
Stall Current Draw 800 mA
Dead Band Width 8 µs

I did find some additional power advice on the Particle community boards, re: servos. 

Consider adding extra capacitors to the power supply +V and GND such as 0.1uF and say 47uF electrolytic near each servo and the Photon.

Servos can generate a good deal of electrical noise back on to their power supply and so extra filtering can't hurt. You might not need all servos to have the extra caps or you might try having them just at the Photon. Experiment and see.

The motors in the servos generate electrical noise on their power supply connection when they run, plus they can draw a lot of current when the motor goes from a stop to a start and that can cause low-voltage conditions for the other things being powered by the same supply. The capacitors filter the noise and help supply extra temporary current when the motor starts up. The low-value 0.1uF helps with high frequency noise and the larger valued (say) 47uF helps with temporary current and low frequency noise.

This is beneficial since the Photon will work better with a "clean" power supply that does not have noise or low-voltage conditions. You might only need one pair or you might need many depending on your wiring and current needs with multiple servos running at the same time or starting and stopping often. Start with one pair and add as needed--they are cheap parts.