Do you need strong servo for Tibia joint or match speed?

[nizee]

Im trying to revive an old project that was put aside since 2008/2009.
A close friend sold me a ridiculous deal (hard for me to pass it) on some Hitec HV servo from his failed Jet Airplane project.
I got 6 HS-7245MH HV servos to use for Coxa joints and 6 HS-7954SH HV to use on Femur joints. Have not decide on the Tibia's servos yet (this i have to buy my own).

I have the older combo board (with p.Brain-HexEngine)
1) p.Brain-SMB V1.0 Motherboard
2) p.Brain-ds24 Robot Controller - (p.Brain-HexEngine).

My question is could i get away with lower torque/speed servos for the Tibia joints? Or i have to match the servo i use for my Femur joints (HS-7954SH)?
Since i'm using Matt's HexEngine firmware, will there be problem with walking gate, timing, etc.... with slower servo speed on the tibia joint compare to faster speed on the femur joint?

Need to use High Voltage servo, so i can 7.4 LiPoly battery for all the servos and separate 9V battery (step down by p.Brain-motherboard) for the boards.

I was thinking of getting 6 HS-5685MH servos for the Tibia's joint. Spec below: -
HS-5685MH
Speed: 6.0V, 0.20 sec/60°
7.4V, 0.17 sec/60°
Torque: 6.0V, 157 oz-in (8.8 kg-cm)
7.4V, 179 oz-in (10 kg-cm)
Weight: 2.1oz (60g)

vs

HS-7954SH
Speed: 6.0V, 0.15 sec/60°
7.4V, 0.12 sec/60°
Torque: 6.0V, 333 oz/in
7.4V, 403 oz/in
Weight: 2.30oz (65.2g)

[Matt Denton]

Hi,

Usually it is best to match the tibia and femur servos, however, the HS-5685 you have specified seems to have a pretty good torque, so I would think it would be ok. As for the mismatch in speed, this shouldn't be an issue for the hexengine, simply because it has no idea of the speed differences, however, if you start to use higher leg transition speeds you may notice the slower servo's lagging which will cause less accurate walking behaviour.

One other point is that very fast servos can sometimes cause oscillation on hexapods due to their smaller deadband and faster response times. If this is the case you may need to swap out the femur servos for slower HS-5685 type servos.