I have had many people asking me for advise on servos over the years, hence this area of the forum.
I will add my views on certain makes and models of servos that have proved them selves in the past within Animatronics and Robotics, along with name and shame those which have failed.
A few notes on selecting servos:
Pick the largest servo you can use within your application. Eg. You may find a standard size servo with the same torque rating as a 1/4 scale servo, however, the gearbox in the 1/4 scale servo will usually be much stronger than the smaller standard size servo gearbox.
Generally speaking, the more expensive the servo is, the better quality it will be, however, you do tend to pay for the name with certain makes of servo.
There are pros and cons for both these servo types. Digital servos usually give better resolution than analogue servos, and usually have higher torque ratings than their analogue counterparts. However, this increased torque usually comes from driving the motor harder which uses more current and generates more heat. Digital servos are often noisier than analogue's which is not a problem for most robot projects, but within animatronics can cause problems.
4) Brush-less Servo Motors
There are a new range of servos coming out which we will probably see more of over the next few years. These servos utilise electronically commutated brush-less motors. As well as the other benefits of digital servos, the coils on a brush-less motor are on the outside, which gives several benefits such as low start up inertia and good heat dissipation.
Some cheap servos use cheap feedback potentiometers which can give very poor linearity over the servo range travel. This means that servo centers will vary more than usual, but more importantly the distance the arm travels in either direction may be different. This is not so much of a problem within general R/C use, but within robotics can soon become a headache. For example, if the servo neutral centers vary (which they will) adding an offset to calibrate each servo is not too difficult, however, if the servo travels a different amount in each direction, you will need to calibrate the center point and end points of the servo and use interpolation do generate a linear motion... yawn! Good servo linearity is one of the most important servo features.
There are many standard size servos which now offer huge torque ratings of 15 kg/cm of torque or higher, however, it's always worth looking at the speed of the servo. Many of these high torque servos have been designed with R/C models in mind, and as such they need to be fast as well as strong. This sounds great, but the way that fast servos achieve high torque is by using much more current, this is fine for a few servos, but if your running an 18DOF hexapod, you'll be eating batteries galore! In my opinion, depending on the application Its better to pick slower servos for robotics use. This has a couple of benefits, such as lower current consumption, cheaper price, less jitter and often stronger gearbox's.