SG90 is one of the cheapest and most common hobby servos available. That is probably because it's also the most commonly cloned hobby servo. If you bought one of these very cheap. there is a very good chance that it wasn't actually produced by Tower Pro, but it was manufactured in one of the hundreds small factories in China based on designs that have been passed from person to person.
As you can expect, the parameters of this servo, including its quality, but also exact dimensions, speed, torque, accuracy, dead band, maximum and minimum voltage, size of the shaft, etc. – will vary wildly depending where it is sourced from, and even when. It is common, for example, for the servo horn from a servo from one batch to not fit the shaft on a servo from a different batch. That means that any designs using those servos have to either have large tolerances, or be adjusted for the particular batch of servos.
Those servo typically come in a plastic bag, with a smaller plastic bag inside containing additional elements: a horn screw, two mounting screws, and three servo horns: a single arm, a double arm and an asymmetrical cross. The holes in the horns are spaced 2mm apart.
The brown, red and yellow wires are typically a little over 15cm long, and end with a standard JR female plug with 2.54mm pitch. That plug is a little thicker than 2.54mm near the top, so when a lot of them are put together, there have to be some additional space between them.
They have shape of the typical 9g microservos: 23mm long, 12.2mm wide and 29mm high. They have two "ears" with mounting holes, but the sizes of the ears and the holes vary greatly. They are held together with 4 long screws. When those are removed, they will break into three parts – the bottom with the wires and electronics, the middle with the motor and the pot, and the top with the gearbox and the shaft.
The gearbox is composed of four nylon double gears, mounted on two shafts – the potentiometer's and an auxiliary shaft. There is a fifth small gear on the motor's shaft. The last gear is connected with the main shaft, which has a piece of plastic preventing the servo from rotating all the way. There is no bearing – the plastic case of the servo holds the top of the shaft, and the brass bushing of the potentiometer holds the bottom. The gears can be very delicate, and it's quite easy to strip their teeth by trying to force the servo's movement by hand.
The electronics used in this servo vary, but are usually analog.The servo expects a PWM signal at around 50Hz, with the pulse width between around 400 and 2400µs, with 1500µs being the middle position. The maximum and minimum pulse widths vary between individual servos, even in the same batch. The exact amount of motion corresponding to a particular change in the pulse width also varies. The total range is usually a little over 180°.
The torque and speed of this servo also vary from batch to batch, but are somewhere around 1.8kg×cm and 0.12s/60° at 4.8V. This is affected both by the power voltage and by the frequency of the control signal – the higher the frequency and voltage, the stronger and faster the servo. However, this also affects its lifetime.
It is possible to modify this servo for continuous rotation, by cutting the limiters on the shaft, drilling the shaft to make it move freely on the potentiometer, and gluing the potentiometer in its center position.
It is also possible to add a wire to the center of the potentiometer to read the servo's position, and to one end of the motor's leads to read the servo's force – both signals will be between 0 and the power voltage.