The MG90S servo may be the most widely copied hobby servo today. Originated by the manufacturer Tower Pro, it is widely copied, cloned, and counterfeited.
This article will explain the differences between genuine Tower Pro MG90S and commonly available clones and counterfeits.
For the purposes of this article, we'll use the following definitions:
- Genuine Tower Pro MG90S: An MG90S servo actually manufactured by Tower Pro.
- Clone MG90S: A servo marked MG90S and generally compatible (same size, etc) with applications that use the genuine Tower Pro MG90s, however its label does not claim it was made by Tower Pro. In other words, the manufacturer is not pretending the servo was made by Tower Pro. This is completely legitimate.
- Counterfeit MG90S: A servo that is marked "Tower Pro MG90S" but was not actually manufactured by Tower Pro. In most countries this violates various intellectual property laws and is illegal. In some cases names very close to "Tower Pro" are used, such as "Tower Prop" or "Turbo Pro". A name that is purposely trying to fool the customer into thinking the servo is made by Tower Pro is also considered a counterfeit in this article.
Tower Pro MG90S Characteristics
The Tower Pro MG90S has these characteristics:
- It is a digital servo, meaning it has more sophisticated internal circuitry that generally provides more torque, more holding power, and faster updates in response to external forces. Digital servos can also generally take faster PWM signals, meaning it can receive updates from the microprocessor more quickly. However, digital servos can also be somewhat more subject to noise on the power rails, see discussion below.
- Very small deadband, 1 microsecond. This is somewhat technical, but the "deadband" is the amount of error the shaft position can have from the commanded position without the servo trying to adjust power to compensate. A deadband of 1 microsecond means the servo essentially will try to adjust the position if it is off by more than about 0.045 degrees. This is an extremely tight deadband and can cause issues, more discussion is below.
- High quality gears made from aircraft grade aluminum. Tower Pro gears rarely bind up and generally turn smoothly without any clicking or grinding noises.
- When running under load, you will typically hear a high pitched squeal as the digital servo circuitry compensates for loads on the shaft. However you will generally not hear clicks, pops and loud buzzing noises. (Exception, see "hunting" below).
Technical specs (from the Tower Pro official spec sheet):
- Weight: 13.4g
- Dimension: 22.8×12.2×28.5mm
- Stall torque: 1.8kg/cm (4.8V); 2.2kg/cm (6.6V)
- Operating speed: 0.10sec/60degree (4.8V); 0.08sec/60degree (6.0V)
- Operating voltage: 4.8V~ 6.6V
- servo wire length: 25 cm
Counterfeit MG90S Characteristics
While it is difficult to be specific due to the large number of counterfeiters, generally speaking this is what we have found after examining thousands of counterfeits:
- Most counterfeits claim to be digital servos but they are actually analog servos.
- Even as analog servos, many of them are defective. For example, there are sometimes batches of counterfeit servos where a large percentage of the servos will exhibit a shaft "drift" when they heat up. In other words, the shaft may drift (usually counterclockwise) anywhere from 2 to 90 degrees as the servo gets warm through use. This drift will revert when the servo cools. If the servo only exhibits a small (2 to 4 degree) drift, then it is usable for many tasks. But clearly, if the drift is as much as 10, 20 or even 90 degrees as we have seen sometimes, the servo is useless for most purposes.
- Gears are metal, but they are of very low quality. About 10 to 15% of all the gearboxes are bad right out of the box and will either fail immediately or within a very short period after use. Even gears that work are of obvious low quality. For example you can see right through the plastic case that there are striations, misshapen teeth, etc. If you carefully turn the shaft by hand, you will frequently feel rough spots, clicks, and pops as bad teeth engage and disengage.
- When running under load, you will frequently hear a lot of chatter, jitter, and noise even when the loads are steady and not changing (such as when the Vorpal Hexapod is standing still).
The technical specs given by vendors who sell counterfeits will match the real Tower Pro specs, however, these specs mean nothing because these are not genuine servos!
Typically the clones will have less than half the torque, be much more sluggish when responding, and will consume more power than the genuine Tower Pro MG90S servos.
Clones vs. Counterfeits
Many clones are identical to counterfeits (other than the label) had have all of the same problems.
However, there are some manufacturers who put their well known brand name on clones, and they do a good job of ensuring quality.
One of these "good clones" is the Turnigy MG90S, sold by Hobby King. This MG90S clone is very good quality. It really is digital and has good torque and speed. Personally, we prefer the Tower Pro MG90S gearboxes though, because from our testing the real Tower Pro MG90s have a little smoother gearing and don't have any noticeable binding when turning by hand. But if you use the Turnigy MG90S you'll get a good servo at a nice price.
How to Tell Genuine Tower Pro MG90S from Counterfeits
If the label is the same, how are you supposed to tell whether you are getting real Tower Pro MG90S or counterfeits?
There are some definite red flags to look for:
- If the price is less than about $4.50 USD, it is a probably counterfeit. If the price is under $2.50 USD, it's almost certain to be a counterfeit unless it is some kind of "clearance sale" where the seller is willing to lose money. Genuine Tower Pro MG90S have a wholesale cost well above $2.50 even in massive quantity purchases (thousands or tens of thousands).
- If the vendor covers up the name on the label in the catalog picture, it's definitely counterfeit. This means Tower Pro went after them and forced them to stop using the Tower Pro name. If you buy such a "label covered in the picture" servo and it says "Tower Pro" on the label when you receive them, it's almost guaranteed not to be genuine Tower Pro.
There are also some physical characteristics to look for that are pretty obvious and can be used even if all you can see is the label. Again, we have to caution that there are many different counterfeits and clones, we're only showing one of the most common ones here.
A few characteristics of the label printing style are a dead give-away as shown here:
From the top view, the genuine Tower Pro MG90S has deep "dimples" around the bracket, and the washer under the plastic where the shaft comes out of the housing is dark in color. By contrast, fake MG90S have little or no dimple marks and have a light copper colored washer under the shaft. Copper is a cheaper metal than the high quality aluminum used in the real Tower Pro servo.
Using Genuine Tower Pro MG90S with Vorpal The Hexapod
While the genuine MG90S are superior in just about every way imaginable, there are a couple of things you need to know when using them with Vorpal The Hexapod.
The information below also applies to "good clones" that are digital like the Turnigy MG90S.
Digital servos have far more torque and are far faster than analog servos. They also have a very narrow "deadband" meaning they generally have much less error when reaching a commanded position. This is all great news! But there is one side effect of that extra power and narrow deadband that is undesirable: hunting.
Let's consider a "hip" servo (the servos attached to the base) swinging a leg to a new position. Because the leg has weight and therefore inertia, what can happen is that the hip servo overshoots the desired position. The greater speed and torque of the digital servo makes this much more likely than with an analog servo. Once the servo overshoots, it needs to reverse power and try to bring the leg back to where it's supposed to be. Because of the very narrow deadband, however, it is almost impossible for the servo to reach the desired position without overshooting!
The result is a rapid quiver around the commanded position. This situation is called "hunting". The servo is searching, or hunting, for the desired angle, but it never finds it because the inertia of the leg keeps causing it to overshoot beyond the deadband range.
Because hip servos have no intrinsic bias, the leg can resonate like that indefinitely. The knee servos, on the other hand, do not exhibit this kind of hunting behavior to any significant extent because gravity biases the movement in one direction and not the other. This causes the hunting resonance to break up before it even starts.
Solution to Hunting
The solution is actually quite simple: add a small rubber washer or O-ring to the shaft of the hip servos. All of the kits we provide that have genuine Tower Pro servos come with the right size washer (some of the first kits we sent that included Tower Pro servos had O-rings instead, which also work). You just slip this on before you put the servo horn on the shaft.
The reason this works is that it provides a little bit of friction that damps out the resonance effect. This friction reduces the amount of overshoot enough to kill the resonance.
Note that without the servo horn screw in place, the leg may still exhibit "hunting". The screw is needed to clamp down on the servo horn and press it against the washer. You should only tighten the screw enough to stop the hunting behavior. We don't want to add so much friction that battery run-time is reduced.
Normally you only need the washers on the hip servos. Sometimes the front two legs will show some of the quiver associated with hunting when in F1 or F2 "fight" mode. If you want, you can add washers to the knee servos on the front two legs to damp out the hunting there, but it's not really necessary. Some people think the knees quivering like that in fight mode actually looks cool!
Both analog and digital servos are subject to oddities that occur due to electrical noise. Because Vorpal The Hexapod has 12 servos all sharing the same power supply, there can be some feedback in the signals going to each servo which causes this noise.
For analog servos, this will usually manifest as little clicks and twitches in the servos. For digital, that can happen, but sometimes a more serious situation occurs where the servo goes off to a weird angle for a moment.
The solution here is to make sure you're running the latest version of Vorpal The Hexapod's robot code, which is optimized for digital servos. This code runs the signals much more quickly out to the servo, at 120 Hertz rather than the 60 Hertz we used to use for analog servos. The faster signal doesn't stop the noise, but it causes new updates to go to the servo more quickly which allows the noisey signal to be overridden by a good signal in a shorter time.