Torque, confused!
 

Okay maybe I'm making more out of servo selection than I need but like lots of people I want the best I can for my money. I have two gliders I'm puting servos in and the wings are my main concern. First my two planes are a 3.4 meter Minimoa and a Multiplex 4 meter Alpina. I have 6 thin servos purchased for the Alpina from Volz (Micro-Max X analog). With 56 oz-in at 4.8 volts its a pretty strong servo for 1/2" thick profile. For my Minimoa I'm looking at 3 servos so far. The Hitec HS-125 analog with 42 oz-in and the Hitec HS-5125 digital with the same torque as his analog brother. Both very thin servos designed specificly for wing installation. The third is the Multiplex Polo Digi 4 digital servo measuring in at 1/2" thickness.

Okay so if I take the specs at face value it looks like the Polo Digi or the Hitec HS125 will both do fine. The HS-5125 is nearly $75 each where the HS-125 is around $40 each. The Polo Digi is in the mid 40's too.

Now for the confusion, I called Hitec customer support and this guy tells me that a digital servo has more "holding" torque than analog. Last I checked when it comes to a "servo" motor, meaning a motor with a closed loop feedback system and volts determines speed and current determines torque. I understand ALL servo motors are DC NOT AC. As I understand from industrial servo motors, a digital circuit is more accurate facilitating smoother and more finite positioning.

Torque as applied is force at a given distance in rotation. As example 56 oz-in equates to a 56 ounces of force at one inch from centerline of rotation. Now how is this measured? Imagine tightening bolts in your engine requiring and equal amount of "torque". With a "torque" wrench we can determin this by tightening just enough for the gauge on the wrench to read what were looking for. In the case of a servo, its the opposite where the bolt might be turning and we hold our wrench stationary. so when force is applied against a load such as a servo motor being pushed by the force of a flying surface, isnt it the same to say a servos operating torque and "holding" torque are pretty much the same?

So the question here is, is it worth the extra money for digital servos if I'm not flying competition? Or is it important to have the latest and greatest for our little babies?

RE: Torque, confused!
 

First of all the digital servo we know is not the same as a digital servo in the industrial world. Our "digital" servo is actually an analog servo. Its the same as the standard servo with a microprocessor that allows it to pulse the motor at a higher rate. It still works on the same principle as the analog servo.
The holding power of a servo is the torque the motor develops at stall multipled by the gear ratio plus the bearing and gear losses.
The stall torque is the the torque the motor develops minus the bearing and gear losses.
If you test the servo you will find they are quite different. They are both measured in oz-in.
Our "digital" servos develop more torque at a given error signal than our "analog" servo. If they have the same motor and gear ratio they will both produce the same holding power and stall torque

RE: Torque, confused!
 

The operating torque is the force which will stop the servo from rotating, the holding torque is the force which can start to drive the servo backwards. Since digitals have much higher holding torque they are much better able to resist being blown back to neutral.

H

RE: Torque, confused!
 

I am curious about how you determined the torque value the airplane required?

RE: Torque, confused!
 

If the servos in question have the same gear ratio and the same motor they will develop the same torque after the amplifier is saturated. The digital servo just seems to have more holding power because it develops its torque at a smaller error signal. I guess you could say it has more holding power if you measure at an error signal less than the saturation point of the servo amplifier say 50us.

RE: Torque, confused!
 

The same motor in a standard and a digital servo do not develop the same power. In both cases the electric voltage supplied to the motor is not constant but switched on and off. The digital is done at a much higher frequency and a better mark/space ratio so the motor is given more electrical power and thus develops more power. Hence two servos with identical motors and gears but with analogue and digital amplifiers develop different torque and have different current consumption.

An analogue servo has a power level that varies over a distance from target, so if the arm is slightly off target it develops low power, and that power rises as the arm is moved further from target until the point is reached where it is developing full power. The distance from target over which that power rises for a digital servo is extremely narrow compared to an analogue servo so that a digi will be developing full power against a tiny movement on the output arm whereas the same movement on the analogue version will only be causing it to develop a small fraction of its power to hold position. You can treat a digi servo as if it is a switch that develops full power or nothing - put the slightest pressure on the output arm and it kicks back with full power, whereas the analogue servo has a sliding response so it can be dragged off position by quite a bit before it responds with full power.

RE: Torque, confused!
 

When the amplifier is in saturation the motor is not switched on and off. I am talking about when the error siganl is greater than 200 us. That is when the stall torque is developed and the servo is rated by the manufacturer. Then there is no mark/space ratio to worry about. Take a look at the Futaba web site where they discuss the digital servo and look at the curves. When the servos are in stall condition the torque developed is the same for digital or analog servos.
The digital servo does not instantally develop its maximum torque. As you can see from tthe curves it takes about 100 us error signal before maximum torque is developed. Thats about twice as good as an analog servo.
I have torque vs error signal curves of JR, Hitec and Futaba servos. Both analog and digital. If you would like to see them send me your address and I will send a copy. I can't email them.
Richard H. Kelly
[email protected]