440GTX

Is there any disadvantage of running 6V for the Reciever and Servos ?. I have a cycler which will charge 6 volts 5 cells.
Cheap extra speed and torque for my servos

Thanks in advance

Giant Scale

No disadvantage I can think of. The 6v packs tend to weigh more. This may be a consideration in a weight sensitive application.

sfaust

My Feedback: (11)

You will also use the capacity faster, thereby having less run time as compated to a 4.8 pack. I think its somewhere like 20% or so. Someone else might have the specific numbers.

Giant Scale

Steve,
Your right..I usually use 2 high capacity nimh packs (4000mah and a 2700mah) so I really never gave much thought to run time. I usually get tired of flying before the packs run out of juice.

Geistware

My Feedback: (16)

I have heard people say that more energy is used. This is only true if the servo consumes more power. if you have 5/4 more voltage, you should use 4/5 less current. The power should be the same and the consumption should be the same. I went from using 2 4.8V 1300maH receiver packs to 2 6.0V 1300maH packs on my Giles. I can fly all weekend on one charge where before I had to recharge at the end of the first day or during the night.

amcross

My Feedback: (2)

Geist, respectfully, the servo IS using more power, and giving you more speed and more torque, and as a result using up more current. ABSOLUTELY your run time is reduced, by roughly 15-20%, when going to 6.0V. You are also shortening the life of your servos slightly by doing so (except digitals which are ideal on 6.0V).

Rodney

Sorry Geistware, you have it backward. Current is voltage divided by impedance. Since the impedance is a constant, current will increase . Power or energy drain is also higher at 6 volts; power equals voltage squared divided by impedance therefore the difference ratio is 36 (6 squared) divided by 23 (4.8 volts squared) or you will draw roughly 1.4 times as much energy from the batteries over any given time period.

Geistware

My Feedback: (16)

amcross,
I agree that the servo has more speed and more available power, but that is "available" power. If it requires 20 oz of torque to move or hold a control surface and the torque output is fairly linear with the current used (I know, there is some loss) then at 6 volts it will take less current to create 20 oz of torque than it does at 4.8 volts. That is the reason you have a higher maximum torque because you have a higher maximum voltage to work from. What I don't know is if the consumption is linear or not. Does anyone have an equation for this?


Rodney,
Torque and power are proportional. THe more torque required, the more power that must be consumed. Power is voltage times current. The resistance is constant. If the voltage goes up 5/4 for the same amount of applied torque, then what happens to the current. Either it goes down, the resistance changes(up), or the motors are less efficient at higher voltages. You have to do the math!

In both cases, this explains why with my flying style, I use less current from the pack which make the pack last longer. (for my Giles, this is a FACT). No other explanation makes since to me but I will stand corrected if someone can show me otherwise.

amcross

My Feedback: (2)

Geist,
The servos don't care how much torque it takes to move hte surface. It puts out its full torque to do the job until it gets where it's going (within reason).

I'm sorry, but your intuition about usage is inaccurate. Having done extensive comparative tests, as well as read and seen information from the best electrical engineers I know, I can tell you the information provided above is 100% accurate. Going to 6.0V undoubtedly drains the pack faster to operate the same model in the same installation/conditions as 4.8V.

ilikeplanes

Errr, the servo does care what the load on the control surface is. Remember "For every action there is an equal and opposite reaction". If there is no load on the surface, like when the airplane is just sitting on the ground, the output torque is nearly zero. However, there is still loss in the servo due to rolling the gears over and turning the motor. Typical dc brush motors are 70% efficient at best, and only about 50% at no load or near stall. The idle current of some small dc motors is not much lower than at lock rotor. Other motors, like ones used in r/c cars and propulsion motors for airplanes, have extremely high lock rotor currents.

I used to fret over 6v systems. I finally tried it and liked the results. Now, after living with it for a while, it seems like a no brainer. To me, 4.8v is like not putting your foot to the floor when you need more power. You're simply not taking advantage of available power capability. Just make sure your mAh's are adequate to do the job. Same as always.

ilikeplanes

After my last post, I became inquisitive about all this servo stuff. I knew that Power = work/time and voltage = current/resistance and Power = voltage * current. But, what was the servo amplifier doing. Obviously, when the servo is standing still with no load, the motor voltage is zero (I confirmed this), but what is the motor voltage with a load?

Test #1
5 cell pack checked at 6.6v
Hung a known load (partially full paint can) on standard servo.
Motor voltage was 3.8v.

Test #2
4 cell pack checked at 5.2v
Hung a known load (partially full paint can) on standard servo.
Motor voltage was 3.8v.

THE SERVO AMPLIFIER APPLIES EXACTLY THE SAME MOTOR VOLTAGE AT A GIVEN LOAD
so, v=ir, v1=v2, r1=r2, i1=i2. 6v pack draws same current as 4.8 at a fixed stationary load.

WHAAATTTT????
Yep, it's true. However, when moving the servo from one position to the other, P=w/t and, as we all know, 6v is faster than 4.8v (the assumption is that the amplifier applies full battery "rail" voltage during a fast stick move). So, power goes up with increasing voltage and the current draw is defined by ohms law v=ir or i=v/r.
THE CURRENT DRAW WHILE MOVING IS PROPORTIONAL TO THE BATTERY VOLTAGE.

Conclusion: 6v packs drain faster than 4.8v because of the increased speed not because of increase output torque (unless you're flying at servo stall in which case the load and torque is also higher).

Ok, I doped my pants, now let me have it.

Geistware

My Feedback: (16)

OK, I was going to get back on and admit I was wrong but ask Ann Marie for any information that she had to explain it (25 yrs of designing automated machines can't be all wrong! )

This above explanation tells me I am wrong. The amplifier regulates the power output when the command and feedback Delta is near zero. But with the higher speed The consumption is higher because you are moving the same mass faster.

What I am curious about is if the pack voltage is higher at the same current, and the motor voltage is the same in both cases, why dissipate all that energy in the amplifier?

Ilikeplanes,
Could you do an experiment for me.
Use two power supplies and the first one controls the voltage to the receiver. The second one controls the voltage to the servo. (makes sure the grounds are tied!) Perform your previous experiment at 6 volts receiver and decrease the servo voltage until it effects the servo motor voltage. That is the minimum voltage that the amplifier needs to control motor voltage. I am going to assume one PN junction drop so the servo voltage should be somewhere around 4.5 volts.

I really would appreciate this. It helps me tie up some loose ends in my understanding.

Thanks in advance!

ilikeplanes

I'll have to bill you for that one.

The servo amplifier doesn't have to dissipate anything except a small amount of heat due to inefficiency (the amplifier is probably 95% efficient of better though). It simply regulates the motor voltage as required to maintain position (command voltage and feedback voltage equal). It's like a throttle on an engine.

Think of the case where you turn on the radio and receiver but never jiggle the sticks. The servo motor voltage is near zero and the current draw is very low. The battery will discharge at a very low rate. I'm sure your own experience verifies this. Remember ohms law. v=i*r If v=0 then i=0. The receiver and servos still have idle current draw though and will eventually drain the pack.

I'm sure the servo will check out at some low voltage level. 4.5v sounds good to me.

Geistware

My Feedback: (16)

I agree with your assessment 100% but I still have one question. Most DC servo control systems will consume the same amount of power for a given load. The big difference is if you can increase the maximum control voltage you can get more power out. At a fixed load, the power consumed by that load is the same regardless of the control voltage (servo amplifier buss voltage). With this said, higher buss voltage does not cause the motor to consume more power. In our case it does and that is what I don't understand. As you said in your example the servo motor voltage was the same 3.8 volts for either 4.8 or 6 volt batteries. We know that with a fixed armature resistance that the power is the same V squared divided by R. The power consumed by the motor is the same. The current is the same, so the power drawn from the 6 volt pack is higher than the one drawn from the 4.8 volt pack. If this is not waste, what happens to it????

I really hate to drag this out and I am not being difficult. I truly want to understand this. Being an EE forces me to want to understand the details!!!!

Another question has come to mind. If they use true PWM to control the servo motor, the response should be as quick but the precision should be better and the dead-band should be smaller! What do you think?

Rodney

A simple test that you can do to show that the higher voltages definitely cause higher current is to put an ammeter in series with the servo power lead (this must be a very good meter with very low internal impedance, not the average cheap multimeter) and then put a load on the servo to simulate air loads or back pressure that you get in flight. You will quickly see that the current is higher for the higher voltages, even with the same load until you get to the stall point at which time the higher voltage will really draw lots more than the lower voltage unit does.

ilikeplanes

Rodney, have you actually tried this? Seems to me that the current draw would be very nearly equal if the motor voltage was equal (as I measured). Of course, near stall, the 6v pack would have higher draw. If the control is PWM, everything is different. My equipment (digital multimeter) is not fast enough to decode the PWM peak voltage.

Ed

Geisty, it's not just "In our case", it's in ANY CASE ! It's simple OHMS LAW ! I = E devided by R ! If you increase E, the voltage, you thereby increase I, the current. Go figure. Use OHMS LAW !

ilikeplanes

Here's a new track...

Is it correct to say that standard "analog" servos have conventional analog circuits, and "digital" servos have PWM circuits?

Is an analog servo circuit more or less efficient than a PWM type?

thomasb

Yes, increasing servo voltage increases the current. The motor has a fixed impedance, so increasing voltage increases current and increases input power, hence increases mechanical torque and speed. Ohms law and the conservation of energy never lie.

In situations where higher voltages yield lower or equal currents, it is because the load had been changed or there had been manipulation of applied power. Given that the electrical load (servo motor) remains unchanged, any situation where you need more mechanical power you will need to force more current into the motor. You do this by increasing the applied voltage.

Despite the reported info, the voltage on the motor is increased when a 6V pack is used. A true RMS meter is needed to read the voltage accurately since it is not purely DC and it is not sinusoidal.

The use of PWM to control a common motor does not ensure any of those benefits. PWM is just another way to buck regulate the motor voltage. I do agree that PWM is found in systems that have precise control, but you can also find purely analog control systems that can do the same.

Both standard and digital servos use some form of PWM to control the motor. The newer digital servos just do it better.

In either case, they do not linearly control (series pass) the motor voltage and instead the bridge amp is either on or off. In the case of the standard servo, the PWM frame is sync'd to the Tx's frame period. Digital servos are sync'd to the frame period chosen by the servo's microcontroller (via firmware) and the period can be much faster than the Tx's frame frequency.

-------------------

Yes, I have done the 4-cell and 5-cell voltage/current measurements. While the servos are idle, the currents do not change on the higher voltage. But, anytime motion is requested, the currents on a 5-cell are about 20% higher. Sport flyers may not recognize any reduced flying time since they tend to not jam the sticks around. A 1/4 scale or 3D flyer certainly will and these are the pilots that tend to go with the 5-cell solution. But with today's high capacity mAH packs, the impact can be difficult to recognize.

I'll make one last stab at convincing you that current AND power does indeed increase when 6 volts is used. Here are some real numbers to play with -- use Ohm's Law to see the impact:

I = E/R
P = I x E
Servo motor impedance 8 ohms (for example only)

Example #1, 4.8VDC (4-cell)
4.8V / 8 ohms = 600mA
600mA x 4.8V = 2.88 watts

Example #2, 6.0VDC (5-cell)
6.0V / 8 ohms = 750mA
750mA x 6.0V = 4.50 watts

150mA increase = 20% more

Increased power input (watts) results in more mechanical speed and torque, even after considering any electrical losses.

Rodney

Thomasb, could not have said it better if I'd tried. A very good and accurate discussion. Yes, I have also done these measurements using good equipment and an oscilloscope. There are many fine points that are to numerous to mention here. I commend Thomas in saying it so accurately in so few words.

ilikeplanes

but, but, but...
The motor and servo amplifier circuit is not just a simple dc resistance like a light bulb. The motor voltage I measured was equal with my slow responding equipment. Since the motor is actually controlled with PWM, and the peak voltage will be different with a 4.8v and 6v pack, will not the duty cycle of the PWM controller be adjusted according to the load? For example: the duty cycle for a 4.8v pack may be 80% while the duty cycle for 6v is 64% at a fixed load?

So, apply the actual measured duty cycle to your simple resistor analogy and we're there.

BTW, I totally agree that 6v systems draw more than 4.8v in real world usage. I use both systems and know from experience. I'm just exercising my inquisitive mind.

thomasb

That is why I reference impedance rather than resistance. Regardless, impedance does NOT change if the source voltage is manipulated -- only the reactive and resistive elements are able to do that. For all practical purposes, whether you use a light bulb or servo motor, the theories discussed here will still hold true.

Remove the servo motor's main pinion gear. Give full Tx stick. You will see a motor voltage difference between 4-cell and 5-cell usage. If you don't, then you will have to drag the o-scope out to see the real amplitudes.

No, there is no additional intelligence in the servo to do that. If there was, then increasing the servo voltage would NOT increase its speed and torque. You can NOT get something for free, even when it comes to mindless electrons.

If you were talking about switch mode power supplies then you would be correct. But servos do not employ those sorts of tricks. If they did, then 5-cell packs would not be able to offer more power at the higher voltages.

I will repeat one last time: You cannot get more power or speed from the servo without paying the price. In this case, it is at the expense of input power. To increase input power, you must increase the source voltage. When you increase source voltage, you increase current in this application. THAT is the whole point.

ilikeplanes

Yes, I fully understand power and speed and the difference between a resistive and reactive load and that the universe is made of all sorts of particles.

I fully agree with you that if you move a given load faster or move a greater load, the power goes up. Power = work/time. I've already established that. I don't think anybody would even attempt to dispute that. I haven't.

But, what is the power consumed when holding a load and not moving it. Work = zero, so power = zero. Obviously not the case where a servo holds the load. And, is more or less power consumed by the servo to simply hold a load if the battery voltage changes?

The servo does have intelligence. It's in the form of the differential amplifier (or equivalent circuit) that gives a servo it's name. The servo motor voltage (or PWM duty cycle) is proportional to the difference between the command and feedback voltage.

thomasb

Yes, but it is clueless to what the source voltage is. The duty cycle will not change merely as a result of the higher battery voltage. Only the time it takes to get from "here to there" is changed, so if that is what you are trying to say then I agree.

Sorry, but I am out of wind on this one.

Geistware

My Feedback: (16)

Jim,
While I have worked with AC and DC motors with various controllers, I can tell you that if your controller is controlling the motor voltage in a closed loop system, that the output of the motor is the same regardless of the buss voltage. The buss voltage will effect the maximum power, but when the applied voltage (to the motor) is less than the smaller of the two buss voltages, and in both examples are equal, the power applied (and current) is the same.