[epineh]

This may be a stupid question and probly isn't the right place to post, but I was wondering if the brushless motors like the kind they use on the R/C planes could be modified for use as a servo motor ?

I have been looking at a lot of sites where people make their own motor's and the end results are quite impressive. I am not too sure how accurate the specs are but the larger motors are pulling serious current, and power in the KW.

The only downfall I can see is that possibly too high rotor inertia, though that may not be the case, coupled with a little too high running speed - 9000 RPM and up... way up.

But say for hobby use could it be a way to get REALLY fast servo's on the cheap, the motors I saw made were dead simple to make, much easier than a brushed DC motor would be.

Just a thought...

Russell.

[jrrdw]

Most of the eletric planes use gear boxes, 4:1 seams to be popular. As for stronger/faster servo's, you can get gear kits for them. Have you looked around www.TowerHobby.com, Hobby Lobby? Ebay is another good place for RC parts, i buy box lots get what i want, re-ebay what i don't want and wind up getting mine for free or real close to it. Been building my entire plane that way, allmost done now.

[Coogrrr]

can you give me a quick explanation on how a servo motor works in a cnc? I have steppers and the logic is simple. 1 pulse 1 step. 100pulses 100 steps.

Servos that run at 9000 rpm and have a gear reduction I understand, but! is the servo a DC electric motor? is it getting pulses and moving therefore 1 step per pulse or equiv?

I know total noob question but I am a noob only built one machine and I want to know if I should make the jump to those motors???

Coogrrr

[neilw20]

I put some hall sensors in the end of a car alternator. Use a MC33035 motrola BLDC controller. Easy to use.
Use 60 or 80 amp MOSFETS, with atleast 220 ohms in series with the gates. At below 30V high side P Channel MOSFETS simplify design enormously.
Optionally use a MC33039 closed loop adapter for excellent speed control.
How do you supply the field? Just put it in series with the DC power supply and put a flywheel diode across it.

Put 3 hall sensors between the slots, one on each phase, wherever you can fit them. To get the phasing right, put a car headlamp in series with the supply to limit the current, then get then keep reconnecting the hall sensor sequence until it works and does not get stuck. The chip has 60/120 phase degree selection, so it must eventually work.

For really fancy control use 100% duty cycle, and vary the supply electronically.
This stops all the sqealing noises inherent in these drive systems.
Only sqeals on current limit now.
Easy to get 1KW out of it and 6000RPM is a breeze. Just up the voltage untll it goes fast enough. You can get even more speed if you weaken the field current via various control means. Naturally the torque reduces.Constant power. Different speed.

[jrrdw]

can you give me a quick explanation on how a servo motor works in a cnc? I have steppers and the logic is simple. 1 pulse 1 step. 100pulses 100 steps.

Servos that run at 9000 rpm and have a gear reduction I understand, but! is the servo a DC electric motor? is it getting pulses and moving therefore 1 step per pulse or equiv?

I know total noob question but I am a noob only built one machine and I want to know if I should make the jump to those motors???

Coogrrr

Basicly, a servo is just a gearbox and motor contained in one unit for remote operations. They can be used for any kind of function needing push pull movment. The servo's i'm refering to are DC, equiped with a shift circute for reversing the main shaft direction, (3rd wire is for direction change). As for movment, basicly like a step motor but smaller, weaker. If you allready have step motors, i wouldn't switch. Hope that helps.

[epineh]

I think I need to clear a few things up, sorry for being vague in my initial post, by servo I mean a positioning servo for CNC use, which in its simplest form is a brushed DC motor with a quadrature encoder mounted on the shaft, this is driven by the electronics/machine controller to give precise positioning used by CNC machines.

R/C servo's are a different thing altogether, they do use feedback and simply adjust their position according to the PWM input from the reciever.

It is usual for CNC servo's to heve encoder counts in the 1000 to 4000 range, I won't go into too much detail, but with quadrature this gives positioning accuracy up to (and above) 16 000 "counts" (or detents) per revolution.

The servo is then bolted directly to a leadscrew / ballscrew or via pulleys, gears whatever, and usually driven by electronics which are in turn controlled by PC.

Neilw20 even though that wasn't really what I was after, that is really cool ! I will definately give that a go one day

What the servo needs to do is accellerate/deccellerate FAST, time in the milliseconds, and reverse direction constantly, which is why the rotor inertia needs to be low.

Anyway, thanks for posting guys.

Russell.

[collibric]

I'm also wondering about ac, 3-phase brushless motors used in rc models in combination with encoder.

Probably, there can be used motor like i.e. "team novak hv 4.5" http://teamnovak.com/products/brushl...5mm/index.html and some quadrature encoder from agilent.
Motor has hall sensors already installed.

In my opinion, the inertia is not a problem here (neodym, 14mm in diameter), but construction stiffness seems to be weak in harder load situations .. also cooling of this construction seems to be problematic ...

May be some modification of motor outer parts can solve this ...

[dmmcnc]

Russel's description for R/C servo is right, the R/C servo is made by a DC brush motor and gears also a forwar/reverse amplifer, is complete open loop
control, really not a servo, servo basically means closed loop control.
For the case of 3 phase DC brushless servo motor or called AC servo motor,
the high resolution encoder is mounted in the motor shaft for the position
feedback control, if the encoder read position is different from the set(desired) position, the calculated voltage will be applied to the motor coils
and let motor goto the desired position. that control is performed by a device
called motor drive.
One pulse sent to motor drive will let motor turn one count of the encoder
for example.

dmmcnc www.dmm-tech.com

[amplexus]

Russell
The larger bldc motors used in planes could be made to work, I have seen one on a minibike that went from 0 to 50 in 4 seconds or so. With an encoder you could do a pid loop. I think granite devices newest controller would work, but bldc motors have few standards so you may be faced with building your own motor controller.
coogrr
There is way more to steppers than 1 pulse one step, check out the cpld tutorial for a bit of insight into what is involved in a good stepper controller or read some of the faq on the gecko website. Servo's typically run in a PID loop. Google it for all the gory details.
The PID controller calculation (algorithm) involves three separate parameters; the proportional, the integral and derivative values. The proportional value determines the reaction to the current error, the integral value determines the reaction based on the sum of recent errors, and the derivative value determines the reaction based on the rate at which the error has been changing. The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve or the power supply of a heating element.

By tuning the three constants in the PID controller algorithm, the controller can provide control action designed for specific process requirements. The response of the controller can be described in terms of the responsiveness of the controller to an error, the degree to which the controller overshoots the setpoint and the degree of system oscillation. Note that the use of the PID algorithm for control does not guarantee optimal control of the system or system stability.

Some applications may require using only one or two modes to provide the appropriate system control. This is achieved by setting the gain of undesired control outputs to zero. A PID controller will be called a PI, PD, P or I controller in the absence of the respective control actions. PI controllers are particularly common, since derivative action is very sensitive to measurement noise, and the absence of an integral value may prevent the system from reaching its target value due to the control action.



Proportional term

Plot of PV vs time, for three values of Kp (Ki and Kd held constant)The proportional term (sometimes called gain) makes a change to the output that is proportional to the current error value. The proportional response can be adjusted by multiplying the error by a constant Kp, called the proportional gain.

The proportional term is given by:


where

Pout: Proportional term of output
Kp: Proportional gain, a tuning parameter
e: Error = SP − PV
t: Time or instantaneous time (the present)
A high proportional gain results in a large change in the output for a given change in the error. If the proportional gain is too high, the system can become unstable (See the section on loop tuning). In contrast, a small gain results in a small output response to a large input error, and a less responsive (or sensitive) controller. If the proportional gain is too low, the control action may be too small when responding to system disturbances.

In the absence of disturbances, pure proportional control will not settle at its target value, but will retain a steady state error that is a function of the proportional gain and the process gain. Despite the steady-state offset, both tuning theory and industrial practice indicate that it is the proportional term that should contribute the bulk of the output change.


[edit] Integral term

Plot of PV vs time, for three values of Ki (Kp and Kd held constant)The contribution from the integral term (sometimes called reset) is proportional to both the magnitude of the error and the duration of the error. Summing the instantaneous error over time (integrating the error) gives the accumulated offset that should have been corrected previously. The accumulated error is then multiplied by the integral gain and added to the controller output. The magnitude of the contribution of the integral term to the overall control action is determined by the integral gain, Ki.

The integral term is given by:



where

Iout: Integral term of output
Ki: Integral gain, a tuning parameter
e: Error = SP − PV
t: Time or instantaneous time (the present)
τ: a dummy integration variable
The integral term (when added to the proportional term) accelerates the movement of the process towards setpoint and eliminates the residual steady-state error that occurs with a proportional only controller. However, since the integral term is responding to accumulated errors from the past, it can cause the present value to overshoot the setpoint value (cross over the setpoint and then create a deviation in the other direction). For further notes regarding integral gain tuning and controller stability, see the section on loop tuning.


[edit] Derivative term

Plot of PV vs time, for three values of Kd (Kp and Ki held constant)The rate of change of the process error is calculated by determining the slope of the error over time (i.e., its first derivative with respect to time) and multiplying this rate of change by the derivative gain Kd. The magnitude of the contribution of the derivative term (sometimes called rate) to the overall control action is termed the derivative gain, Kd.

The derivative term is given by:


where

Dout: Derivative term of output
Kd: Derivative gain, a tuning parameter
e: Error = SP − PV
t: Time or instantaneous time (the present)
The derivative term slows the rate of change of the controller output and this effect is most noticeable close to the controller setpoint. Hence, derivative control is used to reduce the magnitude of the overshoot produced by the integral component and improve the combined controller-process stability. However, differentiation of a signal amplifies noise and thus this term in the controller is highly sensitive to noise in the error term, and can cause a process to become unstable if the noise and the derivative gain are sufficiently large.


[edit] Summary
The proportional, integral, and derivative terms are summed to calculate the output of the PID controller. Defining u(t) as the controller output, the final form of the PID algorithm is:


where the tuning parameters are:

Proportional gain, Kp
larger values typically mean faster response since the larger the error, the larger the Proportional term compensation. An excessively large proportional gain will lead to process instability and oscillation.
Integral gain, Ki
larger values imply steady state errors are eliminated more quickly. The trade-off is larger overshoot: any negative error integrated during transient response must be integrated away by positive error before we reach steady state.
Derivative gain, Kd
larger values decrease overshoot, but slows down transient response and may lead to instability due to signal noise amplification in the differentiation of the error.

Amplexus

[amplexus]

Neil,
I knew people had done this sort of thing but I never took it seriously however I am interested in hearing more about what you have done. Can you post any more details pics etc.
amplexus

[neilw20]

I've made various controllers, including one that uses a PID loop to control heating in a resistor.As applied, the result is a calorimeter measuring how much heat is taken from the resistor with water flow.
The resultant signal, at least until we run out of watts is proportional to the water flow. Most times you only need to know if a flow is above or below a threshold to turn on a pump, so going non linear only stuffs up the time it takes to respond to the off condition.
Motor controllers, using a PIC or similar is not too hard.
Attached are some app notes, that make it easy to figure out how to do the maths with a not to mathematically adept CPU like a cheap PIC
There is a 500K limit for PDFs and zipping them failed too.

http://www.freescale.com/webapp/sps/...p?code=BLDCMTR
And somewhere there find. Google the following and you will easily find a copy.

AN1215.pdf
AN2957.pdf
Apr5.pdf

Digging through these files makes it much easier to implement PID control in cheap CPU's
Plenty of stuff on the web if you search carefully with google...

[amplexus]

Thank you,
what I was most interested in is what you did with a car alternator, any idea how accuratre it is with a decent encoder? Also how did you precisely position the hall sensors? Can you do it without hall effect, many bldc do not use them? how did you do your fancy control, pwm, variable frequency or something else?
Amplexus Ender