CNC servo motors explained?

[mrclauds]

Hi guys

I understand rc servo motors, but see that on my CNC's, there are alot more than 3 wires coming from the servo...

Can anyone offer me links where I can go learn more about these servos?
All searches just keeps showing me rc servos...

I would like to build some drives for my machines but need to learn more about cnc servos...

Thanks
CLauds

[Al_The_Man]

There are basically three type of servo motor, the DC brushed (2 wire) and BLDC, (brushless DC) and AC sinusoidal, both of these have three stator leads and have the identical appearance the difference is in the BLDC has two windings energised at any given time, hence brushless DC, the AC sinusoidal has all three winding energised by three phases 120° apart.
All servo motors should also have one other conductor connected to the frame for Earth Ground.
The other wires on these motors would be the encoder and for the BLDC and AC SS would also be conductors to the drive for the purpose of commutation.
Link to BLDC animation.
http://users.tinyworld.co.uk/flecc/4...otor031102.swf
Al.

[askjerry]

What you are describing are the Stepper type motors, these are not servo motors... I'll explain them shortly.

An typical RC Servo has 3 signal lines...

• 5v (power)
• Pulse Input
• Ground

To operate it, you send a pulse with a certain width... depending on the width the servo will move left or right until it's internal circuit matches the width you are sending.

In the CNC world what you have is very different. You have a DC motor which is connected to a quadrature encoder. Quad means four... so the encoder is comprised of two LEDs and a movable mask... there are four possible states for the output...

• A B <--- Sensor input lines
• 0 0
• 1 0
• 1 1
• 0 1

If you know what state you are at now... then when it changes, you know if it moved forward or backward and can apply a signal to the motor driver to compensate. In a typical installation you send out a STEP and DIRECTION signal to a controller. (Such as a gecko drive.) The controller will then determine where the motor is now (from the quadrature encoder) and which way it needs to go. The further away it is from where it is supposed to be... the more power is applied... this prevents sudden jumps and starts.

Sometimes the quadrature encoder is part of the motor assembly, and sometimes it is a separate device such as those made by US Digital. In any event, these are the typical connections you are likely to see.

• Quadrature Encoder 5v Power
• Quadrature Encoder Ground
• Quadrature Encoder Output A
• Quadrature Encoder Output B
• Quadrature Encoder Index (optional)
• DC Motor +
• DC Motor -

The controller connects to the above plus has these additional connections...

• Step
• Direction
• Power
• Ground
• Motor Power
• Motor Ground
• Enable
• Error / Limit / Halt

A stepper motor is unlike a DC motor in that it doesn't just spin when power is applied. (The same is true for a DC brushless motor.) With these motors you send a signal to each internal coil in sequence which causes the motor to move to that position. If you encounter microstepping, that is where one coil is given 90% power and the next phase gets 10%... then 80%/20% etc... each one causing the motor to step slightly more... hence the name.

A DC brushless motor is usually not used for position as they are not as accurate as a stepper... the coils are pulsed in sequence to control the RPM... these are great for controlling spindle speed or similar constant speed devices.

More details here:

Rotary encoder - Wikipedia, the free encyclopedia

Brushless DC electric motor - Wikipedia, the free encyclopedia

Stepper motor - Wikipedia, the free encyclopedia

I hope that helps.
Jerry

[Al_The_Man]

You have a DC motor which is connected to a quadrature encoder. Quad means four... so the encoder is comprised of two LEDs and a movable mask... there are four possible states for the output...

Although in the differential encoder context, quadrature refers to the two pulses which are 90° apart, there is a common misunderstanding that it refers to 4x the basic pulse/rev.
BTW, I am using DCBL Servo Motor with Galil motion and 2000p/rev (8000P at 2000X4) encoders and can position to one pulse.
Another common mistake that DCBL servo motor cannot be used in servo positioning.
This does not refer to model aircraft type.
Al.

[askjerry]

The encoder I have on my system is a 500 line encoder, that's 2000 pulses per revolution or 1/10000 inch on my machine. (Wells Index 850)

[WCIS]

Have a look at Baldor.com if you want to see industrial type servos. There used to be a good tutorial on their operation there. For inexpensive versions try orientalmotor.com.

By definition a servo is ANY positioning device which uses position feedback to maintain position. This includes pneunmatic and hydraulic devices as well. All the versions previously mentioned fit this description. I might add each type has it's application. Stepper motors are cheap anymore and can offer decent positioning and motion characteristics, but has speed limitations. If not applied correctly they can wreck gear trains from their inherent vibration. Most stepper applications actually DO NOT use position feedback but instead rely on the motor "stepping" each time it's commanded to do so by the driver. A slight bind in the mechanical components can impede movement making this semi-open loop control scheme a risky one. Steppers can be made to hold their position by maintaining current flow through a winding. This too can be risky if not properly executed and can lead to overheating the motor - often to failure. Stepper motors can get quite hot during normal operation anyway and can give you a nasty burn.

AC servo systems offer much more robust and powerful motion control. Typically an AC servo is very much like a three phase squirrel cage motor. BLDC motors are similar in operation as well. Many AC servo motors can tolerate 150% - 300% overcurrent conditions in time limited fashion which can be an advantage to overcome stiction in machine tool applications. I've seen a great many 100 watt Mitsubishi AC servos not much bigger than a baseball break hardened .500" diameter shafts with regularity during the accel portion of the positioning curve.

Most DC and AC servo systems use two types of feedback and therefore, two control loops per motor. One for position, one for armature/rotor angular velocity (shaft speed). In most machine tool applications each axis movement is rate dependent, hence the requirement to use a feedrate during cutting operations. Endcoders are typically used for positioning feedback information while resolvers are used for velocity feedback. Without rate feedback cool things such as circular and helical interpolation wouldn't be possible.

Some clever microcontroller software can extract velocity feedback from an encoder using time measurement techniques, but this requires computational time. Very fast uControllers can pull this off. Resolvers provide an analog output and it's easier to build a very fast A/D converter than to use software.

So, while searching for information on servos use search terms such as BLDC motor, AC servo, etc.

If I were you I would look for controllers and amplifiers that can be purchased off the shelf. Will get your project running much faster. Plus, I would recommend AC servos and amplifers for any positioning if your budget will allow. Try eBay for used, but be careful buying and be careful wiring and using. Industrial versions use coil voltages from 170 vdc up to 700 vdc. And, size your wire properly. Use copper, and be sure the insulation is rated for at least the bus voltage plus some.

[Al_The_Man]

a
AC servo systems offer much more robust and powerful motion control. Typically an AC servo is very much like a three phase squirrel cage motor. BLDC motors are similar in operation as well. .

One main difference of course is a squirrel cage induction motor can never be synchronous, whereas a 3ph AC P.M. servo motor can.
Al.

[WCIS]

One main difference of course is a squirrel cage induction motor can never be synchronous, whereas a 3ph AC P.M. servo motor can.
Al.

Alas, my good man. But, an inverter rated squirrel cage motor driven by a vector mode inverter can come extremely close to synchronous servo systems. This scheme has been applied quite successfully. One example is rigid tapping. A machine that comes to mind in which this is employed is a Fadal VMC. The spindle is driven by a 15 HP squirrel cage motor.

[Al_The_Man]

But the very principle of induction motor theory is that the closest an induction motor can come to syncronism is within about 6 cycles at no load minimum otherwise motion (torque) will fail.
It does not matter whether it is driven by a VFD or not.
In rigid tapping, the Z axis is usually geared to the spindle encoder, so the Z will follow the spindle, not vice-versa.
An induction motor with VFD and encoder or pulse generator feedback can offer very tight speed control, but not synchronism, it will increase the frequency in order that the motor runs at a commanded rpm. but it is impossible to run at the same frequency as the applied frequency (synchronism would be zero torque!).
Al.

[mike_Kilroy]

cool discussion guys. i had to but in to say you are both right and wrong

of course induction motor has slip and will not go synchronous speed, but a good vector drive will make it go the required speed and position to do the job - if it is capable of it.... just dont need to get hung up on synchronous speed vs slip definition.

that said, an induction motor BY DESIGN will have a much LARGER rotor inertia than an equiv torque rated PM ac motor (AKA synchronous motor). since T=Jw/t, it will take a LOT more Torque to move that larger J inertia in a given t time; hence, it cannot be as responsive as its equiv sized PM ac motor. that in a nut shell is the only difference between the PERFORMANCE comparison of the two.

BTW, one way that vector ac induction motor drive makes the slip a non issue is by adding the slip in as a steady behind the scene frequency at all times. It is interesting when repairing real vector drives that if broke they often run at this 10rpm or so as the slip freq is rotating around the rotor at zero speed making it into a responsive motor

[mrclauds]

Thanks for all the info guys...
Seems another steep learning curve is on the horizon for me...

Let me give you guys an idea of where I am and what I plan to do so maybe I can get some answers to questions that have arised...

I own a high volume production engineering workshop, 18 CNC lathes, 9 CNC milling machines.
None are below 2kw motors.
All motors say 'servo motor' on label
I also have 3 robots that feed 9 CNC lathes amongst many other more manual machines.

I have made mechanical machines that have optomised the manufacturing of our main product, so I know mechanics.

About 3 months ago I delved into the world of electronics. I designed and have just completed a production monitoring rfid system (Built up from PIC's, and designed my own software to match)

Now I am looking at building a multispindle milling machine, and this is where I need the help...

1. Does anyone know which servo motor type is most common with fanuc, siemens,daewoo,Johnford,MC800 or kia CNC's? (ac,dc,brushless servos etc)

2. I see many guys have built small milling type machines, if I followed those guidelines and simply increased the motor power and matched the drive, would that work just as well?

3. How come the industrial CNC machines are so much more complicated that what is being done by the DIY builder?

Thanks
Clauds

[andypugh]

Hi guys
I understand rc servo motors, but see that on my CNC's, there are alot more than 3 wires coming from the servo...s

A motor makes torque. In a servo system you want to control position.
Some of the wires are power leads to supply the current tocreate the torque, hall sensors (or equivalent) to tell the drive where to send the current to make the torque and encoder/resolver leads to measure the position to close the control loop so that you get position control.

There will be two power leads on a DC, brushed servo motor and 3 on an AC servo, Permanent Magnets AC motor or Brushless DC motor. I am going to be contentious and state that BLDC and PMAC motors are effectively identical as far as we are concerned. BLDC motors often only drive one lead high and one low, but they work a lot more smoothly on a sinusoidal 3-phase drive.

Brushed DC motors use a commutator to send the current to the right windings to make torque and induction motors make torque regardless of rotor position, but the other motors need to know where the rotor magnets are so that they can energise the right coils in the armature to make torque. For this you need an absolute system, which gives the right answer from power-up. This is often done with 3 Hall (magnetic) sensors which cycle through 6 combinations of states to indicate where the rotor magnets are in the cycle. There are 42 possible combinations of these 6 states with 3 sensors, and so not all drives work with all motors. (deliberately).This 6-step sequence is often calles trapezoidal commutation. Fanuc use 4 signals in a Gray-code for 16 positions around the electrical cycle, and a semi-sinusoidal excitation sequence.

Resolvers give absolute feedback too. They have 6 wires, 2 for excitation and 2 x 2 phase outputs. By comparing the relative amplitudes of the sin and cos phases, and computing an arctan, you can tell exactly where the rotor is to arbitrary precision. Resolvers are tough, reliable and contamination resistant. They would be the perfect feedback system, if they weren't such a pain to interface. Resolvers can be used for both position feedback and motor commutation.

The most common position feedback system is the quadrature encoder, 2 channels (A and B) which output square waves 90 degrees (1/4 wavelength) apart. If A goes high with B low then you are going CW, if A goes high with B high then you are going CCW, so you get a sense of both how far (number of edges) and what direction the motor has moved. However, you don't know where you started from, so there is normally (but not always) an Index or Z phase to give absolute motor position. Often encoders use differential line-drivers, to you have +V, 0V, A and /A, B and /B and Z and /Z wires. Otherwise you will have +V. 0V, A, B, C. Or maybe +V, 0V, -V, A, B, C.

There are other encoder interfaces, serial, SPI, EnDAT, many proprietory. It's a lot easier to keep motors and drives together as pairs. However, if you want to mix and match then it can all be sorted out by software For example in EMC2 you can use the "bldc" component to convert any input to any output (including swapping between Hall commutation patterns)
LinuxCNC.org - BLDC

A you have mentioned, it is a large and complicated area.