Back to Back Motor Control
Back to Back Motor Control
(OP)
We are building a high inertia robot that requires very high precision on several rotary axis. We need a a system to negate the backlash and have decided that Back To Back motors would be best. Because one motor is "dragging" on the other Indramat and Siemens solutions only allow one motor to drive in one direction ie. both motors must be capable of producing the full torque required for the axis. Does anyone know of a controller that would allow torque "sharing" (so that we could have two smaller motors) between the two drives?
any help gratefully received!
any help gratefully received!





RE: Back to Back Motor Control
RE: Back to Back Motor Control
http://www.engineeringtalk.com/news/len/len120.html
http://www.nasatech.com/Briefs/June01/NPO20904.html
http://161.58.192.251/_fe_files/PDF/Fine_Cyclo/FineCyclo-004_Overview.pdf
etc. for more info
A motor controller can be developed precise enough to produce the negligible backlash
RE: Back to Back Motor Control
I have done applications like this where the customer took 1 motor into 2 gearboxes, then on the output of the gearbox, the preloaded the two against each other. Think of a rack and pinion, then having two pinions on it. Preload the 2 against each other to eliminate backlash.
In this case, we want to load the motors against each other. For control, I think using more like a cam table to do this rather than gearing because we want position based, not speed based. Some how the system would have to on power up, rotate the motors in oppisite direction to preload them, then enable camming and keep one motor always behind the other, keeping the mechanics stiff, but also still both of them are moving the load.
Can you define more in detail what your trying to do exactly.
Thank you,
Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. - www.aerotech.com
"Dedicated to the Science of Motion"
RE: Back to Back Motor Control
RE: Back to Back Motor Control
I think that possibly in this application, that 3D error mapping may be very benificial. Mapping out the positions of the end-effoctor and createing a calibration table to compensate for errors in mechanics...still can't full compensate for backlash...so large direct-drive motors may be the way to go.
Maybe look at how GE Fanuc builds their robots. Now when you say robot, are you talking about Gantry/Cartesian, SCARA, or Articulated? Or is this a non-traditional robot system that you just call a robot?
More details when you can please.
So we don't get on a "Robot" argument....
ro·bot n. (Courtesy of Dictonary.com)
A mechanical device that sometimes resembles a human and is capable of performing a variety of often complex human tasks on command or by being programmed in advance.
A machine or device that operates automatically or by remote control.
A person who works mechanically without original thought, especially one who responds automatically to the commands of others.
Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. - www.aerotech.com
"Dedicated to the Science of Motion"
RE: Back to Back Motor Control
The Dual Motor system came from my boss who has used them in the past. The controller that he used previously meant that the combined power of the motors was equal to running torque required. Indramat and Siemens systems on offer today do not support this configuration meaning that you must spec each motor to the full running torque thus putting the price up.
The robot structure is a little unorthodox. It has a central column with linear vertical movement (rack and pinion) and a carriage that a set of grippers will be mounted upon that is moved horizontally on another rack and pinion. The gripper head will have a "yaw" movement (double drive system), -45degree to +15degree pitch, 180degree roll movement and the grippers themselves. The complete structure will rotate at the central column by a double drive Slew. The maximum distance from the center of the robot to the gripper tips is 6m with a max payload of 500kg and an accuracy of +/- 1mm.
You can see why we need to remove as much of the backlash as possible but as an OEM we also need to keep the costs down. There will have to be interpolation between the slew (main rotation), horizontal and the yaw axis to keep the center line of the grippers perpendicular to the 3m long work piece as the robot rotates left and right. The pitch, roll and vertical functions will only be for special manipulations and will be in one position for most of the working life. The robot is completely automated with the option of an operator taking over if something goes wrong.
So, we are basically looking for a motion controller/double drive controller that can interpolate between 3 axis, control a further single axis (vertical) and be happy with an Siemens S7 PLC doing the sequencing :)
Thanks again for the help and ideas!
RE: Back to Back Motor Control
I believe that "gears boxes" or gears when meshing,have to have a certain amount of Backlash.There is no getting away from it.As far as the motor torque being equal to load torque,just like backlash ,motor TQ has to be > load Tq otherwise we have a Tie.No "Work" can be produced.
Good roboting
GusD
RE: Back to Back Motor Control
So any backlash in the mechanics will be compensated for, you will strictly position to the encoder on the load.
Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. - www.aerotech.com
"Dedicated to the Science of Motion"
RE: Back to Back Motor Control
RE: Back to Back Motor Control
If you had the servo vendor supply half the torque on each motor with a separate power unit, you should be able to run one as lead and the second as loadshare. The load drawn by the lead would limit the torque on the slave to the same value. The limitation of this comes when you are close to zero torque or the torque is reversing. You have to do some fancy software maniputlation to guarantee the slave torque is the same direction and magnitude as the master.
GusD is correct when he says all gear boxes have backlash. Even those that are sold as zero backlash have some. I guess I will have to go over your description again to see why two motors is helping this.
ON another issue, Servocam, I agree with Eoghann about the encoder mounted on the machine instead of on the motor. I have never been successful in getting the performance I wanted out of doing this for the exact reasons he mentioned, although all servo manufactureres say you can do this. Have you ever been successful with it and was the application demanding?
RE: Back to Back Motor Control
Yes, we do it all the time with our controllers on our stages. Rotary encoder on motor & linear encoder in the stage. Of course we are using a preloaded nut, but there is still a degree of backlash. I have done it in other applications on belt drives using an rotary encoder on the other end, or even a linear encoder. We have also put linear/ring encoders on worm-drive rotary stages to close the position loop.
Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. - www.aerotech.com
"Dedicated to the Science of Motion"
RE: Back to Back Motor Control
RE: Back to Back Motor Control
I believe if you can get a handle on describing your system mathematically, you should be able to design a control loop which monitors motor current and thereby control torque. Just remember if/when the system changes direction, that kinetic energy has to go somewhere (regardless of the control system), and protection on the drive electronics also needs to be implemented.
Good Luck!
RE: Back to Back Motor Control
We have found two companies who have implemented this system: NUM-Schneider and SEW Eurodrive, applied mainly to large CNC machinery.
Initial figures for the motors show that in single-direction motor movement we need a little over 100NM (at the motor) to accelerate but with the two drives running in tandem this is dropped to 38NM!! Cost difference of about £2000 per axis.
As far as I understand it (still deep in talks with their Apps guys) the motion controller keeps both motors driving until the very last part of the decelerate-to-position movement then switches one of the motors to lag the other thus pulling the robot consitently to one side of the backlash and letting us hit our repeatability targets :)
Anyone intrested drop me an email and I'll send out information/results once the project is completed/moving (soon-ish).
Cheers for the ideas!
RE: Back to Back Motor Control
Just curious.
RE: Back to Back Motor Control
If you're ever in California, I invite you to stop by the Point Magu Naval Air Station and have a look at the antennae drives for the pedestal mounted dishes.
They have precision gearboxes and back-to-back motors controlled by a dc servo drive system that results in .....ZERO backlash..... and hold position in varying wind conditions.
RE: Back to Back Motor Control
RE: Back to Back Motor Control
p.s. I was there for the commissioning of the drive system and witnessed the measurements....a low frequency bi-polar squarewave was applied to the input of the servo loop to test the system response and check for backlash.
p.p.s. Perhaps you were absent the day this lesson was taught.
p.p.p.s. There has been much written on ZERO BACKLASH systems over the last 50 years.... A lot of it in the Servo Drive community. Perhaps you'd be inclined to crank up your search engine and probe for Antennae Drives. Makes for some fascinating reading.
RE: Back to Back Motor Control
RE: Back to Back Motor Control
http://%20www.avltech.com/Pages/RotoLokWEBpg2.html
for "zero to nearly zero backlash"
Of course, in some instances or statistically, there may happen to be a zero backlash, however, the worst case is the nearly zero backlash.
RE: Back to Back Motor Control
In using a search engine to investigate "Zero Backlash" ... one of the things found would be:
Textron Power Transmission - AccuDrive, Benzlers, Cone Drive, ...
Absolute Zero Backlash. Double-enveloping ... Many Cone Drive products are
available with Absolute Zero Backlash worm gearsets. Absolute Zero ...
www.textronpt.com/cgi-bin/ products.cgi?prod=zero&group=worm
And there are many other such products, you'll find, that afford ZERO backlash.
One wonders if after splitting hairs, spltting atoms is the next task ..
RE: Back to Back Motor Control
ftp://ftp.kollmorgen.com/dist/techdocs/app_prod_notes/600rdpbacklash.pdf
We are finalising the supplier list at the moment and gathering quotes. It is more than likely that NUM (www.schneider-num.com) will be chosen due to the turnkey solution they have offered. They propose a full CNC controller which also has full PLC capabilities so we no longer need a PLC for interlocking with support equipment. They have many of the Back to Back systems in operation (one in Glasgow that I shall be visiting).
RE: Back to Back Motor Control
The backlash between both the left motor and the load, and the right motor and the load, was adjusted to be approximately 0.010” (0.4mm).
The backlash is adjustable. Apparently, it is somewhat greater than zero due to the manufacturing tolerances of the equipment.