AC vs DC servo motors in large scale cable robot design

Click For Summary

Discussion Overview

The discussion revolves around the selection of AC versus DC servo motors for a large-scale cable-driven parallel robot design. Participants explore the implications of motor choice on control systems, accuracy, and operational efficiency, considering both theoretical and practical aspects of the design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant highlights the importance of motor selection being dependent on the available controllers and the system's control requirements.
  • Another suggests using PWM-controlled DC servo motors with off-the-shelf H-bridges and shaft encoders for continuous monitoring of speed, position, torque, and load.
  • A participant proposes that rotary encoders should be independent of the winches due to the variable axle size created by cable winding on the drum.
  • Concerns are raised about potential slippage between the cable and the idler driving the encoder, suggesting alternative methods for accurate measurement, such as a follower arm on the cable.
  • One participant emphasizes the need for independent position measurement systems to avoid reliance on energy transfer systems, citing safety concerns and the risk of operator injury.
  • Another suggests using a modern equivalent to “ball chain” for reliable position indexing, indicating a preference for a system that minimizes slippage.

Areas of Agreement / Disagreement

Participants express differing views on the best approach to encoder placement and the implications of motor choice, indicating that multiple competing views remain without a clear consensus.

Contextual Notes

Participants mention potential complications with motor controls and the importance of independent measurement systems, but specific limitations or assumptions regarding the design and implementation are not fully resolved.

Who May Find This Useful

Engineers and designers involved in robotics, automation, and control systems may find this discussion relevant, particularly those interested in the practical applications of servo motors in complex mechanical systems.

cpcoe
Messages
2
Reaction score
0
hi, I'm designing a 50ftx50ft, 8-cable Cable Driven Parallel Robot and am thinking this is a pivotal decision. the corner posts are 10ft tall and the machine should be able to lift 100lbs at least 5ft off the ground. traveling speed of the end effector is less important than accuracy but I'm not trying to build a slug either! I am not attached to AC or DC, and the prices aren't that different in the 1/2hp range, will the controls and drivers present any hidden complications? what are the pros and cons of each system?
thanks for your help
 
Engineering news on Phys.org
Welcome to PF.
The motor selection will be dependent on the controllers available to you, and what will be controlling the system.
It is important that you make it work ASAP. The details can be changed later if you need to manufacture in quantity.
I would use PWM, DC servo motors, controlled by off the shelf H-bridges, with shaft encoders on the wire drums.
That will give a continuous estimate of speed, position, torque and load.
 
  • Like
Likes   Reactions: jim hardy
Thanks, I'm thinking the rotary encoders should be independent of the winches because the cable winding on the drum creates a variable axle size. I imagine a pulley wheel/ encoder mounted between the winch and end-effector for cable length and velocity measurement. In regards to controllers and controlling systems, I would like to be able to manually 'drive' the machine with a joystick (or something) and record the drive for repetitive playback. Any insight as to how this might look in hardware and software?
 
cpcoe said:
rotary encoders should be independent of the winches because the cable winding on the drum creates a variable axle size.
Taking into account Murphy's Law*, there will be slippage between the cable and the idler driving the encoder. I suggest a follower arm riding on the cable as it winds on the winch, and putting the encoder somewhere on the winch drive. Servo motors typically have an encoder mounted on their back end (it makes the gearing to the driven element simpler).

* Anything that can go wrong, will.

Cheers,
Tom
 
cpcoe said:
Thanks, I'm thinking the rotary encoders should be independent of the winches because the cable winding on the drum creates a variable axle size.
Correct. The rotary encoders are needed on the motor / drum to control the motor direction and velocity. The position of the load needs to be measured quite independently of any loaded positioning cable. A slipping or broken positioning cable will cause a detectable difference between the drum estimate and the real position. How else can it detect the next fault to occur?

Do not use the energy transfer systems to obtain critical positioning data. It will crush or strangle the operator.

If you must use cables to measure position, use a totally independent and parallel system that maintains the loop length without a spool. Make sure that the tension in the sensor cable is within narrow specified limits by using a spring or mass with switches to detect any length error. Kill the power and sound the alarm if it goes outside range.

Take a look at the modern equivalent to “ball chain”, as used on vertical window blinds. The string of spaced plastic balls moulded onto the fiber will not slip on a position index encoder wheel made with holes to accommodate the beads and a Vee groove to accommodate the fibre.
 

Similar threads

Microcontroller for DC Motor Controller
  • · Replies 9 ·
Replies
9
Views
8K