RE: Re: Low cost Servo Controller

Kirk, The S-Curve aspects of working with variable amounts of back lash under the constraints here with mean that it is impossible to match the performance of real servo systems. The firmware algroythm must keep the table motion cooridinated, point for point at the desired feed rates and solve the slop in the system. Of equal challange, is that fact that the feed rates must be able to be changed under the control of the program driving the controller. This means that while the control system must solve the problems encounted with the mill, it must also recognize the fact that the data rates of the signals from the computer represent feed rates that it must reconstruct while solving problems.

The worse case becomes when the job of keeping the mill doing exactly what it must do, takes too long and the data from the computer overflows the controllers ability to manage both. In this case, the speed at which the system can function will be limited by how difficult it becomes for the controller to hold the intended accuracy. A mill table that is so bad, that it travels like the S-curve itself (in 3 demintions) must be run at lower speeds (just like in real life manual machining). The challange for the control system is to operate within the envelop of accuracy first and speed second. A stepper system is limited in accuracy without feedback, and is limited in speed from it's own limitations and that of the driver controlling it. Naturally other speed and resolution limitations exist in all types of systems.

The potential advantage may come in that the controller can hold accuracy on worn out tables which are not worth ball screws and major service.... like the ones in many hobby shops. That is really what I'm trying to do. My bridgeport also has 0.030" of back lash, and exactly as I have been told, the best thing I can do at the moment to retrofit it, is to trade it in on a mill with the servos and leed screws in place. In the end, that is probably what I will do. Mean while, I believe there is a need to upgrade these things if only the cost were low enough.

Thank you for the links to both the information sites you listed. They will be useful if and when this becomes a project. I'll pass them along to the firmware department (if he shows up for lunch).

David Howland

-----Original Message-----
From: Kirk W. Fraser [SMTP:kirk@...]
Sent: Tuesday, September 14, 1999 10:01 AM
To: CAD_CAM_EDM_DRO@onelist.com
Subject: [CAD_CAM_EDM_DRO] Re: Low cost Servo Controller

From: "Kirk W. Fraser" <kirk@...>

Dave,

Sounds like what you're looking for is right at the state of the art in control. Two pieces of information to consider:

1) S-curves. This is the reduction of jerk in start and stop by dividing the motion curve into several parts: zero to maximum accelleration, maximum accelleration to maximum speed, maintaining top speed, maximum speed to maximum deccelleration, and maximum decelleration to zero motion. These stages of each motion can be controlled without complex math by giving a time in milliseconds to spend in each of the first two stages. Chips to do this aren't as cheap as your own custom circuit but here's a supplier: http://www.pmdcorp.com/

2) Adding a spring after the actuator. The idea here is to use a fairly stiff spring and a strain gauge to measure the distortion as the tool is engaging the work. The values of strain can be tuned by software control to a specific combination of tool and stock. Another advantage is safety as you can detect when the strain exceeds expected levels at any point and thus if something (like a human) gets in the way or a tool breaks it can shut down. A thesis describing this technology is at: http://www.ai.mit.edu/projects/cog/publications.html near the bottom of the page -- Williamson, Matthew. Series Elastic Actuators.

Combining these two technologies should help provide the best performance possible. The S-curve technology alone is supposed to double performance of CNC sewing machines. The spring technology can be used to tune it even better.

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