Using a spreadsheet for complex motion

Mike Caputo, Applications Engineer, Parker Hannifin Corp., Compumotor Div., Rohnert Park, CA

Traditionally implemented by doing complex programming, parabolic, S-curve, or arbitrary-path profiles can now be solved using standard spreadsheets to design and simulate them.

Compumotor's 2-axis, PC-based AT6200 Indexer provides timed-data streaming mode (SD mode) with variable update interval, 64K on-board RAM, and Windows-based support software. The SD mode allows sending asynchronous data to the indexer, which synchronously executes it in 10 msecond intervals. On-board RAM allows storing data on the indexer card, and the Windows-based software allows generating data points in a spreadsheet and sending them to the indexer card.

Consider the work needed to implement a sine wave profile on Axis 1 and a cosine profile on Axis 2 (this system, mechanically implemented, follows a circular path). The formulae to produce this motion when the period of each profiles is 2 seconds and the amplitude 25,000 step motor microsteps, works out to:

D₁=25,000x[sin({pi}t)-sin{{pi}(t-0.01)}]

and

D₂=25,000x[cos({pi}t)-cos{{pi}(t-0.01)}].

Next calculate the derivatives of these position equations with respect to time.

Set up a column for the update interval of 10 mseconds in an Excel spreadsheet. Leave the first line blank, enter 0.010 in the second row, and use Excel's "Auto Fill" feature to fill the column down 200 rows. Next, enter the formulae for Axis 1 and Axis 2, and the table presents the position to which each axis must move.

Users can employ DOS-based spreadsheets as well as Excel. Macros could fully automate this process. With minimal experience in math, spreadsheets, and motion, anyone can use these tools to achieve a complex motion profile.

To contact a Compumotor applications engineer, call 800-358-9070 or FAX 707-584-3793.

Taking advantage of microprocessor-based speed control

Jeff Duncan, Applications Engineering,Danfoss Electronic Drives, Rockford, IL

Microprocessor-based variable speed controllers often require motor derating. They use PWM switching to modify the input waveform. Voltage harmonics and ripple current result because of the difference between the waveform the motor winding requires and the PWM drive's switch output. Ripple generates heat because of resistive losses.

In contrast, the Danfoss Voltage Vector Control (VVC) system uses a microprocessor to calculate real-time pulse widths. Experiments at Danfoss demonstrated that controller switching frequency determines speed control output quality. Higher switching frequency provides better sine wave approximation, but increases energy losses. Danfoss' research indicates that the optimum PVC on/off switch time is 200 museconds.

In the system, as switching frequency increases, it becomes less important to place the switching angles correctly, and less processing power is needed. The VVC system operates through the entire frequency range without loss of output voltage, reducing the harmonic current that generates ripple.

To control three-phase motors, the PWM switching must control the six switches in a three-phase inverter. Danfoss determined that the best quality PWM involves switching only during the 0- to 60-degree part of the cycle.

Used in the Danfoss VLT Series 3000 speed control, the VVC technology employs a 68000 series microprocessor and an ASIC for waveform generation. No motor derating is necessary.

To contact a Danfoss Electronic Drives applications engineer, call 800-432-6367 or FAX 815-398-2748.