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Chapter 3: DSPpro Controller Design
The error (En) is the basis for changes in control voltage (On). To review how each element of
the PID algorithm affects motion, refer to appendix C, “Tuning Your System.” The same PID
algorithm is used for open-loop and closed-loop servo and step motor control. This feature makes
the programming the same for both motor types.
3.3 Hardware Features
3.3.1 Step Motor Control via VFCs
Step motors are controlled via the analog control voltage. The analog control voltage (DAC) is
connected to a voltage to frequency converter (VFC). The VFC generates a pulse train directly
proportional to the input voltage. The relationship between the analog control voltage and the
pulse output rate is constant and linear over the entire frequency range.
This is a major performance enhancement over timer/divider pulse generators that have
exponential pulse output rates and will cause large changes in step rate at high frequencies.
Since a step motor's torque curve is inversely related to its speed, it is more susceptible to stalling
at high speeds. Thus, a VFC at high pulse rates is less likely to stall a step motor. Another
advantage of the VFC is the high pulse output resolution.
The step pulse output supports the following speed ranges:
Slow
0 to 23 kHz
Medium
0 to 94 kHz
Fast
0 to 375 kHz
The voltage level at which steps are produced by the VFCs is determined by the internal DAC
offset set by the CONFIG program. This offset is usually required so that the DAC’s output zero
agrees with the VFC’s input zero. If the analog zero volt output is above the VFC zero input, the
VFC will begin to output step pulses.
The firmware guarantees the VFC will not output steps when command velocity is zero and the
position error is zero. If the offset is set too high, the firmware will automatically “shut down”
the step pulse output at the end of a move. If the offset was set too low, a small delay would
occur between the time the command position changes and the step motor actually starts moving.
The delay would be minimal, rarely exceeding two sample periods and would be based on the
acceleration and velocity figures.
One great design advantage to using VFCs based on servo DAC outputs is the same
programming is used to control either step or servo motors. If no motors are connected to the
DSPpro-Series controller, you can simulate motors by configuring the axes as open-loop
steppers.
3.3.2 Communication via three words in I/O Space
On a hardware level, the controller is a small microcomputer unto itself. It has its own address
and data bus that the DSP, its data memory, and peripherals (dedicated and non-dedicated I/O,
analog inputs and outputs, encoder inputs, and timer) use for communication. The 386EX has
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