27961 Parallax Inc, 27961 Datasheet - Page 8

• Manufacturer Part Number: 27961
• Description: MODULE MOTOR MIND SIP
• Manufacturer: Parallax Inc

Specifications of 27961

• Product: Microcontroller Accessories
• Operating Supply Voltage: 6 to 36 V
• Board Size: 30.48 mm x 25.4 mm
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Motor Mind B enhanced
Data Sheet
3.8 The PI Filter
Speed control mode can be used with an increment or decrement control scheme, or a Proportional
Integral (PI) filter may be applied to the error signal. In the standard increment/decrement mode the
Master unit issues a desired frequency with the SPDCON command, and the motor speed is incremented
(if the tachometer is lower than the desired value) or decremented (if the tachometer is higher than the
desired value). When configured for PI mode the error signal (desired frequency – actual frequency) is
multiplied by a Proportional term, then a sum of errors-over-time is multiplied by an Integral term, and
added to the previous result. The resulting 32-bit two’s compliment number is scaled down to a value
between +255 to -255. This value is applied to the motor as the duty cycle or speed control. It is
recommended that PI mode be used with the 15.5KHz frequency setting.
If tuned correctly the PI filter will provide responsive control of the motor speed and account for changes
in the motor load.
frequencies in the hundreds of Hertz or higher range. The components of the PI filter are modified with
the WRITE_PI command. The STATUS.PIMODE bit must be set before the PI filter will be used in
SPDCON mode. This bit may be toggled with the SPDCON_MODE command. SPDCON motor drive
updates occur 8 times per second.
Component of PID
PTERM
ITERM
PISCALAR
STATUS.PIMODE
3.9 Tuning the PI Filter
Tuning the PI requires a bit of trial and error. For starters you can change the ITERM to zero, and then
modify the PTERM until you get roughly the correct speed. Then increment the ITERM to fine tune
control. The PISCALAR is the “number of divide-by-twos” applied to the PI output to get the motor speed
update (PWM = PI/2
decreasing it increases the PWM (doubles it). Therefore, the PISCALAR will have a large effect on the
way PI mode drives the motor. If your motor is ramping (zooms up to top speed, then back to zero)
increase the PISCALAR. If changes in the PTERM do not create motor movement, then decrease the
PISCALAR.
doesn’t drive the motor at all. That’s okay, this would be the point where adjusting the PTERM would
take place.
3.10 External Capacitance on 5V / VMOTOR Pins
Electrolytic capacitors may be important components in your circuit.
located near the MMBe to provide short bursts of current and to smooth power supply spikes.
connections to your power supply or battery exceed 6” you should consider adding capacitance to the 5V
and VMOTOR pins. The VMOTOR capacitance should be capable of providing a ripple-current of ½ of
your peak current. Low ESR capacitors are good candidates for use at the VMOTOR pin. See Figure 2
for an example of capacitor placement
You may find that one value of the PISCALAR causes cycling, and the next higher value
Very low frequencies are difficult to control, so the PI filter is best used with
Description
The Proportional term is multiplied by the error signal (desired frequency – actual
frequency). This term has the greatest effect on changing the motor speed, and
will typically be much larger than the Integral term. This is an 8-bit value.
While the PI mode is in operation the error signals are summed in a register. The
Integral term is then multiplied by the sum of errors and the result is added to the
proportional term and error signal product. The integral term allows for small
corrections to motor speed to occur over time. This value will be much smaller
than the Proportional term. This is an 8-bit value.
The result of the previous multiplications will be much greater than the +/-255 used
for setting the motor speed duty cycle. Therefore the result must be scaled down.
The resulting output is divided by 2
Set this bit to enable PI mode for use with SPDCON operation.
PISCALAR
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). So making the PISCALAR larger reduces the PWM (divides it by 2),
PISCALAR
. This is an 8-bit value.
Some capacitance should be
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If