nodemcu-firmware/docs/en/modules/switec.md

switec Module

This module controls a Switec X.27 (or compatible) instrument stepper motor. These are the stepper motors that are used in modern automotive instrument clusters. They are incredibly cheap and can be found at your favorite auction site or Chinese shopping site. There are varieties which are dual axis -- i.e. have two stepper motors driving two concentric shafts so you can mount two needles from the same axis.

These motors run off 5V (some may work off 3.3V). They draw under 20mA and are designed to be driven directly from MCU pins. Since the nodemcu runs at 3.3V, a level translator is required. An octal translator like the 74LVC4245A can perfom this translation. It also includes all the protection diodes required.

These motors can be driven off three pins, with pin2 and pin3 being the same GPIO pin. If the motor is directly connected to the MCU, then the current load is doubled and may exceed the maximum ratings. If, however, a driver chip is being used, then the load on the MCU is neglible and the same MCU pin can be connected to two driver pins. In order to do this, just specify the same pin for pin2 and pin3.

These motors do not have absolute positioning, but come with stops at both ends of the range. The startup procedure is to drive the motor anti-clockwise until it is guaranteed that the needle is on the step. Then this point can be set as zero. It is important not to let the motor run into the endstops during normal operation as this will make the pointing inaccurate. This module does not enforce any range limiting.

Sources

These stepper motors are available at the following locations:

• Amazon: These are X27 Stepper motors
• Aliexpress: These are X27 Stepper motors. They also have the Dual shaft version
• Ebay: These are X27 Stepper motors

Note

This module uses the hardware timer interrupt and hence it cannot be used at the same time as the PWM module. Both modules can be compiled into the same firmware image, but an application can only use one. It may be possible for an application to alternate between switec and pwm, but care must be taken.

switec.setup()

Initialize the nodemcu to talk to a switec X.27 or compatible instrument stepper motor. The default slew rate is set so that it should work for most motors. Some motors can run at 600 degress per second.

Syntax

switec.setup(channel, pin1, pin2, pin3, pin4 [, maxDegPerSec])

Parameters

• channel The switec module supports three stepper motors. The channel is either 0, 1 or 2.
• pin1 This is a GPIO number and connects to pin 1 on the stepper.
• pin2 This is a GPIO number and connects to pin 2 on the stepper.
• pin3 This is a GPIO number and connects to pin 3 on the stepper.
• pin4 This is a GPIO number and connects to pin 4 on the stepper.
• maxDegPerSec (optional) This can set to limit the maximum slew rate. The default is 400 degrees per second.

Returns

Nothing. If the arguments are in error, or the operation cannot be completed, then an error is thrown.

Note

Once a channel is setup, it cannot be re-setup until the needle has stopped moving.

Example

switec.setup(0, 5,6,7,8)

switec.moveto()

Starts the needle moving to the specified position. If the needle is already moving, then the current motion is cancelled, and the needle will move to the new position. It is possible to get a callback when the needle stops moving. This is not normally required as multiple moveto operations can be issued in quick succession. During the initial calibration, it is important. Note that the callback is not guaranteed to be called -- it is possible that the needle never stops at the target location before another moveto operation is triggered.

Syntax

switec.moveto(channel, position[, stoppedCallback)

Parameters

• channel The switec module supports three stepper motors. The channel is either 0, 1 or 2.
• position The position (number of steps clockwise) to move the needle. Typically in the range 0 to around 1000.
• stoppedCallback (optional) callback to be invoked when the needle stops moving.

Errors

The channel must have been setup, otherwise an error is thrown.

Example

switec.moveto(0, 1000, function ()
    switec.moveto(0, 0)
end)

switec.reset()

This sets the current position of the needle as being zero. The needle must be stationary.

Syntax

switec.reset(channel)

Parameters

• channel The switec module supports three stepper motors. The channel is either 0, 1 or 2.

Errors

The channel must have been setup and the needle must not be moving, otherwise an error is thrown.

Example

switec.reset(0)

switec.getpos()

Gets the current position of the needle and whether it is moving.

Syntax

switec.getpos(channel)

Parameters

• channel The switec module supports three stepper motors. The channel is either 0, 1 or 2.

Returns

• position the current position of the needle
• moving 0 if the needle is stationary. 1 for clockwise, -1 for anti-clockwise.

Example

print switec.getpos(0)

switec.close()

Releases the resources associated with the stepper.

Syntax

switec.close(channel)

Parameters

• channel The switec module supports three stepper motors. The channel is either 0, 1 or 2.

Errors

The needle must not be moving, otherwise an error is thrown.

Example

switec.close(0)

Calibration

In order to set the zero point correctly, the needle should be driven anti-clockwise until it runs into the end stop. Then the zero point can be set. The value of -1000 is used as that is larger than the range of the motor -- i.e. it drives anti-clockwise through the entire range and onto the end stop.

switec.setup(0, 5,6,7,8)
calibration = true
switec.moveto(0, -1000, function() 
  switec.reset(0)
  calibration = false
end)

Other moveto operations should not be performed while calibration is set.