POTs Calibration – RAMPS 1.4

The first batch of the Rostock 3D Printer BI Edition is driven by the Arduino Mega 2560 and the RAMPS 1.4 electronic package. This package is installed upside down under the top plate of the Rostock BI inside a protective PLA case.

Ramps 1.4 Installed on the Top Plate

Ramps 1.4 installed on the top plate

The Arduino is located under the Ramps 1.4

The Arduino is located under the Ramps 1.4

When you remove the protective PLA case and take a look at the RAMPS board you will find four A4988 Pololu Stepper Drivers equipped with heat sink.

There are four A4988 on the Ramps 1.4

There are four A4988 drivers on the Ramps 1.4

The potentiometers (POTs) found on each stepper driver are used to adjust the power delivered to their respective stepper motors. The initial adjustment of each POT is done at BI Labs (except for the DIY 3D printer), but you may find that over time they might require fine tuning. There is a small margin of adjustment for each POT that is optimal for your Rostock 3D printer. In this article we will cover the steps required to properly adjust the POTs.

If a POT is set too high then the associated stepper driver will tend to overheat and go into over-temperature thermal shutdown (to prevent damage to its components). The first sign of overheating is erratic stepper motor behavior. Typically, this can be recognized by the sounds of the stepper motor suddenly losing power (thermal shutdown). If no load or movement is required of the motor, it is hard to detect whether it is over-powered as the driver is barely producing any heat. To help you better understand, we’ve included a short video that shows the different behaviors of an improperly powered stepper motor.

 

please install flash

 

We talked about the over-powered state that can lead to erratic stepper motor control and thermal shutdown. Conversely, if the POT is set too low, the stepper motor can enter an under-powered state. This can be recognized by a lack of holding torque and a stepper motor that is skipping steps because the necessary movement  requires a higher power demand than the POT setting allows for.

Both situations are remedied by fine tuning the POT adjustment so that the stepper can provide enough power without overheating. To adjust the POT screw we recommend using a non-conductive flat screwdriver (#0).

Typical non conducting screwdriver

Typical non conducting screwdriver

If you turn the POT adjustment screw clockwise you will:

  1. Increase the power delivered to the stepper; and
  2. Increase the heat generated by the stepper driver.

Turning the POT counter-clockwise will have the opposite effect.

POTs screw with a clockwise sign

Increasing power output

POTs screw with a counter-clockwise sign

Decreasing power output

It’s important to note that some POTs do not have a physical stop at the minimum and maximum power setting. In the absence of a physical stop, you must be aware that there is a dead zone of rotation where the POT screw will be ineffective. In other words, making a full revolution will bring you back to the same setting but only a certain percentage of the revolution is effectively controlling the power output. Note: The image below depicts the “dead-zone” as 180 degrees. This is not necessarily the case, just know that there is a dead zone where your input will have no effect.

POTs with a dead zone illustration depicted in red

Dead zone depicted in red

The best way to calibrate a POT is to launch a print and adjust the POTs until you are satisfied with the power delivery. The ideal point is reached when your POT is set slightly higher than the minimal setting required to accomplish the task. The three tower stepper motors won’t require as much power as the extruder stepper motor.

Finally, we should point out that the fan enclosed in the PLA protective case plays a key role in keeping your POTs at a low temperature. As such, make sure to re-install the case when you are done.

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