Marlin + RAMBo: Unpredictable moves of the Z carriage;

Hi folks!
I'm trying to commission a new Delta printer and am struggling with a problem wherein whatever motor attached to either of RAMBo's Z dual drivers will move the carriage at a faster rate than X,Y carriages. The printer will home normally most of the time on the G28 command. However, issuing a relative by 100 units command (G91 G1 Z-100 F1000) will result in X and Y travelling the correct distance (as measured by a ruler) with Z intermittently accelerating during the move, thus travelling more than 100 mm. From what i can tell running multiple trials is that the distance traveled every time is different on Z. Swapping and connecting a different stepper motor to either of the Z drivers moves the problem to that column.

So far I've tried different microstepping and SPU settings with very conservative homing and feed rates and reduced XY and Z jerk to 10 but was unable to solve this yet by just tweaking the settings. I tried both 1/16th and 1/4th microstepping with the same symptoms. 1/8th doesn't seem to be a configurable option with Marlin and my Rambo as Marlin seems to represent MICROSTEP8 and MICROSTEP16 using the same HIGH,HIGH combination of the pins.

The current settings I'm on are:

#define MICROSTEP_MODES {4,4,4,4,4} // [1,2,4,8,16]
#define DEFAULT_AXIS_STEPS_PER_UNIT {20, 20, 20, 760*1.1}
#define HOMING_FEEDRATE {200*3, 200*3, 200*3, 0} // set the homing speeds (mm/min)
#define DEFAULT_MAX_FEEDRATE {1500, 1500, 1500, 25} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {9000,9000,9000,10000} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, for older versions raise them a lot.
#define DEFAULT_XYJERK 10.0 // (mm/sec)
#define DEFAULT_ZJERK 10.0 // (mm/sec) Must be same as XY for delta

Is there anything else I'm missing?

Thanks for all your help!
Val

RAMBo only has one Z driver, it just has two connections to the same driver for when you want to drive two motors in parallel (such as with a Prusa Mendel-style printer). It seems like the Z driver may be the culprit here. An easy way to check is to swap the pin numbers in pins.h so that the Z axis in firmware points to the E1 driver, and you can then plug your Z axis motor into the E1 port. It looks like RAMBo's pin settings for Z and E1 are as follows:

#define Z_STEP_PIN 35
#define Z_DIR_PIN 47
#define Z_MIN_PIN 10
#define Z_MAX_PIN 30
#define Z_ENABLE_PIN 27
#define Z_MS1_PIN 68
#define Z_MS2_PIN 67

#define E1_STEP_PIN 33
#define E1_DIR_PIN 42
#define E1_ENABLE_PIN 25
#define E1_MS1_PIN 63
#define E1_MS2_PIN 64

So swapping the pins for Z and E1 would look like this:

#define Z_STEP_PIN 33
#define Z_DIR_PIN 42
#define Z_MIN_PIN 10
#define Z_MAX_PIN 30
#define Z_ENABLE_PIN 25
#define Z_MS1_PIN 63
#define Z_MS2_PIN 64

#define E1_STEP_PIN 35
#define E1_DIR_PIN 47
#define E1_ENABLE_PIN 27
#define E1_MS1_PIN 68
#define E1_MS2_PIN 67

If your Z axis now moves properly when connected to the E1 port, then it confirms the problem is with the Z driver. Either the chip is bad, or there is a bad solder joint somewhere, or possibly something else. If the problem is with the Z driver, and you recently purchased the electronics, I would request a replacement. Alternatively, if you never plan on running dual extruders, you may just choose to run the printer with the Z motor running on the E1 port.

---

Help improve the RepRap wiki! Just click "Edit" in the top-right corner of the page and start typing. Anyone can edit the wiki!

Cameron,
Thank you very much for your help! This is exactly what was needed. All carriages now are moving uniformly.
For the benefit of folks reading this later, the board was made by Geeetech and IS NOT the one made/sold by Ultimachine.
Thanks again for your help. This is a fantastic community.
Val