Setting up parellel motors (two motors driving one gantry)

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Gantry robot with two motored linear stages driving one axis

Controlling paralleled servo motors (two or more servo motors driving same mechanically linked axis) is possible but requires special attention due to following reasons:

Problem Solution
Motors tend to fight against each other during operation due to lead screw pitch error or feedback device inaccuracies This is not a problem in systems where some mechanical flexibility is present between the parallel motors. Flexibility reduces the fighting force to insignificantly small.

More advanced control may be required with highly rigid machines.

Servo motor tuning may need to be done for single axis at the time. This is possible due to the same mechanical flexibility reason as above servo fighting issue. The trick is to use very small travel when tuning the motors to avoid causing excess bending forces to the mechanics. See more info in the next chapter.
Motor fault stopping must be synchronized to avoid only one motor driving the axis and second acting as a load which may cause excessive bending stress to mechanics. This is possible by electrically wiring fault stop output signal to parallel axis enable input (some external logic may be needed). Another way is to connect fault output to the master controller which will disable all axis with low latency once fault occurs in any axis.
Motor initialization must not produce any uncontrolled motion. This is problem only with AC/BLDC motors without Hall sensors and homing function with automatic start after drive power-on. See more info in the next chapter.

Setting up[edit | edit source]

Servo tuning for parallel motors requires special attention as motors are not allowed to move freely without synchronism. However tuning the motors separately is usually still possible as gantry has some flexibility allowing one motor move just enough for tuning purposes.

Note on AC/BLDC/linear motors[edit | edit source]

Unlike DC motors, AC & BLDC & linear motors (synchronous motors) require alignment of magnetic angle/position to windings, a.k.a phasing. This can be generally achieved by using one of the following methods:

  1. By driving current to windings and letting motor to settle to certain magnetic angle
  2. By utilizing absolute feedback device, such as Hall sensors or resolver, to give all needed information directly

Method 1 is not suitable with parallel axis as motor needs to be able to move in random direction a little but during drive power-up. The only option is to use absolute sensors which allows motor to stand still at power up. Typically Hall sensors are used in parallel motor configurations.

Tuning[edit | edit source]

After drives & motors are successfully powered and holding position, tuning is straightforward: tune one motor normally by use only very little amplitude of step response tests to prevent gantry flexing too much (to avoid damage). After one motor is tuned, the settings typically can be transferred 1:1 to the another motor.

It is highly recommended to do basic setup of motors with unconnected load (non mounted shaft). Basic setup includes setting motor parameters, torque tuning and, most importantly, testing motor's ability to stand still in position mode from the moment of power-up to entering into step response tuning.

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