Controller Feature: Parking

In order to move a motion control device to an absolute position, a reference must be established to define the coordinate frame. A common way of doing this is to have it search for a limit sensor. All Zaber’s devices support this, but it does require returning to that limit with each power-up or reset to re-establish the reference. Avoiding this movement is sometimes desirable or necessary. Zaber's parking feature is designed to assist with this. Read on to understand more about how a reference is obtained, where parking can be used to compliment this, and what limitations there are.

Note that the information below assumes some basic understanding of Zaber products and how they are connected and controlled from a computer. If you're not familiar with this you may want to review our Quick Setup Guide first.

Obtaining a reference position by homing

For Zaber motion control devices to move accurately, each axis needs a reference position - a known position from which all other positions are measured. This is the purpose of the home command. On receiving a home command a Zaber device will move the specified axis towards its home sensor which, when detected, will set the reference position for the axis. Until the home reference position is set, the axis cannot perform calibrated moves (compensating for mechanical error) and may also move at a reduced speed. Therefore it is always recommended to have a reference position.

However, sending the home command every time the device powers up is not always practical or even possible. For instance,

• in some configurations an enclosure or other physical obstruction may prevent an axis from reaching its home sensor, or
• two or more axes that are mechanically linked and set up in lockstep often cannot all be homed after they are set up.

Obtaining a reference position by parking

In order to allow accurate, calibrated motion in these kinds of situations, the position and reference state of a Zaber device axis can be saved and later restored. This process is what we call "parking" an axis.

The tools parking park command can be used to save the current position and reference state of the specified axis to non-volatile memory before powering down the device. Then, after powering up, instead of sending the home command, the tools parking unpark command can be sent to restore the saved position and reference state. This means a device axis only needs to ever be homed once and thereafter the device can be made to "remember" the axis position using the parking commands.

For this to work properly, however, the axis must not be moved between the calls to tools parking park and tools parking unpark (this is where the name "parking" comes from). The device will enforce this constraint as best as it can. It will

• reject any motion command sent to an axis after the tools parking park command with the reason PARKED (except for the home command) and
• reject parking commands for axes that are very susceptible to backdriving, such as linear motors or voice coils.

Limitations

While a device is powered down there is no hold current to maintain position. It is the user's responsibility to ensure that devices are not moved after they are powered down. Even minute shifts in position may cause the reference state to become invalid, resulting in poor performance or incorrect motion when the device is again powered up and the axis is unparked. To prevent an axis from being inadvertently moved while the device is powered down, it is advisable to remove any large, unnecessary loads and avoid any significant vibrations.