Positioning mechanism

A method of aligning a detection zone on a liquid handling cartridge with an image capture device, wherein the cartridge is secured to a first member defining an axis of rotation for the cartridge, the method comprising:

• engaging the cartridge with a second member;
• driving the second member to align a detection zone on the cartridge with the image capture device by rotating the cartridge at one or more rotational frequencies below a limit frequency to substantially avoid centrifugally driven liquid flow in the cartridge;
• receiving images from the image capture device while the detection zone is aligned with the image capture device;
• disengaging the second member from the cartridge;
• and
• driving the first member to spin the cartridge at one or more rotational frequencies exceeding the limit frequency to centrifugally drive liquid flow in the cartridge,
• wherein the second member comprises a wheel and a perimeter of the wheel is substantially circular over a first portion of its angular extent and the wheel has a radial extent less than a radial extent of the first portion over a second portion of its annular extent, the first portion facing the cartridge in the first configuration and the second portion facing the cartridge in the second configuration.

A drive system for spinning a liquid handling cartridge and aligning a detection zone of the cartridge with an image capture device, the drive system comprising:

• a cartridge-securing member being configured to securely engage a feature of the cartridge to enable rotation of the cartridge about an axis of rotation;
• a first motor coupled to the cartridge-securing member to drive rotation of the cartridge when the cartridge is engaged with the cartridge-securing member;
• a cartridge-aligning member configurable in a first configuration such that movement of the cartridge-aligning member causes rotation of the cartridge about the axis, when the cartridge is engaged with the cartridge-securing member, and in a second configuration such that the cartridge, when engaged with the cartridge-securing member, is free to rotate about the axis independent of movement of the cartridge-aligning member;
• and
• a second motor coupled to the cartridge-aligning member to drive the cartridge-aligning member to align the detection zone of the cartridge and the image capture device,
• wherein the cartridge-aligning member comprises a wheel, the cartridge-aligning member is configured to align the detection zone of the cartridge by rotation of the wheel and the cartridge-aligning member is configured to transition between the first and the second configuration by rotation of the wheel.