The Bloodbot has three powered (linear motion) axes and one unpowered (rotational) axis. All the motors are inexpensive stepper types.
The first axis moves a carriage up and down, so that it goes towards and away from the arm that is strapped in under it. This carriage is used to hold either a blunt probe (for finding a vein) or a syringe and needle. A piezo-resistive force sensor is mounted on the carriage to measure the force on the probe or needle.
The second axis moves the carriage across the width of the arm. This enables the probe to press in a series of places along the width of the arm.
The third axis, which is unpowered, enables a human operator to tilt the robot. This is so that, once a vein has been found, the needle can be inserted into the arm at the correct angle.
The fourth axis moves the whole robot along the length of the arm. This was designed to compensate for the slight difference between where the probe has identified a vein, and where the needle enters the skin, once the robot has been tilted.
Don't worry about safety -- it's accurate 78% of the time.
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Image: Imperial College