I'm thinking about the quad H-bridge board I want to build to match my quad DC servo controller.

Standard thinking is a discrete bridge built from four MOSFETs and a pair of half-bridge drivers: 4 x IRFP260 @ ~$5 + 2 x IR2184 @ ~$3 = 200V 30A ~$26
But for lower voltages STMicroelectronics make the VNH3SP30-E a "Automotive fully integrated H-bridge motor driver": 1 x VNH3SP30-E = 36V 30A ~$8

These are used by the H-bridge boards I've currently got (packing a lot of power in to a small space). I was initially scared by SMD devices like this, but having seen people achieve amazing results without specialised equipment, I think it's worth the effort.

The only feature I feel is missing is current limiting. The VNH3SP30-E has internal thermal, over-voltage and over-current protection, but that won't stop it delivering 30A to your stalled 2A motor. Dropping a resistor between the VNH3SP30-E and GND would allow you to determine the current by measuring the voltage at VNH3SP30-E's GND pins and combining that with your knowledge of the resistor's value.

Let's use a low 0.01Ω resistor:

• V = IR = 30A x 0.01Ω = 0.3V
• P = I²R = (30A)² x 0.01Ω = 9W 

Therefore, for a 30A draw to show 0.3V, a 0.01Ω 9W current sensing resistor is needed: let's say 2 x 0.02Ω 5W resistors in parrallel.

For low current systems (<3A), the 0.03V drop may be too small to adjust. Let's use a slightly higher (but still very low) 0.1Ω resistor:

• V = IR = 3A x 0.1Ω = 0.3V
• P = I²R = (3A)² x 0.1Ω = 0.9W

A single 1W 0.1Ω current sensing resistor could be used in low current situations to make the drop more pronounced.

Add an LM339 to compare the 4 drops against a reference voltage and pass that thru a 4081 quad 2-input AND chip with the PWM signal for the H-bridge.

Piece of cake!