H-bridge - concern about mosfets
To turn on any of the MOSFETs properly in your circuit, the gate voltage has to be significantly higher than the source voltage. For the lower position devices Q2 and Q4, the sources are grounded to 0V therefore they can easily turn on into "saturation" by applying a gate voltage of a few volts above ground.

For the devices connected to the top rail, if you want the on-resistance between drain and source to be really low you have to obey the same rule - the drive voltage to the gate has to be several volts above the source. Now for low volt drop you want the source to be switched virtually to 6V - where in the circuit can the gate receive a voltage of maybe 9 or 10 volts?

There isn't one so please consider two options: -


Using P channel MOSFETs at the top rail, source to 6V. P MOS is slower than N MOS but in your application it won't make a difference (1.7kHz).
Using a drive circuit derived from a supply voltage that is at least 9V and quite possibly higher to get the best saturation from the MOSFETs.


Now the relays. What are you hoping to achieve here? The MOSFET whose gate is disconnected will float to some almost random voltage level and possibly turn on unexpectedly or just get hot. Get rid of the relays unless you have some cunning plan for their use which eludes me.

This is a standard H bridge using P and N MOSFETs: -



Please ask if you need recommendations for devices.