Controlling transitions in optically switchable devices

1. A controller comprising a microprocessor, the controller configured to control an optical transition of an optically switchable device, by:

(a) applying a voltage and/or a current for driving a first optically switchable device to transition from a starting optical state to an ending optical state;

(b) before the transition is complete, determining an electrical characteristic of the first optically switchable device; and, responsive to the determined electrical characteristic:

(c) ramping, over time, the applied voltage and/or the current at a ramp rate to a target voltage value; and

(d) making an adjustment to the ramp rate, to maintain the determined electrical characteristic between threshold levels, wherein the adjustment comprises:

(i) stopping the ramping, reversing the direction of the ramping, decreasing a slope of the ramp rate, or increasing the slope of the ramp rate; and

(ii) substantially matching, during the optical transition, a tint level of the first optically switchable device to that of a second optically switchable device proximate the first optically switchable device.

2. The controller of claim 1 , wherein the first optically switchable device and/or the second optically switchable device is an electrochromic (EC) window.

3. The controller of claim 2 , wherein the EC window is included in a folding curtain wall comprising at least two EC windows coupled by ribbon connectors.

4. The controller of claim 1 , wherein the determined electrical characteristic comprises an open circuit voltage across two electrodes of the optically switchable device.

5. The controller of claim 1 , wherein the determined electrical characteristic comprises a current flowing between two electrodes of the optically switchable device.

6. The controller of claim 1 , wherein the determined electrical characteristic comprises a voltage and/or a current, and wherein using the determined electrical characteristic comprises adjusting the applied voltage and/or current used to drive the transition based at least in part on the determined electrical characteristic to ensure that the optically switchable device is maintained during the optical transition (i) within a safe operating range of current density and/or (ii) within a safe operating voltage range, wherein the safe operating range of current density and the safe operating voltage range facilitate operation such that the optically switchable device is not degraded.

7. The controller of claim 6 , wherein the safe operating range of current density has a maximum current density magnitude between (I) about seventy (70) microamperes per centimeter square and (II) about two hundred and fifty (250) microamperes per centimeter square (μA/cm 2 ).

8. The controller of claim 6 , wherein the safe operating voltage range has a maximum magnitude between (I) about five (5) Volts and (II) about nine (9) Volts (V).

9. The controller of claim 1 , wherein the determined electrical characteristic comprises a voltage and/or a current, and wherein using the determined electrical characteristic comprises adjusting the applied voltage and/or current used to drive the transition based at least in part on the determined electrical characteristic to ensure that the optical transition is occurring at a rate of optical transition that is at least as high as a target rate of optical transition.

10. The controller of claim 9 , wherein using the determined electrical characteristic further comprises adjusting the applied voltage and/or current based at least in part on the determined electrical characteristic to ensure that the optical transition occurs within a target timeframe.

11. The controller of claim 1 , wherein the determined electrical characteristic comprises a voltage and/or a current, and wherein using the determined electrical characteristic comprises adjusting the applied voltage and/or current used to drive the transition based at least in part on the determined electrical characteristic to determine whether the optically switchable device is at or near the ending optical state.

12. The controller of claim 11 , wherein the determined electrical characteristic comprises a current that occurs in response to open circuit voltage conditions applied to the optically switchable device.

13. The controller of claim 12 , the controller further configured to determine a quantity of charge delivered to drive the optical transition, and, based at least in part on the determined quantity of charge delivered, determining whether the optically switchable device is at or near the ending optical state.

14. The controller of claim 1 , the controller further configured to receive a command to transition the optically switchable device to a third optical state after initiation of the optical transition from the starting optical state to the ending optical state, wherein the third optical state is different from the ending optical state, and wherein using the determined electrical characteristic comprises adjusting the applied voltage and/or current used to drive the optical transition is based at least in part on the determined electrical characteristic to thereby drive the optically switchable device to the third optical state.

15. The controller of claim 1 , wherein the target voltage value is (i) a target drive voltage or (ii) a target hold voltage.

16. The controller of claim 1 , wherein the determined electrical characteristic comprises a conducted current, and the controller is further configured to:

ramp the applied voltage and/or current at an increased rate in response to a decrease in the conducted current.

17. The controller of claim 1 , wherein the determined electrical characteristic comprises a conducted current, and the controller is further configured to:

ramp the applied voltage and/or current at a decreased rate in response to an increase in the conducted current.