Opening and closing device

1. An opening and closing device comprising:

an electrically conductive fixed body that defines an opening portion therein;

a movable body that is displaceable between a full closed position and a full open position, wherein:

the movable body closes the opening portion of the fixed body at the full closed position; and

the movable body opens the opening portion of the fixed body at the full open position;

a drive motor that drives the movable body to open and close the opening portion of the fixed body;

a speed control device that controls a rotational speed of the drive motor to change a speed of displacement of the movable body;

a detection assembly that includes a sensor electrode, which is located on an end portion of the movable body toward the full closed position, wherein the detection assembly detects a change of a capacitance formed between the sensor electrode and an electrically conductive object adjacent to the sensor electrode; and

a determining device that computes, as a detection value, a change amount of the capacitance at the sensor electrode per a predetermined constant measuring time based on the change of the capacitance detected by the sensor electrode, wherein:

the determining device compares the detection value with a constant threshold value, which is used for determining whether the object exists between the movable body and a periphery of the opening portion;

the determining device determines whether the object exists between the movable body and the periphery of the opening portion based on the comparison result, and

the threshold value is determined based on an actual change of the capacitance detected in advance by the sensor electrode in a condition, where the object is absent between the movable body and the periphery of the opening portion during a closing operation of the movable body.

2. The opening and closing device according to claim 1 , further comprising:

a position detector that outputs a position detection signal in accordance with a position of the movable body, wherein the speed control device controls the rotational speed of the drive motor based on the position detection signal.

3. The opening and closing device according to claim 2 , wherein:

the speed control device controls the rotational speed of the drive motor such that the movable body is displaced at a high speed in a high speed operation range defined between the full open position and a closed-side position, which is located on a side of the full open position toward the full closed position;

the speed control device controls the rotational speed of the drive motor such that the movable body is displaced at a low speed in a low speed operation range that ranges from the closed-side position to a nearly closed position, which is located between the closed-side position and the full closed position; and

the movable body is displaced faster at the high speed than at the low speed.

4. The opening and closing device according to claim 1 , wherein:

the fixed body is a body of a vehicle; and

the movable body is a door panel slidably displaceable along a longitudinal axis of the vehicle for opening and closing the opening portion disposed on a side of the vehicle.

5. The opening and closing device according to claim 1 , wherein:

the fixed body is a body of a vehicle; and

the movable body is a back door operable for opening and closing the opening portion disposed on a rear portion of the vehicle.

6. The opening and closing device according to claim 1 , wherein:

the end portion of the movable body toward the full closed position faces forward in a displacing direction during a closing operation of the movable body for closing the opening portion.