1. A rear-projection screen, comprising:
a total-internal-reflection Fresnel lens configured to aim light received at an incidence angle toward a central axis of said total-internal-reflection Fresnel lens at a convergence angle, wherein said total-internal-reflection Fresnel lens has elements having reflection faces of uniform angle relative to said central axis; and
a refractive Fresnel lens configured to refract said light received from said total-internal-reflection Fresnel lens and at least to reduce said convergence angle.
2. The rear-projection screen as recited in claim 1 wherein said refractive Fresnel lens reduces said convergence angle to about 0\xb0 thereby to collimate said light.
3. The rear-projection screen as recited in claim 1 wherein said total-internal-reflection Fresnel lens and said refractive Fresnel lens are coaxial about said central axis and have concentric, corresponding elements.
4. The rear-projection screen as recited in claim 1 wherein said incidence angle varies between about 55\xb0 and about 75\xb0.
5. The rear-projection screen as recited in claim 1 wherein said total-internal-reflection Fresnel lens has a groove angle between about 40\xb0 and about 70\xb0.
6. The rear-projection screen as recited in claim 1 wherein said total-internal-reflection Fresnel lens and said refractive Fresnel lens are embodied in a unitary plastic structure.
7. A method of rear-projection collimation, comprising:
receiving light at an incidence angle into a total-internal-reflection Fresnel lens, wherein reflection faces of elements in said total-internal-reflection Fresnel lens are of a uniform angle;
employing said total-internal-reflection Fresnel lens to aim said light toward a central axis thereof at a convergence angle;
receiving said light from said total-internal-reflection Fresnel lens into a refractive Fresnel lens; and
employing said refractive Fresnel lens to collimate said light.
8. The method as recited in claim 7 wherein said total-internal-reflection Fresnel lens and said refractive Fresnel lens have corresponding elements and share a central axis.
9. The method as recited in claim 7 wherein said incidence angle is at least about 55\xb0 and at most about 75\xb0.
10. The method as recited in claim 7 wherein a groove angle of said total-internal-reflection Fresnel lens is at least about 40\xb0 and at most about 70\xb0.
11. The method as recited in claim 8 wherein a unitary structure contains both said total-internal-reflection Fresnel lens and said refractive Fresnel lens.
12. A projection visual display (PVD) system, comprising:
a light source configured to generate light;
a projection lens;
a digital micro-mirror device (DMD) configured to receive and reflect portions of said light toward or away from said projection lens;
DMD control circuitry coupled to said DMD and configured to control said DMD in response to data derived from a video stream; and
a projection screen, including:
a total-internal-reflection Fresnel lens configured to redirect said light received at an incidence angle toward a central axis of said total-internal-reflection Fresnel lens at a convergence angle, wherein said central axis is of a substantially uniform angle with respect to reflection faces of elements of said total-internal-reflection Fresnel lens; and
a refractive Fresnel lens that configured to collimate said light.
13. The PVD system as recited in claim 12 wherein said refractive Fresnel lens has said central axis as its central axis.
14. The PVD system as recited in claim 12 wherein said incidence angle lies in a range between about 55\xb0 and about 75\xb0.
15. The PVD system as recited in claim 12 wherein said total-internal-reflection Fresnel lens has a groove angle that lies in a range between about 40\xb0 and about 70\xb0.
16. The PVD system as recited in claim 12 wherein at least one of said total-internal-reflection Fresnel lens and said refractive Fresnel lens contains polymethyl methacrylate.
17. The PVD system as recited in claim 12 further comprising a single-piece, double-fold mirror configured to fold an optical path between said projection lens and said projection screen.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A method for reducing audible motor noise in a motor drive, wherein the motor drive includes a motor controller driving a PWM space vector modulator providing gating pulses to an inverter providing phase currents to the phase windings of the motor, and wherein phase currents of the motor are determined by taking samples of the DC link current in a DC link powering the inverter, the method comprising:
determining the speed of the motor and comparing the speed to a preset threshold, said preset threshold defining a speed at which audible noise is generated by the motor due to a minimum pulse constraint being imposed on the motor phase currents in order to reliably sample the DC link current to measure the phase currents; and
if the speed is below the threshold, sampling the DC link current in the DC link using a reduced number of current samples for a PWM cycle of said PWM space vector modulator to reduce audible motor noise.
2. The method of claim 1, wherein, when the motor speed is above the threshold, four current samples in a PWM cycle in the DC link are taken, and when the motor speed is below the threshold, the current samples are reduced to two current samples.
3. The method of claim 2, wherein the first two samples in a PWM cycle are eliminated thereby utilizing the last two samples in the PWM cycle for determination of the motor phase currents.
4. The method of claim 2, wherein the last two samples in the PWM cycle are eliminated, thereby utilizing the first two samples for determination of the motor phase currents.
5. The method of claim 2, wherein the minimum pulse constraint is imposed on the motor phase currents, and whereby, as a result of the step of reduced current sampling, there are eliminated current samples, and wherein, for the eliminated current samples, phase voltage transitions near the eliminated current samples do not require the minimum pulse constraint.
6. The method of claim 1, wherein the minimum pulse constraint is imposed on the motor phase currents, and whereby, as a result of the step of reduced current sampling, there are eliminated current samples, and wherein, for the eliminated current samples, phase voltage transitions near the eliminated current samples do not require the minimum pulse constraint.
7. The method of claim 1, wherein, when the motor speed is below the threshold, current averaging is eliminated and the current samples are directly used to determine the phase currents.
8. Apparatus for reducing audible motor noise in a motor drive, comprising:
a motor controller driving a PWM space vector modulator providing gating pulses to an inverter, the inverter providing phase currents to the phase windings of the motor, and wherein phase currents of the motor are determined by taking samples of a DC link current in a DC link powering the inverter;
the motor controller determining the speed of the motor, further comprising a comparator stage comparing the speed to a preset threshold, said preset threshold defining a speed at which audible noise is generated by the motor due to a minimum pulse constraint being imposed on the motor phase currents in order to reliably sample the DC link current to measure the phase currents; and
a system for sampling the DC link current in the DC link and if the speed is below the threshold, for reducing the number of current samples of said DC link current in said DC link for a PWM cycle of said PWM space vector modulator to reduce audible motor noise.
9. The apparatus of claim 8, wherein the sampling system operates such that when the motor speed is above the threshold, four current samples in a PWM cycle in the DC link are taken, and when the motor speed is below the threshold, the current samples are reduced to two current samples.
10. The apparatus of claim 9, wherein the sampling system operates such that the first two samples in a PWM cycle are eliminated thereby utilizing the last two samples in the PWM cycle for determination of the motor phase currents.
11. The apparatus of claim 10, wherein the minimum pulse constraint is imposed on the motor phase currents, and whereby, as a result of the reduced current sampling, there are eliminated current samples, and wherein, for the eliminated current samples, phase voltage transitions near the eliminated current samples do not require the minimum pulse constraint.
12. The apparatus of claim 9, wherein the sampling system operates such that the last two samples in the PWM cycle are eliminated, thereby utilizing the first two samples for determination of the motor phase currents.
13. The apparatus of claim 8, wherein the minimum pulse constraint is imposed on the motor phase currents, and whereby, as a result of the reduced current sampling, there are eliminated current samples, and wherein, for the eliminated current samples, phase voltage transitions near the eliminated current samples do not require the minimum pulse constraint.
14. The apparatus of claim 8, wherein, when the motor speed is below the threshold, current averaging is eliminated and the current samples are directly used to determine the phase currents.