8.4.1.2 BEMF Constant
The BEMF constant, Kt[6:0] describes the phase-to-phase BEMF voltage of the motor as a function of the motor velocity.
Figure 15 shows the measurement technique for this constant as used in the DRV10983-Q1 device.
With the motor coasting, use an oscilloscope to capture the differential voltage waveform between any two phases. Derive the BEMF constant used by the DRV10983-Q1 device as shown in Equation 1.
Equation 1. Kt
PH = E
p × t
e
where
- Ep is ½ the peak-to-peak amplitude of the measured voltage
- te is the electrical period
The measured BEMF constant (KtPH) must be converted to a 7-bit digital register value Kt[6:0] (combination of KtShift[2:0] and KtValue[3:0]) to program the BEMF constant value. The digital register value can be determined as follows:
- Convert the measured KtPH to a weighted digital value: Ktph_dig = 1090 × KtPH
- Encode the digital value such that Ktph_dig = KtValue[3:0] << KtShift[2:0].
The maximum KtPH that can be programmed is 1760 mV/Hz. This represents a digital value of 1920 and an encoded Kt[6:0] value of 0x7Fh. The minimum KtPH that can be programmed is 0.92 mV/Hz, which represents a digital value of 1 and an encoded Kt[6:0] value of 0x01h.
For convenience, the encoded value of Kt[6:0] may also be obtained from Table 4.
Table 4. BEMF Constant Look-Up Table
Kt[6:0] {KtShift[2:0], KtValue[3:0]} |
KtPH (mV/Hz) |
|
Kt [6:0] {KtShift[2:0], KtValue[3:0]} |
KtPH (mV/Hz) |
|
Kt [6:0] {KtShift[2:0], KtValue[3:0]} |
KtPH (mV/Hz) |
BINARY |
HEX |
BINARY |
HEX |
BINARY |
HEX |
000 0000 |
0x00 |
0 |
|
010 1000 |
0x28 |
29.44 |
|
101 1000 |
0x58 |
235.52 |
000 0001 |
0x01 |
0.92 |
|
010 1001 |
0x29 |
33.12 |
|
101 1001 |
0x59 |
264.96 |
000 0010 |
0x02 |
1.84 |
|
010 1010 |
0x2A |
36.8 |
|
101 1010 |
0x5A |
294.4 |
000 0011 |
0x03 |
2.76 |
|
010 1011 |
0x2B |
40.48 |
|
101 1011 |
0x5B |
323.84 |
000 0100 |
0x04 |
3.68 |
|
010 1100 |
0x2C |
44.16 |
|
101 1100 |
0x5C |
353.28 |
000 0101 |
0x05 |
4.6 |
|
010 1101 |
0x2D |
47.84 |
|
101 1101 |
0x5D |
382.72 |
000 0110 |
0x06 |
5.52 |
|
010 1110 |
0x2E |
51.52 |
|
101 1110 |
0x5E |
412.16 |
000 0111 |
0x07 |
6.44 |
|
010 1111 |
0x2F |
55.2 |
|
101 1111 |
0x5F |
441.6 |
000 1000 |
0x08 |
7.36 |
|
011 1000 |
0x38 |
58.88 |
|
110 1000 |
0x68 |
471.04 |
000 1001 |
0x09 |
8.28 |
|
011 1001 |
0x39 |
66.24 |
|
110 1001 |
0x69 |
529.92 |
000 1010 |
0x0A |
9.2 |
|
011 1010 |
0x3A |
73.6 |
|
110 1010 |
0x6A |
588.8 |
000 1011 |
0x0B |
10.12 |
|
011 1011 |
0x3B |
80.96 |
|
110 1011 |
0x6B |
647.68 |
000 1100 |
0x0C |
11.04 |
|
011 1100 |
0x3C |
88.32 |
|
110 1100 |
0x6C |
706.56 |
000 1101 |
0x0D |
11.96 |
|
011 1101 |
0x3D |
95.68 |
|
110 1101 |
0x6D |
765.44 |
000 1110 |
0x0E |
12.88 |
|
011 1110 |
0x3E |
103.04 |
|
110 1110 |
0x6E |
824.32 |
000 1111 |
0x0F |
13.8 |
|
011 1111 |
0x3F |
110.4 |
|
110 1111 |
0x6F |
883.2 |
001 1000 |
0x18 |
14.72 |
|
100 1000 |
0x48 |
117.76 |
|
111 1000 |
0x78 |
942.08 |
001 1001 |
0x19 |
16.56 |
|
100 1001 |
0x49 |
132.48 |
|
111 1001 |
0x79 |
1059.84 |
001 1010 |
0x1A |
18.4 |
|
100 1010 |
0x4A |
147.2 |
|
111 1010 |
0x7A |
1177.6 |
001 1011 |
0x1B |
20.24 |
|
100 1011 |
0x4B |
161.92 |
|
111 1011 |
0x7B |
1295.36 |
001 1100 |
0x1C |
22.08 |
|
100 1100 |
0x4C |
176.64 |
|
111 1100 |
0x7C |
1413.12 |
001 1101 |
0x1D |
23.92 |
|
100 1101 |
0x4D |
191.36 |
|
111 1101 |
0x7D |
1530.88 |
001 1110 |
0x1E |
25.76 |
|
100 1110 |
0x4E |
206.08 |
|
111 1110 |
0x7E |
1648.64 |
001 1111 |
0x1F |
27.6 |
|
100 1111 |
0x4F |
220.8 |
|
111 1111 |
0x7F |
1766.4 |